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Title 14Chapter ISubchapter JPart 171 → Subpart J


Title 14: Aeronautics and Space
PART 171—NON-FEDERAL NAVIGATION FACILITIES


Subpart J—Microwave Landing System (MLS)


Contents
§171.301   Scope.
§171.303   Definitions.
§171.305   Requests for IFR procedure.
§171.307   Minimum requirements for approval.
§171.309   General requirements.
§171.311   Signal format requirements.
§171.313   Azimuth performance requirements.
§171.315   Azimuth monitor system requirements.
§171.317   Approach elevation performance requirements.
§171.319   Approach elevation monitor system requirements.
§171.321   DME and marker beacon performance requirements.
§171.323   Fabrication and installation requirements.
§171.325   Maintenance and operations requirements.
§171.327   Operational records.

Source: Docket No. 20669, 51 FR 33177, Sept. 18, 1986, unless otherwise noted.

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§171.301   Scope.

This subpart sets forth minimum requirements for the approval, installation, operation and maintenance of non-Federal Microwave Landing System (MLS) facilities that provide the basis for instrument flight rules (IFR) and air traffic control procedures.

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§171.303   Definitions.

As used in this subpart:

Auxiliary data means data transmitted in addition to basic data that provide ground equipment siting information for use in refining airborne position calculations and other supplementary information.

Basic data means data transmitted by the ground equipment that are associated directly with the operation of the landing guidance system.

Beam center means the midpoint between the −3 dB points on the leading and trailing edges of the scanning beam main lobe.

Beamwidth means the width of the scanning beam main lobe measured at the −3 dB points and defined in angular units on the boresight, in the horizontal plane for the azimuth function and in the vertical plane for the elevation function.

Clearance guidance sector means the volume of airspace, inside the coverage sector, within which the azimuth guidance information provided is not proportional to the angular displacement of the aircraft, but is a constant fly-left or fly-right indication of the direction relative to the approach course the aircraft should proceed in order to enter the proportional guidance sector.

Control Motion Noise (CMN) means those fluctuations in the guidance which affect aircraft attitude, control surface motion, column motion, and wheel motion. Control motion noise is evaluated by filtering the flight error record with a band-pass filter which has corner frequencies at 0.3 radian/sec and 10 radians/sec for azimuth data and 0.5 radian/sec and 10 radians/sec for elevation data.

Data rate means the average number of times per second that transmissions occur for a given function.

Differential Phase Shift Keying (DPSK) means differential phase modulation of the radio frequency carrier with relative phase states of 0 degree or 180 degrees.

Failure means the inability of an item to perform within previously specified limits.

Guard time means an unused period of time provided in the transmitted signal format to allow for equipment tolerances.

Integrity means that quality which relates to the trust which can be placed in the correctness of the information supplied by the facility.

Mean corrective time means the average time required to correct an equipment failure over a given period, after a service technician reaches the facility.

Mean course error means the mean value of the azimuth error along a specified radial of the azimuth function.

Mean glide path error means the mean value of the elevation error along a specified glidepath of the elevation function.

Mean-time-between-failures (MTBF) means the average time between equipment failures over a given period.

Microwave Landing System (MLS) means the MLS selected by ICAO for international standardization.

Minimum glidepath means the lowest angle of descent along the zero degree azimuth that is consistent with published approach procedures and obstacle clearance criteria.

MLS Approach Reference Datum is a point at a specified height located vertically above the intersection of the runway centerline and the threshold.

MLS back azimuth reference datum means a point 15 meters (50 feet) above the runway centerline at the runway midpoint.

MLS datum point means a point defined by the intersection of the runway centerline with a vertical plane perpendicular to the centerline and passing through the elevation antenna phase center.

Out of coverage indication (OCI) means a signal radiated into areas outside the intended coverage sector, where required, to specifically prevent invalid removal of an airborne warning indication in the presence of misleading guidance information.

Path Following Error (PFE) means the guidance perturbations which could cause aircraft displacement from the desired course or glidepath. It is composed of the path following noise and of the mean course error in the case of azimuth functions, or the mean glidepath error in the case of elevation functions. Path following errors are evaluated by filtering the flight error record with a second order low pass filter which has a corner frequency at 0.5 radian/sec for azimuth data or 1.5 radians/sec for elevation data.

Path following noise (PFN) means that portion of the guidance signal error which could cause displacement from the actual mean course line or mean glidepath as appropriate.

Split-site ground station means the type of ground station in which the azimuth portion of the ground station is located near the stop end of the runway, and the elevation portion is located near the approach end.

Time division multiplex (TDM) means that each function is transmitted on the same frequency in time sequence, with a distinct preamble preceding each function transmission.

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§171.305   Requests for IFR procedure.

(a) Each person who requests an IFR procedure based on an MLS facility which that person owns must submit the following information with that request:

(1) A description of the facility and evidence that the equipment meets the performance requirements of §§171.309, 171.311, 171.313, 171.315, 171.317, 171.319, and 171.321 and is fabricated and installed in accordance with §171.323.

(2) A proposed procedure for operating the facility.

(3) A proposed maintenance organization and a maintenance manual that meets the requirements of §171.325.

(4) A statement of intent to meet the requirements of this subpart.

(5) A showing that the facility has an acceptable level of operational reliability and an acceptable standard of performance. Previous equivalent operational experience with a facility with identical design and operational characteristics will be considered in showing compliance with this subparagraph.

(b) FAA inspects and evaluates the MLS facility; it advises the owner of the results, and of any required changes in the MLS facility or in the maintenance manual or maintenance organization. The owner must then correct the deficiencies, if any, and operate the MLS facility for an in-service evaluation by the FAA.

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§171.307   Minimum requirements for approval.

(a) The following are the minimum requirements that must be met before the FAA approves an IFR procedure for a non-Federal MLS facility:

(1) The performance of the MLS facility, as determined by flight and ground inspection conducted by the FAA, must meet the requirements of §§171.309, 171.311, 171.313, 171.315, 171.317, 171.319, and 171.321.

(2) The fabrication and installation of the equipment must meet the requirements of §171.323.

(3) The owner must agree to operate and maintain the MLS facility in accordance with §171.325.

(4) The owner must agree to furnish operational records as set forth in §171.327 and agree to allow the FAA to inspect the facility and its operation whenever necessary.

(5) The owner must assure the FAA that he will not withdraw the MLS facility from service without the permission of the FAA.

(6) The owner must bear all costs of meeting the requirements of this section and of any flight or ground inspection made before the MLS facility is commissioned.

(b) [Reserved]

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§171.309   General requirements.

The MLS is a precision approach and landing guidance system which provides position information and various ground-to-air data. The position information is provided in a wide coverage sector and is determined by an azimuth angle measurement, an elevation angle measurement and a range (distance) measurement.

(a) An MLS constructed to meet the requirements of this subpart must include:

(1) Approach azimuth equipment, associated monitor, remote control and indicator equipment.

(2) Approach elevation equipment, associated monitor, remote control and indicator equipment.

(3) A means for the encoding and transmission of essential data words, associated monitor, remote control and indicator equipment. Essential data are basic data words 1, 2, 3, 4, and 6 and auxiliary data words A1, A2 and A3.

(4) Distance measuring equipment (DME), associated monitor, remote control and indicator equipment.

(5) Remote controls for paragraphs (a) (1), (2), (3), and (4) of this section must include as a minimum on/off and reset capabilities and may be integrated in the same equipment.

(6) At locations where a VHF marker beacon (75 MHz) is already installed, it may be used in lieu of the DME equipment.

(b) In addition to the equipment required in paragraph (a) of this section the MLS may include:

(1) Back azimuth equipment, associated monitor, remote control and indicator equipment. When Back Azimuth is provided, a means for transmission of Basic Data Word 5 and Auxiliary Data Word A4 shall also be provided.

(2) A wider proportional guidance sector which exceeds the minimum specified in §§171.313 and 171.317.

(3) Precision DME, associated monitor, remote control and indicator equipment.

(4) VHF marker beacon (75 MHz), associated monitor, remote control and indicator equipment.

(5) The MLS signal format will accommodate additional functions (e.g., flare elevation) which may be included as desired. Remote controls for paragraphs (b) (1), (3) and (4) of this section must include as a minimum on/off and reset capabilities, and may be integrated in the same equipment.

(6) Provisions for the encoding and transmission of additional auxiliary data words, associated monitor, remote control and indicator equipment.

(c) MLS ground equipment must be designed to operate on a nominal 120/240 volt, 60 Hz, 3-wire single phase AC power source and must meet the following service conditions:

(1) AC line parameters, DC voltage, elevation and duty:

120 VAC nominal value—102 V to 138 V (±1 V)*

240 VAC nominal value—204 V to 276 V (±2 V)*

60 Hz AC line frequency—57 Hz to 63 Hz (±0.2 Hz)*

*Note: Where discrete values of the above frequency or voltages are specified for testing purposes, the tolerances given in parentheses indicated by an asterisk apply to the test instruments used to measure these parameters.

Elevation—0 to 3000 meters (10,000 feet) above sea level

Duty—Continuous, unattended

(2) Ambient conditions within the shelter for electronic equipment installed in shelters are:

Temperature, −10 °C to + 50 °C

Relative humidity, 5% to 90%

(3) Ambient conditions for electronic equipment and all other equipment installed outdoors (for example, antenna, field detectors, and shelters):

Temperature, −50 °C to + 70 °C

Relative humidity, 5% to 100%

(4) All equipment installed outdoors must operate satisfactorily under the following conditions:

Wind Velocity: The ground equipment shall remain within monitor limits with wind velocities of up to 70 knots from such directions that the velocity component perpendicular to runway centerline does not exceed 35 knots. The ground equipment shall withstand winds up to 100 knots from any direction without damage.

Hail Stones: 1.25 centimeters ( 12 inch) diameter.

Rain: Provide required coverage with rain falling at a rate of 50 millimeters (2 inches) per hour, through a distance of 9 kilometers (5 nautical miles) and with rain falling at the rate of 25 millimeters (1 inch) per hour for the additional 28 kilometers (15 nautical miles).

Ice Loading: Encased in 1.25 centimeters ( 12 inch) radial thickness of clear ice.

Antenna Radome De-Icing: Down to −6 °C (20 °F) and wind up to 35 knots.

(d) The transmitter frequencies of an MLS must be in accordance with the frequency plan approved by the FAA.

(e) The DME component listed in paragraph (a)(4) of this section must comply with the minimum standard performance requirements specified in subpart G of this part.

(f) The marker beacon components listed in paragraph (b)(4) of this section must comply with the minimum standard performance requirements specified in subpart H of this part.

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§171.311   Signal format requirements.

The signals radiated by the MLS must conform to the signal format in which angle guidance functions and data functions are transmitted sequentially on the same C-band frequency. Each function is identified by a unique digital code which initializes the airborne receiver for proper processing. The signal format must meet the following minimum requirements:

(a) Frequency assignment. The ground components (except DME/Marker Beacon) must operate on a single frequency assignment or channel, using time division multiplexing. These components must be capable of operating on any one of the 200 channels spaced 300 KHz apart with center frequencies from 5031.0 MHz to 5090.7 MHz and with channel numbering as shown in Table 1a. The operating radio frequencies of all ground components must not vary by more than ±10 KHz from the assigned frequency. Any one transmitter frequency must not vary more than ±50 Hz in any one second period. The MLS angle/data and DME equipment must operate on one of the paired channels as shown in Table 1b.

Table 1a—Frequency Channel Plan

Channel No. Frequency (MHz)
5005031.0
5015031.3
5025031.6
5035031.9
5045032.2
5055032.5
5065032.8
5075033.1
5085033.4
5095033.7
5105034.0
5115034.3
*            *            *            *            *
5985060.4
5995060.7
6005061.0
6015061.3
*            *            *            *            *
6985090.4
6995090.7

Table 1b—Channels

Channel pairingDME parameters
DME No.VHF freq. MHzMLS angle freq. MHzMLS Ch. No.InterrogationReply
Freq. MHzPulse codesFreq. MHzPulse codes µs
DME/N µsDME/P Mode
IA µsFA µs
*1X10251296212
**1Y102536108830
*2X10261296312
**2Y102636108930
*3X10271296412
**3Y102736109030
*4X10281296512
**4Y102836109130
*5X10291296612
**5Y102936109230
*6X10301296712
**6Y103036109330
*7X10311296812
**7Y103136109430
*8X10321296912
**8Y103236109530
*9X10331297012
**9Y103336109630
*10X10341297112
**10Y103436109730
*11X10351297212
**11Y103536109830
*12X10361297312
**12Y103636109930
*13X10371297412
**13Y103736110030
*14X10381297512
**14Y103836110130
*15X10391297612
**15Y103936110230
*16X10401297712
**16Y104036110330
17X108.0010411297812
17Y108.055043.05401041363642110430
17Z5043.354110412127110415
18X108.105031.0500104212121897912
18W5031.35011042243097924
18Y108.155043.65421042363642110530
18Z5043.954310422127110515
19X108.2010431298012
19Y108.255044.25441043363642110630
19Z5044.554510432127110615
20X108.305031.6502104412121898112
20W5031.95031044243098124
20Y108.355044.85461044363642110730
20Z5045.154710442127110715
21X108.4010451298212
21Y108.455045.45481045363642110830
21Z5045.754910452127110815
22X108.505032.2504104612121898312
22W5032.55051046243098324
22Y108.555046.05501046363642110930
22Z5046.355110462127110915
23X108.6010471298412
23Y108.655046.65521047363642111030
23Z5046.955310472127111015
24X108.705032.8506104812121898512
24W5033.15071048243098524
24Y108.755047.25541048363642111130
24Z5047.555510482127111115
25X108.8010491298612
25Y108.855047.85561049363642111230
25Z5048.155710492127111215
26X108.905033.4508105012121898712
26W5033.75091050243098724
26Y108.955048.45581050363642111330
26Z5048.755910502127111315
27X109.0010511298812
27Y109.055049.05601051363642111430
27Z5049.356110512127111415
28X109.105034.0510105212121898912
28W5034.35111052243098924
28Y109.155049.65621052363642111530
28Z5049.956310522127111515
29X109.2010531299012
29Y109.255050.25641053363642111630
29Z5050.556510432127111615
30X109.305034.6512105412121899112
30W5034.95131054243099124
30Y109.355050.85661054363642111730
30Z5051.156710542127111715
31X109.4010551299212
31Y109.455051.45681055363642111830
31Z5051.756910552127111815
32X109.505035.2514105612121899312
32W5035.55151056243099324
32Y109.555052.05701056363642111930
32Z5052.357110562127111915
33X109.6010571299412
33Y109.655052.65721057363642112030
33Z5052.957310572127112015
34X109.705035.8516105812121899512
34W5036.15171058243099524
34Y109.755053.25741058363642112130
34Z5053.557510582127112115
35X109.8010591299612
35Y109.855053.85761059363642112230
35Z5054.157710592127112215
36X109.905036.4518106012121899712
36W5036.75191060243099724
36Y109.955054.45781060363642112330
36Z5054.757910602127112315
37X110.0010611299812
37Y110.055055.05801061363642112430
37Z5055.358110612127112415
38X110.105037.0520106212121899912
38W5037.35211062243099924
38Y110.155055.65821062363642112530
38Z5055.958310622127112515
39X110.20106312100012
39Y110.255056.25841063363642112630
39Z5056.558510632127112615
40X110.305037.65221064121218100112
40W5037.952310642430100124
40Y110.355056.85861064363642112730
40Z5057.158710642127112715
41X110.40106512100212
41Y110.455057.45881065363642112830
41Z5057.758910652127112815
42X110.505038.25241066121218100312
42W5038.552510662430100324
42Y110.555058.05901066363642112930
42Z5058.359110662127112915
43X110.60106712100412
43Y110.655058.65921067363642113030
43Z5058.959310672127113015
44X110.705038.85261068121218100512
44W5039.152710682430100524
44Y110.755059.25941068363642113130
44Z5059.559510682127113115
45X110.80106912100612
45Y110.855059.85961069363642113230
45Z5060.159710692127113215
46X110.905039.45281070121218100712
46W5039.752910702430100724
46Y110.955060.45981070363642113330
46Z5060.759910702127113315
47X111.00107112100812
47Y111.055061.06001071363642113430
47Z5061.360110712127113415
48X111.105040.05301072121218100912
48W5040.353110722430100924
48Y111.155061.66021072363642113530
48Z5061.960310722127113515
49X111.20107312101012
49Y111.255062.26041073363642113630
49Z5062.560510732127113615
50X111.305040.65321074121218101112
50W5040.953310742430101124
50Y111.355062.86061074363642113730
50Z5063.160710742127113715
51X111.40107512101212
51Y111.455063.46081075363642113830
51Z5063.760910752127113815
52X111.505041.25341076121218101312
52W5041.553510762430101324
52Y111.555064.06101076363642113930
52Z5064.361110762127113915
53X111.60107712101412
53Y111.655064.66121077363642114030
53Z5064.961310772127114015
54X111.705041.85361078121218101512
54W5042.153710782430101524
54Y111.755065.26141078363642114130
54Z5065.561510782127114115
55X111.80107912101612
55Y111.855065.86161079363642114230
55Z5066.161710792127114215
56X111.905042.45381080121218101712
56W5042.753910802430101724
56Y111.955066.46181080363642114330
56Z5066.761910802127114315
57X112.00108112101812
57Y112.05108136114430
58X112.10108212101912
58Y112.15108236114530
59X112.20108312102012
59Y122.25108336114630
**60X108412102112
**60Y108436114730
**61X108512102212
**61Y108536114830
**62X108612102312
**62Y108636114930
**63X103712102412
**63Y108736115030
**64X108812115112
**64Y108836102530
**65X108912115212
**65Y108936102630
**66X109012115312
**66Y109036102730
**67X109112115412
**67Y109136102830
**68X109212115512
**68Y109236102930
**69X109312115612
**69Y109336103030
70X112.30109412115712
**70Y112.35109436103130
71X112.40109512115812
**71Y112.45109536103230
72X112.50109612115912
**72Y112.55109636103330
73X112.60109712116012
**73Y112.65109736103430
74X112.70109812116112
**74Y112.75109836103530
75X112.80109912116212
**75Y112.85109936103630
76X112.90110012116312
**76Y112.95110036103730
77X113.00110112116412
**77Y113.05110136103830
78X113.10110212116512
**78Y113.15110236103930
79X113.20110312116612
**79Y113.25110336104030
80X113.30110412116712
80Y113.355067.06201104363642104130
80Z5067.362111042127104115
81X113.40110512116812
81Y113.455067.66221105363642104230
81Z5067.962310052127104215
82X113.50110612116912
82Y113.555068.26241106363642104330
82Z5068.562511062127104315
83X113.60110712117012
83Y113.655068.86261107363642104430
83Z5069.162711072127104415
84X113.70110812117112
84Y113.755069.46281108363642104530
84Z6069.762911082127104515
85X113.80110912117212
85Y113.855070.06301109363642104630
85Z5070.363111092127104615
86X113.90111012117312
86Y113.955070.66321110363642104730
86Z5070.963311102127104715
87X114.00111112117412
87Y114.055071.26341111363642104830
87Z5071.563511112127104815
88X114.10111212117512
88Y114.155071.86361112363642104930
88Z5072.163711122127104915
89X114.20111312117612
89Y114.255072.46381113363642105030
89Z5072.763911132127105015
90X114.30111412117712
90Y114.355073.06401114363642105130
90Z5073.364111142127105115
91X114.40111512117812
91Y114.455073.66421115363642105230
91Z5073.964311152127105215
92X114.50111612117912
92Y114.555074.26441116363642105330
92Z5074.564511162127105315
93X114.60111712118012
93Y114.655074.86461117363642105430
93Z5075.164711172127105415
94X114.70111812118112
94Y114.755075.46481118363642105530
94Z5075.764911182127105515
95X114.80111912118212
95Y114.855076.06501119363642105630
95Z5076.365111192127105615
96X114.90112012118312
96Y114.955076.66521120363642105730
96Z5076.965311202127105715
97X115.00112112118412
97Y115.055077.26541121363642105830
97Z5077.565511212127105815
98X115.10112212118512
98Y115.155077.86561122363642105930
98Z5078.165711222127105915
99X115.20112312118612
99Y115.255078.46581123363642106030
99Z5078.765911232127106015
100X115.30112412118712
100Y115.355079.06601124363642106130
100Z5079.366111242127106115
101X115.40112512118812
101Y115.455079.66621125363642106230
101Z5079.966311252127106215
102X115.50112612118912
102Y115.555080.26641126363642106330
102Z5080.566511262127106315
103X115.60112712119012
103Y115.655080.B6661127363642106430
103Z5081.166711272127106419
104X115.70112812119112
104Y115.755081.46681128363642106530
104Z5081.766911282127106519
105X115.80112912119212
105Y115.855082.06701129363642106630
105Z5082.367111292127106615
106X115.90113012119312
106Y115.955082.66721130363642106730
106Z5082.967311302127106715
107X116.00113112119412
107Y116.055083.26741131363642106830
107Z5083.567511312127106815
108X116.10508113212119512
108Y116.155083.86761132363642106930
108Z5084.167711322127106915
109X116.20113312119612
109Y116.255084.46781133363642107030
109Z5084.767911332127107015
110X116.30113412119712
110Y116.355085.06801134363642107130
110Z5085.368111342127107115
111X116.40113512119812
111Y116.455086.66821135363642107230
111Z5085.968311352127107215
112X116.50113612119912
112Y116.555086.26841136363642107330
112Z5086.568511362127107315
113X116.60113712120012
113Y116.655086.86861137363642107430
113Z5087.168711372127107415
114X116.70113812120112
114Y116.755087.46881138363642107530
114Z5087.768911382127107515
115X116.80113912120212
115Y116.855088.06901139363642107630
115Z5088.369111392127107615
116X116.90114012120312
116Y116.955088.66921140363642107730
116Z5088.969311402127107715
117X117.00114112120412
117Y117.055089.26941141363642107830
117Z5089.569511412127107815
118X117.1011421212.512
118Y117.155089.86961142363642107930
118Z5090.169711422127107912
119X117.20114312120612
119Y117.255090.46981143363642108030
119Z5090.769911432127108015
120X117.30114412120712
120Y117.35114436108130
121X117.40114512120812
121Y117.45114536108230
122X117.50114612120912
122Y117.55114636108330
123X117.60114712121012
123Y117.65114736108430
124X117.70114812121112
**124Y117.75114836108530
125X117.80114912121212
**125Y117.85114936108630
126X117.90115012121312
**126Y117.95115036108730

Notes:

*These channels are reserved exclusively for national allotments.

**These channels may be used for national allotment on a secondary basis. The primary reason for reserving these channels is to provide protection for the secondary Surveillance Radar (SSR) system.

108.0 MHz is not scheduled for assignment to ILS service. The associated DME operating channel No. 17X may be assigned to the emergency service.

(b) Polarization. (1) The radio frequency emissions from all ground equipment must be nominally vertically polarized. Any horizontally polarized radio frequency emission component from the ground equipment must not have incorrectly coded angle information such that the limits specified in paragraphs (b) (2) and (3) of this section are exceeded.

(2) Rotation of the receiving antenna thirty degrees from the vertically polarized position must not cause the path following error to exceed the allowed error at that location.

(c) Modulation requirements. Each function transmitter must be capable of DPSK and continuous wave (CW) modulations of the RF carrier which have the following characteristics.

(1) DPSK. The DPSK signal must have the following characteristics:

bit rate15.625 KHz
bit length64 microseconds
logic “0”no phase transition
logic “1”phase transition
phase transitionless than 10 microseconds
phase tolerance±10 degrees

The phase shall advance (or retard) monotonically throughout the transition region. Amplitude modulation during the phase transition period shall not be used.

eCFR graphic ec15se91.005.gif

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(2) CW. The CW pulse transmissions and the CW angle transmissions as may be required in the signal format of any function must have characteristics such that the requirements of paragraph (d) of this section are met.

(d) Radio frequency signal spectrum. The transmitted signal must be such that during the transmission time, the mean power density above a height of 600 meters (2000 feet) does not exceed −100.5 dBW/m2 for angle guidance and −95.5 dBW/m2 for data, as measured in a 150 KHz bandwidth centered at a frequency of 840 KHz or more from the assigned frequency.

(e) Synchronization. Synchronization between the azimuth and elevation components is required and, in split-site configurations, would normally be accomplished by landline interconnections. Synchronization monitoring must be provided to preclude function overlap.

(f) Transmission rates. Angle guidance and data signals must be transmitted at the following average repetition rates:

FunctionAverage data rate (Hertz)
Approach Azimuth13 ±0.5
High Rate Approach Azimuth139 ±1.5
Approach Elevation39 ±1.5
Back Azimuth6.5 ±0.25
Basic Data(2)
Auxiliary Data(3)

1The higher rate is recommended for azimuth scanning antennas with beamwidths greater than two degrees. It should be noted that the time available in the signal format for additional functions is limited when the higher rate is used.

2Refer to Table 8a.

3Refer to Table 8c.

(g) Transmission sequences. Sequences of angle transmissions which will generate the required repetition rates are shown in Figures 2 and 3.

eCFR graphic ec15se91.006.gif

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eCFR graphic ec15se91.007.gif

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(h) TDM cycle. The time periods between angle transmission sequences must be varied so that exact repetitions do not occur within periods of less than 0.5 second in order to protect against synchronous interference. One such combination of sequences is shown in Figure 4 which forms a full multiplex cycle. Data may be transmitted during suitable open times within or between the sequences.

eCFR graphic ec15se91.008.gif

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(i) Function Formats (General). Each angle function must contain the following elements: a preamble; sector signals; and a TO and FRO angle scan organized as shown in Figure 5a. Each data function must contain a preamble and a data transmission period organized as shown in Figure 5b.

eCFR graphic ec15se91.009.gif

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(1) Preamble format. The transmitted angle and date functions must use the preamble format shown in Figure 6. This format consists of a carrier acquisition period of unmodulated CW transmission followed by a receiver synchronization code and a function identification code. The preamble timing must be in accordance with Table 2.

eCFR graphic ec15se91.010.gif

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(i) Digital codes. The coding used in the preamble for receiver synchronization is a Barker code logic 11101. The time of the last phase transition midpoint in the code shall be the receiver reference time (see Table 2). The function identification codes must be as shown in Table 3. The last two bits (I11 and I12) of the code are parity bits obeying the equations:

I6 + I7 + I8 + I9 + I10 + I11 = Even

I6 + I8 + I10 + I12 = Even

(ii) Data modulation. The digital code portions of the preamble must be DPSK modulated in accordance with §171.311(c)(1) and must be transmitted throughout the function coverage volume.

(2) Angle function formats. The timing of the angle transmissions must be in accordance with Tables 4a, 4b, and 5. The actual timing of the TO and FRO scans must be as required to meet the accuracy requirements of §§171.313 and 171.317.

(i) Preamble. Must be in accordance with requirements of §171.311(i)(1).

Table 2—Preamble Timing1

Event Event time slot begins at—
15.625 kHz clock pulse (number) Time (milliseconds)
Carrier acquisition:
(CW transmission)00
Receiver reference time code:
I1 = 1130.832
I2 = 1140.896
I3 = 1150.960
I4 = 0161.024
I5 = 11721.088
Function identification:
I6181.152
I7191.216
I8201.280
I9211.344
I10 (see table 1)221.408
I11231.472
I12241.536
END PREAMBLE251.600

1Applies to all functions transmitted.

2Reference time for receiver synchronization for all function timing.

Table 3—Function Identification Codes

Function Code
I6 I7 I8 I9 I10 I11 I12
Approach azimuth0011001
High rate approach azimuth0010100
Approach elevation1100001
Back azimuth1001001
Basic data 10101000
Basic data 20111100
Basic data 31010000
Basic data 41000100
Basic data 51101100
Dasic data 60001101
Auxiliary data A1110010
Auxiliary data B1010111
Auxiliary data C1111000

(ii) Sector signals. In all azimuth formats, sector signals must be transmitted to provide Morse Code identification, airborne antenna selection, and system test signals. These signals are not required in the elevation formats. In addition, if the signal from an installed ground component results in a valid indication in an area where no valid guidance should exist, OCI signals must be radiated as provided for in the signal format (see Tables 4a, 4b, and 5). The sector signals are defined as follows:

(A) Morse Code. DPSK transmissions that will permit Morse Code facility identification in the aircraft by a four letter code starting with the letter “M” must be included in all azimuth functions. They must be transmitted and repeated at approximately equal intervals, not less than six times per minute, during which time the ground subsystem is available for operational use. When the transmissions of the ground subsystem are not available, the identification signal must be suppressed. The audible tone in the aircraft is started by setting the Morse Code bit to logic “1” and stopped by a logic “0” (see Tables 4a and 4b). The identification code characteristics must conform to the following: the dot must be between 0.13 and 0.16 second in duration, and the dash between 0.39 and 0.48 second. The duration between dots and/or dashes must be one dot plus or minus 10%. The duration between characters (letters) must not be less than three dots. When back azimuth is provided, the code shall be transmitted by the approach azimuth and back azimuth within plus or minus 0.08 seconds.

(B) Airborne antenna selection. A signal for airborne antenna selection shall be transmitted as a “zero” DPSK signal lasting for a six-bit period (see Tables 4a and 4b).

Table 4a—Approach Azimuth Function timing

Event Event time slot
begins at—
15.625 kHz clock pulse (number) Time
(milliseconds)
Preamble00
Morse code251.600
Antenna select261.664
Rear OCI322.048
Left OCI342.176
Right OCI362.304
To test382.432
To scan1402.560
Pause8.760
Midscan point9.060
FRO scan19.360
FRO test15.560
End Function (Airborne)15.688
End guard time; end function (ground)15.900

AA1The actual commencement and completion of the TO and the FRO scan transmissions are dependent on the amount of proportional guidance provided. The time slots provided shall accommodate a maximum scan of plus or minus 62.0 degrees. Scan timing shall be compatible with accuracy requirements.

Table 4b—High Rate Approach Azimuth and Back Azimuth Function Timing

EventEvent time slot
begins at—
15.625 kHz clock pulse (number) Time
(milliseconds)
Preamble00
Morse Code251.600
Antenna select261.664
Rear OCI322.048
Left OCI342.176
Right OCI362.304
To test382.432
To scan1402.560
Pause6.760
Midscan point7.060
FRO scan17.360
FRO test pulse11.560
End function (airborne)11.688
End guard time; end function (ground)11.900

1The actual commencement and completion of the TO and the FRO scan transmissions are dependent on the amount of proportional guidance provided. The time slots provided will accommodate a maximum scan of plus or minus 42.0 degrees. Scan timing shall be compatible with accuracy requirements.

(C) OCI. Where OCI pulses are used, they must be: (1) greater than any guidance signal in the OCI sector; (2) at least 5 dB less than the level of the scanning beam within the proportional guidance sector; and (3) for azimuth functions with clearance signals, at least 5 dB less than the level of the left (right) clearance pulses within the left (right) clearance sector.

Table 5—Approach Elevation Function Timing

EventEvent time slot
begins at:
15.625 kHz clock pluse (number)Time
(milliseconds)
Preamble00
Processor pause251.600
OCI271.728
To scan1291.856
Pause3.406
Midscan point3.606
FRO scan13.806
End function (airborne)5.356
End guard time; end function (ground)5.600

1The actual commencement and completion of the TO and FRO scan transmissions are dependent upon the amount of proportional guidance provided. The time slots provided will accommodate a maximum scan of −1.5 degrees to + 29.5 degrees. Scan timing shall be compatible with accuracy requirements.

The duration of each pulse measured at the half amplitude point shall be at least 100 microseconds, and the rise and fall times shall be less then 10 microseconds. It shall be permissible to sequentially transmit two pulses in each out-of-coverage indication time slot. Where pulse pairs are used, the duration of each pulse shall be at least 50 microseconds, and the rise and fall times shall be less then 10 microseconds. The transmission of out-of-coverage indication pulses radiated from antennas with overlapping coverage patterns shall be separated by at least 10 microseconds.

Note: If desired, two pulses may be sequentially transmitted in each OCI time slot. Where pulse pairs are used, the duration of each pulse must be 45 (±5) microseconds and the rise and fall times must be less than 10 microseconds.

(D) System test. Time slots are provided in Tables 4a and 4b to allow radiation of TO and FRO test pulses. However, radiation of these pulses is not required since the characteristics of these pulses have not yet been standardized.

(iii) Angle encoding. The encoding must be as follows:

(A) General. Azimuth and elevation angles are encoded by scanning a narrow beam between the limits of the proportional coverage sector first in one direction (the TO scan) and then in the opposite direction (the FRO scan). Angular information must be encoded by the amount of time separation between the beam centers of the TO and FRO scanning beam pulses. The TO and FRO transmissions must be symmetrically disposed about the midscan point listed in Tables 4a, 4b, 5, and 7. The midscan point and the center of the time interval between the TO and FRO scan transmissions must coincide with a tolerance of ±10 microseconds. Angular coding must be linear with angle and properly decoded using the formula:

eCFR graphic ec15se91.011.gif

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where:

θ = Receiver angle in degrees.

V = Scan velocity in degrees per microsecond.

T0 = Time separation in microseconds between TO and FRO beam centers corresponding to zero degrees.

t = Time separation in microseconds between TO and FRO beam centers.

The timing requirements are listed in Table 6 and illustrated in Figure 7.

eCFR graphic ec15se91.012.gif

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(B) Azimuth angle encoding. Each guidance angle transmitted must consist of a clockwise TO scan followed by a counterclockwise FRO scan as viewed from above the antenna. For approach azimuth functions, increasing angle values must be in the direction of the TO scan; for the back azimuth function, increasing angle values must be in the direction of the FRO scan. The antenna has a narrow beam in the plane of the scan direction and a broad beam in the orthogonal plane which fills the vertical coverage.

(C) Elevation angle encoding. The radiation from elevation equipment must produce a beam which scans from the horizon up to the highest elevation angle and then scans back down to the horizon. The antenna has a narrow beam in the plane of the scan direction and a broad beam in the orthogonal plane which fills the horizontal coverage. Elevation angles are defined from the horizontal plane containing the antenna phase center; positive angles are above the horizontal and zero angle is along the horizontal.

(iv) Clearance guidance. The timing of the clearance pulses must be in accordance with Figure 8. For azimuth elements with proportional coverage of less than ±40 degrees (±20 degrees for back azimuth), clearance guidance information must be provided by transmitting pulses in a TO and FRO format adjacent to the stop/start times of the scanning beam signal. The fly-right clearance pulses must represent positive angles and the fly-left clearance pulses must represent negative angles. The duration of each clearance pulse must be 50 microseconds with a tolerance of ±5 microseconds. The transmitter switching time between the clearance pulses and the scanning beam transmissions must not exceed 10 microseconds. The rise time at the edge of each clearance pulse must be less than 10 microseconds. Within the fly-right clearance guidance section, the fly-right clearance guidance signal shall exceed scanning beam antenna sidelobes and other guidance and OCI signals by at least 5 dB; within the fly-left clearance guidance sector, the fly left clearance guidance signal shall exceed scanning beam antenna sidelobes and all other guidance and OCI signals by at least 5 dB; within the proportional guidance sector, the clearance guidance signals shall be at least 5dB below the proportional guidance signal. Optionally, clearance guidance may be provided by scanning throughout the approach guidance sector. For angles outside the approach azimuth proportional coverage limits as set in Basic Data Word One (Basic Data Word 5 for back azimuth), proper decode and display of clearance guidance must occur to the limits of the guidance region. Where used, clearance pulses shall be transmitted adjacent to the scanning beam signals at the edges of proportional coverage as shown in Figure 8. The proportional coverage boundary shall be established at one beamwidth inside the scan start/stop angles, such that the transition between scanning beam and clearance signals occurs outside the proportional coverage sector. When clearance pulses are provided in conjunction with a narrow beamwidth (e.g., one degree) scanning antenna, the scanning beam antenna shall radiate for 15 microseconds while stationary at the scan start/stop angles.

(3) Data function format. Basic data words provide equipment characteristics and certain siting information. Basic data words must be transmitted from an antenna located at the approach azimuth or back azimuth site which provides coverage throughout the appropriate sector. Data function timing must be in accordance with Table 7a.

Table 6—Angle Scan Timing Constants

FunctionMax value of t(usec)To(usec)V(deg/usec)Tm (usec)Pause time (usec)Tt (usec)
Approach azimuth13,0006,8000.027,97260013,128
High rate approach azimuth9,0004,8000.025,9726009,128
Approach elevation3,5003,3500.022,518400N/A
Back azimuth9,0004,800−0.025,9726009,128

Table 7a—Basic Data Function Timing

EventEvent time slot
begins at:1
15.625 kHz clock pulse (number)Time
(milliseconds)
Preamble00
Data transmission (bits I13-I30)251.600
Parity transmission (bits I31-I32)432.752
End function (airborne)452.880
End guard time: end function (ground)3.100

1The previous event time slot ends at this time.

Table 7b—Auxiliary Data Function Timing—(Digital)

EventEvent time slot
begins at:
15.625 kHz clock pulse (number)Time
(milliseconds)
Preamble00
Address transmission (bits I13-I20)251.600
Data transmission: (bits I21-I69)332.112
Parity transmission (bits I70-I76)825.248
End function (airborne)895.696
End guard time; end function (ground)5.900

Table 7c—Auxiliary Data Function Timing—(Alphanumeric)

EventEvent time slot
begins at:
15.615 kHz clock pulse (number)Time
(milliseconds)
Preamble00
Address transmission (bits I13-I20)251.600
Data transmission: (bits I21-I76332.112
End function (airborne)895.696
End guard time; (end function ground)5.900

(i) Preamble. Must be in accordance with requirements of §171.311(i)(1).

(ii) Data transmissions. Basic data must be transmitted using DPSK modulation. The content and repetition rate of each basic data word must be in accordance with Table 8a. For data containing digital information, binary number 1 must represent the lower range limit with increments in binary steps to the upper range limit shown in Table 8a. Data containing digital information shall be transmitted with the least significant bit first.

(j) Basic Data word requirements. Basic Data shall consist of the items specified in Table 8a. Basic Data word contents shall be defined as follows:

(1) Approach azimuth to threshold distance shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane perpendicular to the centerline which contains the landing threshold.

(2) Approach azimuth proportional coverage limit shall represent the limit of the sector in which proportional approach azimuth guidance is transmitted.

(3) Clearance signal type shall represent the type of clearance when used. Pulse clearance is that which is in accordance with §171.311 (i) (2) (iv). Scanning Beam (SB) clearance indicates that the proportional guidance sector is limited by the proportional coverage limits set in basic data.

eCFR graphic ec15se91.013.gif

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Table 8a—Basic Data Words

Data bit #Data item definitionLSB valueData bit value
Basic Data Word No. 1
1PreambleN/A1
21
31
40
51
60
71
80
91
100
110
120
13Approach azimuth to threshold distance (Om−630m)100m100m
14200m
15400m
16800m
171600m
183200m
19Approach azimuth proportional coverage limit (negative limit) (0° to −62°)−2°
20−4°
21−8°
22−16°
23−32°
24Approach azimuth proportional coverage limit (positive limit) (0° to + 62°)
25
26
2716°
2832°
29Clearance signal typeN/A0 = pulse; 1 = SB
30SpareTransmit zero
31Parity: (13 + 14 + 15.  .  . + 30 + 31 = odd)N/AN/A
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 1.0 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute value of the coded parameter unless otherwise noted.
Basic Data Word No. 2
1PreambleN/A1
21
31
40
51
60
71
81
91
101
110
120
13Minimum glide path (2.0° to 14.7°)0.1°0.1°
140.2°
150.4°
160.8°
171.6°
183.2°
196.4°
20Back azimuth statussee note 4
21DME statussee note 6
22
23Approach azimuth statussee note 4
24Approach azimuth statussee note 4
25SpareTransmit zero
26......do      Do.
27......do      Do.
28......do      Do.
29......do      Do.
30......do      Do.
31Parity: (13 + 14 + 15.  .  . + 30 + 31) = odd)N/AN/A
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 0.16 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 3
1PreambleN/A1
21
31
40
51
61
70
81
90
100
110
120
13Approach azimuth beamwidth (0.5°−4.0°) See note 70.5°0.5°
141.0°
152.0°
16Approach elevation beamwidth (0.5° to 2.5°) See note 70.5°0.5°
171.0°
18Note: values greater than 2.5° are invalid2.0°
19DME distance (Om to 6387.5m12.5m12.5m
2025.0m
2150.0m
22100.0m
23200.0m
24400.0m
25800.0m
261600.0m
273200.0m
28SpareTransmit zero
29......do      Do.
30......do      Do.
31Parity: (13 + 14 + 15.  .  . + 30 + 31 = odd)
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 1.0 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 4
1PreambleN/A1
21
31
40
51
61
70
80
90
101
110
120
13Approach azimuth magnetic orientation (0° to 359°)
14
15
16
1716°
1832°
1964°
20128°
21256°
22Back azimuth magnetic orientation (0° to 359°)
23
24
25
2616°
2732°
2864°
29128°
30256°
31Parity: (13 + 14 + 15.  .  . + 30 + 31 = odd)N/AN/A
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
Note 1: Transmit at intervals of 1.0 second or less throughout the Approach Azimuth guidance sector, except when Back Azimuth guidance is provided. See Note 8.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 5
1PreambleN/A1
21
31
40
51
61
71
80
91
101
110
120
13Back azimuth proportional coverage negative limit (0° to −42°)−2°
14−4°
15−8°
16−16°
17−32°
18Back azimuth proportional coverage positive limit (0° to + 42°)
19
20
2116°
2232°
23Back azimuth beamwidth (0.5° to 4.0°) See note 70.5°0.5°
241.0°
252.0°
26Back azimuth statusSee Note 10
27......do      Do.
28......do      Do.
29......do      Do.
30......do      Do.
31Parity: (13 + 14 + 15.  .  . + 30 + 31 = odd)N/AN/A
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
Note 1: Transmit only when Back Azimuth guidance is provided. See note 9.
Note 2: The all zero state of the data filed represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 6
1PreambleN/A1
21
31
40
51
60
70
80
91
101
110
121
(13-
30)
MLS ground equipment identification (Note 3)
13Character 2N/AB1
14B2
15B3
16B4
17B5
18B6
19Character 3N/AB1
20B2
21B3
22B4
23B5
24B6
25Character 4N/AB1
26B2
27B3
28B4
29B5
30B6
31Parity: (13 + 14 + 15.  .  . + 30 + 31 = odd)N/AN/A
32Parity: (14 + 16 + 18.  .  . + 30 + 32 = odd)N/AN/A
   

Note 1: Transmit at intervals of 1.0 second or less throughout the Approach Azimuth guidance sector, except when Back Azimuth guidance is provided. See note 8.

   

Note 3: Characters are encoded using the International Alphabet Number 5, (IA-5):

Note 4: Coding for status bit:

0 = Function not radiated, or radiated in test mode (not reliable for navigation).

1 = Function radiated in normal mode (for Back Azimuth, this also indicates that a Back Azimuth transmission follows).

Note 5: Date items which are not applicable to a particular ground equipment shall be transmitted as all zeros.

Note 6: Coding for status bits:

I21I22
00DME transponder inoperative or not available.
10Only IA mode or DME/N available.
00FA mode, Standard 1, available.
11FA mode, Standard 2, available.

Note 7: The value coded shall be the actual beamwidth (as defined in §171.311 (j)(9) rounded to the nearest 0.5 degree.

Note 8: When back Azimuth guidance is provided, Data Words 4 and 6 shall be transmitted at intervals of 1.33 seconds or less throughout the Approach Azimuth coverage and 4 seconds or less throughout the Back Azimuth coverage.

Note 9: When Back Azimuth guidance is provided, Data Word 5 shall be transmitted at an interval of 1.33 seconds or less throughout the Back Azimuth coverage sector and 4 seconds or less throughout the Approach Azimuth coverage sector.

Note 10: Coding for status bit:

0 = Function not radiated, or radiated in test mode (not reliable for navigation).

1 = Function radiated in normal mode.

(4) Minimum glidepath the lowest angle of descent along the zero degree azimuth that is consistent with published approach procedures and obstacle clearance criteria.

(5) Back azimuth status shall represent the operational status of the Back Azimuth equipment.

(6) DME status shall represent the operational status of the DME equipment.

(7) Approach azimuth status shall represent the operational status of the approach azimuth equipment.

(8) Approach elevation status shall represent the operational status of the approach elevation equipment.

(9) Beamwidth the width of the scanning beam main lobe measured at the −3 dB points and defined in angular units on the antenna boresight, in the horizontal plane for the azimuth function and in the vertical plane for the elevation function.

(10) DME distance shall represent the minimum distance between the DME antenna phase center and the vertical plane perpendicular to the runway centerline which contains the MLS datum point.

(11) Approach azimuth magnetic orientation shall represent the angle measured in the horizontal plane clockwise from Magnetic North to the zero-degree angle guidance radial originating from the approach azimuth antenna phase center. The vertex of the measured angle shall be at the approach azimuth antenna phase center.

Note: For example, this data item would be encoded 090 for an approach azimuth antenna serving runway 27 (assuming the magnetic heading is 270 degrees) when sited such that the zero degree radial is parallel to centerline.

(12) Back azimuth magnetic orientation shall represent the angle measured in the horizontal plane clockwise from Magnetic North to the zero-degree angle guidance radial originating from the Back Azimuth antenna. The vertex of the measured angle shall be at the Back Azimuth antenna phase center.

Note: For example, this data item would be encoded 270 for a Back Azimuth Antenna serving runway 27 (assuming the magnetic heading is 270 degrees) when sited such that the zero degree radial is parallel to centerline.

(13) Back azimuth proportional coverage limit shall represent the limit of the sector in which proportional back azimuth guidance is transmitted.

(14) MLS ground equipment identification shall represent the last three characters of the system identification specified in §171.311(i)(2). The characters shall be encoded in accordance with International Alphabet No. 5 (IA-5) using bits b1 through b6.

Note: Bit b7 of this code may be reconstructed in the airborne receiver by taking the complement of bit b6.

(k) Residual radiation. The residual radiation of a transmitter associated with an MLS function during time intervals when it should not be transmitting shall not adversely affect the reception of any other function. The residual radiation of an MLS function at times when another function is radiating shall be at least 70 dB below the level provided when transmitting.

(l) Symmetrical scanning. The TO and FRO scan transmissions shall be symmetrically disposed about the mid-scan point listed in Tables 4a, 4b and 5. The mid-scan point and the center of the time interval between the TO and FRO scan shall coincide with a tolerance of plus or minus 10 microseconds.

(m) Auxiliary data—(1) Addresses. Three function identification codes are reserved to indicate transmission of Auxiliary Data A, Auxiliary Data B, and Auxiliary Data C. Auxiliary Data A contents are specified below, Auxiliary Data B contents are reserved for future use, and Auxiliary Data C contents are reserved for national use. The address codes of the auxiliary data words shall be as shown in Table 8b.

(2) Organization and timing. The organization and timing of digital auxiliary data must be as specified in Table 7b. Data containing digital information must be transmitted with the least significant bit first. Alphanumeric data characters must be encoded in accordance with the 7-unit code character set as defined by the American National Standard Code for Information Interchange (ASCII). An even parity bit is added to each character. Alphanumeric data must be transmitted in the order in which they are to be read. The serial transmission of a character must be with the lower order bit transmitted first and the parity bit transmitted last. The timing for alphanumeric auxiliary data must be as shown in Table 7c.

(3) Auxiliary Data A content: The data items specified in Table 8c are defined as follows:

(i) Approach azimuth antenna offset shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane containing the runway centerline.

(ii) Approach azimuth to MLS datum point distance shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane perpendicular to the centerline which contains the MLS datum point.

(iii) Approach azimuth alignment with runway centerline shall represent the minimum angle between the approach azimuth antenna zero-degree guidance plane and the runway certerline.

(iv) Approach azimuth antenna coordinate system shall represent the coordinate system (planar or conical) of the angle data transmitted by the approach azimuth antenna.

(v) Approach elevation antenna offset shall represent the minimum distance between the elevation antenna phase center and the vertical plane containing the runway centerline.

(vi) MLS datum point to threshold distance shall represent the distance measured along the runway centerline from the MLS datum point to the runway threshold.

(vii) Approach elevation antenna height shall represent the height of the elevation antenna phase center relative to the height of the MLS datum point.

(viii) DME offset shall represent the minimum distance between the DME antenna phase center and the vertical plane containing the runway centerline.

(ix) DME to MLS datum point distance shall represent the minimum distance between the DME antenna phase center and the vertical plane perpendicular to the centerline which contains the MLS datum point.

(x) Back azimuth antenna offset shall represent the minimum distance between the back azimuth antenna phase center and the vertical plane containing the runway centerline.

(xi) Back azimuth to MLS datum point distance shall represent the minimum distance between the Back Azimuth antenna and the vertical plane perpendicular to the centerline which contains the MLS datum point.

(xii) Back azimuth antenna alignment with runway centerline shall represent the minimum angle between the back azimuth antenna zero-degree guidance plane and the runway centerline.

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§171.313   Azimuth performance requirements.

This section prescribes the performance requirements for the azimuth equipment of the MLS as follows:

(a) Approach azimuth coverage requirements. The approach azimuth equipment must provide guidance information in at least the following volume of space (see Figure 9):

Table 8b—Auxiliary Data Word Address Codes

No.I13I14I15I16I17I18I19I20
1.00000111
2.00001010
3.00001101
4.00010011
5.00010100
6.00011001
7.00011110
8.00100010
9.00100101
10.00101000
11.00101111
12.00110001
13.00110110
14.00111011
15.00111100
16.01000011
17.01000100
18.01001001
19.01001110
20.01010000
21.01010111
22.01011010
23.01011101
24.01100001
25.01100110
26.01101011
27.01101100
28.01110010
29.01110101
30.01111000
31.01111111
32.10000010
33.10000101
34.10001000
35.10001111
36.10010001
37.10010110
38.10011011
39.10011100
40.10100000
41.10100111
42.10101010
43.10101101
44.10110011
45.10110100
46.10111001
47.10111110
48.11000001
49.11000110
50.11001011
51.11001100
52.11010010
53.11010101
54.11011000
55.11011111
56.11100011
57.11100100
58.11101001
59.11101110
60.11110000
61.11110111
62.11111010
63.11111101
64.00000000

Note 1: Parity bits I19 and I20 are chosen to satisfy the equations:

I13 + I14 + I15 + I16 + I17 + I18 + I19 = EVEN

I14 + I16 + I18 + I20 = EVEN

Table 8c—Auxiliary Data

Word (See note 6)Data contentType of dataMaximun time between transmissions (Seconds)Bits usedRange of valuesLeast significant bit
A1PreambleDigital1.012
   Address8
   Approach azimuth antenna offset10−511 m to + 511 m (See note 3)1 m
   Approach azimuth to MLS datum point distance130 m to 8 191 m1 m
   Approach azimuth antenna alignment with runway centerline12−20.47° to 20.47° (See note 3)0.01°
   Approach azimuth antenna coordinate system1(See note 2)
   Spare13
   Parity7(See note 1)
A2PreambleDigital1.012
   Address8
   Approach elevation antenna offset10−511 m to + 511 m (See note 3)1 m
   MLS datum point to threshold distance100 m to 1 023 m1 m
   Approach elevation antenna height7−6.3 m to + 6.3 m (See note 3)0.1 m
   Spare22
   Parity7(See note 1)
A3PreambleDigital(See note 4)12
   Address8
   DME offset10−511 m to + 511 m1 m
   DME to MLS datum point distance14−8 191 m to + 8 191 m (See note 3)1 m
   Spare25
   Parity7(See note 1)
A4PreambleDigital(See note 5)12
   Address8
   Back azimuth antenna10−511 m to + 511 m (See note 3)1 m
   Back azimuth to MLS datum point distance110 m to 2 047 m1 m
   Back azimuth antenna alignment with runway centerline12−20.47° to 20.47° (See note 3)0.01°
   Spare16
   Parity7(See note 1)

Note 1: Parity bits I70 to I76 are chosen to satisfy the equations which follow:

For BIT I70:

Even = (I13 + ... + I18) + I20 + I22 + I24 + I25 + I28 + I29 + I31 + I32 + I33 + I35 + I36 + I38 + I41 + I44 + I45 + I46 + I50 + (I52 + ... + I55) + I58 + I60 + I64 + I65 + I70

For BIT I71:

Even = (I14 + ... + I19) + I21 + I23 + I25 + I26 + I29 + I30 + I32 + I33 + I34 + I36 + I37 + I39 + I42 + I45 + I46 + I47 + I51 + (I53 + ... + I56) + I59 + I61 + I65 + I66 + I71

For BIT I72:

Even = (I15 + ... + I20) + I22 + I24 + I26 + I27 + I30 + I31 + I33 + I34 + I35 + I37 + I38 + I40 + I43 + I46 + I47 + I48 + I52 + (I54 + ... + I57) + I60 + I62 + I66 + I67 + I72

For BIT I73:

Even = (I16 + ... + I21) + I23 + I25 + I27 + I28 + I31 + I32 + I34 + I35 + I36 + I38 + I39 + I41 + I44 + I47 + I48 + I49 + I53 + (I55 + ... + I58) + I61 + I63 + I67 + I68 + I73

For BIT I74:

Even = (I17 + ... + I22) + I24 + I26 + I28 + I29 + I32 + I33 + I35 + I36 + I37 + I39 + I40 + I42 + I45 + I48 + I49 + I50 + I54 + (I56 + ... + I59) + I62 + I64 + I68 + I69 + I74

For BIT I75:

Even = (I13 + ... + I17) + I19 + I21 + I23 + I24 + I27 + I28 + I30 + I31 + I32 + I34 + I35 + I37 + I40 + I43 + I44 + I45 + I49 + (I51 + ... + I54) + I57 + I59 + I63 + I64 + I69 + I75

For BIT I76:

Even = I13 + I14 + ... + I75 + I76

Note 2: Code for I56 is: 0 = conical; 1 = planar.

Note 3: The convention for the coding of negative numbers is as follows: − MSB is the sign bit; 0 = + ; 1 = −.

—Other bits represent the absolute value.

The convention for the antenna location is as follows: As viewed from the MLS approach reference datum looking toward the datum point, a positive number shall represent a location to the right of the runway centerline (lateral offset) or above the runway (vertical offset), or towards the stop end of the runway (longitudinal distance).

The convention for the antenna alignment is as follows: As viewed from above, a positive number shall represent clockwise rotation from the runway centerline to the respective zero-degree guidance plane.

Note 4: Data Word A3 is transmitted at intervals of 1.0 seconds or less throughout the approach Azimuth coverage sector, except when back Azimuth guidance is provided. Where back Azimuth is provided transmit at intervals of 1.33 seconds or less throughout the approach Azimuth sector and 4.0 seconds or less throughout the back Azimuth coverage sector.

Note 5: When back Azimuth guidance is provided, transmit at intervals of 1.33 seconds or less throughout the back Azimuth coverage sector and 4.0 seconds or less throughout the approach Azimuth coverage sector.

Note 6: The designation “A1” represents the function identification code for “Auxiliary Data A” and address code number 1.

eCFR graphic ec15se91.014.gif

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(1) Horizontally within a sector plus or minus 40 degrees about the runway centerline originating at the datum point and extending in the direction of the approach to 20 nautical miles from the runway threshold. The minimum proportional guidance sector must be plus or minus 10 degrees about the runway centerline. Clearance signals must be used to provide the balance of the required coverage, where the proportional sector is less than plus or minus 40 degrees. When intervening obstacles prevent full coverage, the ±40° guidance sector can be reduced as required. For systems providing ±60° lateral guidance the coverage requirement is reduced to 14 nm beyond ±40°.

(2) Vertically between:

(i) A conical surface originating 2.5 meters (8 feet) above the runway centerline at threshold inclined at 0.9 degree above the horizontal.

(ii) A conical surface originating at the azimuth ground equipment antenna inclined at 15 degrees above the horizontal to a height of 6,000 meters (20,000 feet).

(iii) Where intervening obstacles penetrate the lower surface, coverage need be provided only to the minimum line of sight.

(3) Runway region:

(i) Proportional guidance horizontally within a sector 45 meters (150 feet) each side of the runway centerline beginning at the stop end and extending parallel with the runway centerline in the direction of the approach to join the approach region. This requirement does not apply to offset azimuth installations.

(ii) Vertically between a horizontal surface which is 2.5 meters (8 feet) above the farthest point of runway centerline which is in line of sight of the azimuth antenna, and in a conical surface originating at the azimuth ground equipment antenna inclined at 20 degrees above the horizontal up to a height to 600 meters (2,000 feet). This requirement does not apply to offset azimuth installations.

(4) Within the approach azimuth coverage sector defined in paragraphs (a) (1), and (2) and (3) of this section, the power densities must not be less than those shown in Table 9 but the equipment design must also allow for:

(i) Transmitter power degradation from normal by −1.5 dB;

Table 9—Minimum Power Density Within Coverage Boundaries(dBW/m2)

Function Data signals Angle signals for various antenna beamwidths Clearance signals
1.5°
Approach azimuth−89.5−88−85.5−82−88
High rate approach azimuth−89.5−88−88−86.5−88
Back azimuth−89.5−88−85.5−82−88
Approach elevation−89.5−88−88−88

(ii) Rain loss of −2.2 dB at the longitudinal coverage extremes.

(b) Siting requirements. The approach azimuth antenna system must, except as allowed in paragraph (c) of this section:

(1) Be located on the extension of the centerline of the runway beyond the stop end;

(2) Be adjusted so that the zero degree azimuth plane will be a vertical plane which contains the centerline of the runway served;

(3) Have the minimum height necessary to comply with the coverage requirements prescribed in paragraph (a) of this section;

(4) Be located at a distance from the stop end of the runway that is consistent with safe obstruction clearance practices;

(5) Not obscure any light of an approach lighting system; and

(6) Be installed on frangible mounts or beyond the 300 meter (1,000 feet) light bar.

(c) On runways where limited terrain prevents the azimuth antenna from being positioned on the runway centerline extended, and the cost of the land fill or a tall tower antenna support is prohibitive, the azimuth antenna may be offset.

(d) Antenna coordinates. The scanning beams transmitted by the approach azimuth equipment within ±40° of the centerline may be either conical or planar.

(e) Approach azimuth accuracy. (1) The system and subsystem errors shall not exceed those listed in Table 10 at the approach reference datum.

At the approach reference datum, temporal sinusoidal noise components shall not exceed 0.025 degree peak in the frequency band 0.01 Hz to 1.6 Hz, and the CMN shall not exceed 0.10 degree. From the approach reference datum to the coverage limit, the PFE, PFN and CMN limits, expressed in angular terms, shall be allowed to linearly increase as follows:

(i) With distance along the runway centerline extended, by a factor of 1.2 for the PFE and PFN limits and to ±0.10 degree for the CMN limits.

(ii) With azimuth angle, by a factor of 1.5 at the ±40 degree and a factor of 2.0 at the ±60 degree azimuth angles for the PFE, PFN and CMN limits.

(iii) With elevation angle from + 9 degrees to + 15 degrees, by a factor of 1.5 for the PFE and PFN limits.

(iv) Maximum angular limits. The PFE limits shall not exceed ±0.25 degree in any coverage region below an elevation angle of + 9 degrees nor exceed ±0.50 degree in any coverage region above that elevation angle. The CMN limits shall not exceed ±0.10 degree in any coverage region within ±10 degrees of runway centerline extended nor exceed ±0.20 degree in any other region within coverage.

Note: It is desirable that the CMN not exceed ±0.10 degree throughout the coverage.

(f) Approach azimuth antenna characteristics are as follows:

(1) Drift. Any azimuth angle as encoded by the scanning beam at any point within the proportional coverage must not vary more than ±0.07 degree over the range of service conditions specified in §171.309(d) without the use of internal environmental controls. Multipath effects are excluded from this requirement.

(2) Beam pointing errors. The azimuth angle as encoded by the scanning beam at any point within ±0.5 degree of the zero degree azimuth must not deviate from the true azimuth angle at that point by more than ±.05 degree. Multipath and drift effects are excluded from this requirement.

Table 10—Approach Azimuth Accuracies at the Approach Reference Datum

Error type System Angular error (degrees)
Ground subsystem Airborne subsystem
PFE±20 ft. (6.1m)1 2±0.118°3±0.017°
CMN±10.5 ft. (3.2m)1 2 4±0.030°±0.050°

Notes:

1Includes errors due to ground and airborne equipment and propagation effects.

2The system PFN component must not exceed ±3.5 meters (11.5 feet).

3The mean (bias) error component contributed by the ground equipment should not exceed ±10 feet.

4The system control motion noise must not exceed 0.1 degree.

5The airborne subsystem angular errors are provided for information only.

(3) Antenna alignment. The antenna must be equipped with suitable optical, electrical or mechanical means or any combination of the three, to bring the zero degree azimuth radial into coincidence with the approach reference datum (for centerline siting) with a maximum error of 0.02 degree. Additionally, the azimuth antenna bias adjustment must be electronically steerable at least to the monitor limits in steps not greater than 0.005 degree.

(4) Antenna far field patterns in the plane of scan. On boresight, the azimuth antenna mainlobe pattern must conform to Figure 10, and the beamwidth must be such that, in the installed environment, no significant lateral reflections of the mainlobe exist along the approach course. In any case the beamwidth must not exceed three degrees. Anywhere within coverage the −3 dB width of the antenna mainlobe, while scanning normally, must not be less than 25 microseconds (0.5 degree) or greater than 250 microseconds (5 degrees). The antenna mainlobe may be allowed to broaden from the value at boresight by a factor of 1/cosθ, where θ is the angle off boresight. The sidelobe levels must be as follows:

(i) Dynamic sidelobe levels. With the antenna scanning normally, the dynamic sidelobe level that is detected by a receiver at any point within the proportional coverage sector must be down at least 10 dB from the peak of the main beam. Outside the coverage sector, the radiation from the scanning beam antenna must be of such a nature that receiver warning will not be removed or suitable OCI signals must be provided.

(ii) Effective sidelobe levels. With the antenna scanning normally, the sidelobe levels in the plane of scan must be such that, in the installed environment, the CMN contributed by sidelobe reflections will not exceed the angular equivalent of 9 feet at approach reference datum over the required range of aircraft approach speeds.

eCFR graphic ec15se91.015.gif

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(5) Antenna far field pattern in the vertical plane. The azimuth antenna free space radiation pattern below the horizon must have a slope of at least −8 dB/degree at the horizon and all sidelobes below the horizon must be at least 13 dB below the pattern peak. The antenna radiation pattern above the horizon must satisfy both the system coverage requirements and the spurious radiation requirement.

(6) Data antenna. The data antenna must have horizontal and vertical patterns as required for its function.

(g) Back azimuth coverage requirements. The back azimuth equipment where used must provide guidance information in at least the following volume of space (see Figure 11):

eCFR graphic ec15se91.016.gif

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(1) Horizontally within a sector ±40 degrees about the runway centerline originating at the back azimuth ground equipment antenna and extending in the direction of the missed approach at least to 20 nautical miles from the runway stop end. The minimum proportional guidance sector must be ±10 degrees about the runway centerline. Clearance signals must be used to provide the balance of the required coverage where the proportional sector is less than ±40 degrees.

(2) Vertically in the runway region between:

(i) A horizontal surface 2.5 meters (8 feet) above the farthest point of runway centerline which is in line of sight of the azimuth antenna, and,

(ii) A conical surface originating at the azimuth ground equipment antenna inclined at 20 degrees above the horizontal up to a height of 600 meters (2000 feet).

(3) Vertically in the back azimuth region between:

(i) A conical surface originating 2.5 meters (8 feet) above the runway stop end, included at 0.9 degree above the horizontal, and,

(ii) A conical surface orginating at the missed approach azimuth ground equipment antenna, inclined at 15 degrees above the horizontal up to a height of 1500 meters (5000 feet).

(iii) Where obstacles penetrate the lower coverage limits, coverage need be provided only to minimum line of sight.

(4) Within the back azimuth coverage sector defined in paragraph (q) (1), (2), and (3) of this section the power densities must not be less than those shown in Table 9, but the equipment design must also allow for:

(i) Transmitter power degradation from normal −1.5 dB.

(ii) Rain loss of −2.2 dB at the longitudinal coverage extremes.

(h) Back azimuth siting. The back azimuth equipment antenna must:

(1) Normally be located on the extension of the runway centerline at the threshold end;

(2) Be adjusted so that the vertical plane containing the zero degree course line contains the back azimuth reference datum;

(3) Have minimum height necessary to comply with the course requirements prescribed in paragraph (g) of this section;

(4) Be located at a distance from the threshold end that is consistent with safe obstruction clearance practices;

(5) Not obscure any light of an approach lighting system; and

(6) Be installed on frangible mounts or beyond the 300 meter (1000 feet) light bar.

(i) Back azimuth antenna coordinates. The scanning beams transmitted by the back azimuth equipment may be either conical or planar.

(j) Back azimuth accuracy. The requirements specified in §171.313(e) apply except that the reference point is the back azimuth reference datum.

(k) Back azimuth antenna characteristics. The requirements specified in §171.313(f) apply.

(l) Scanning conventions. Figure 12 shows the approach azimuth and back azimuth scanning conventions.

eCFR graphic ec15se91.017.gif

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eCFR graphic ec15se91.018.gif

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eCFR graphic ec15se91.019.gif

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(m) False guidance. False courses which can be acquired and tracked by an aircraft shall not exist anywhere either inside or outside of the MLS coverage sector. False courses which exist outside of the minimum coverage sector may be suppressed by the use of OCI.

Note: False courses may be due to (but not limited to) MLS airborne receiver acquisition of the following types of false guidance: reflections of the scanning beam, scanning beam antenna sidelobes and grating lobes, and incorrect clearance.

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§171.315   Azimuth monitor system requirements.

(a) The approach azimuth or back azimuth monitor system must cause the radiation to cease and a warning must be provided at the designated control point if any of the following conditions persist for longer than the periods specified:

(1) There is a change in the ground equipment contribution to the mean course error component such that the path following error at the reference datum or in the direction of any azimuth radial, exceeds the limits specified in §§171.313(e)(1) or 171.313(j) for a period of more than one second.

Note: The above requirement and the requirement to limit the ground equipment mean error to ±10 ft. can be satisfied by the following procedure. The integral monitor alarm limit should be set to the angular equivalent of ±10 ft. at the approach reference datum. This will limit the electrical component of the mean course error to ±10 ft. The field monitor alarm limit should be set such that with the mean course error at the alarm limit the total allowed PFE is not exceeded on any commissioned approach course from the limit of coverage to an altitude of 100 feet.

(2) There are errors in two consecutive transmissions of Basic Data Words 1, 2, 4 or 5.

(3) There is a reduction in the radiated power to a level not less than that specified in §§171.313(a)(4) or 171.313(g)(4) for a period of more than one second.

(4) There is an error in the preamble DPSK transmissions which occurs more than once in any one second period.

(5) There is an error in the time division multiplex synchronization of a particular azimuth function that the requirement specified in §171.311(e) is not satisfied and if this condition persists for more than one second.

(6) A failure of the monitor is detected.

(b) Radiation of the following fuctions must cease and a warning provided at the designated control point if there are errors in 2 consecutive transmissions:

(1) Morse Code Identification,

(2) Basic Data Words 3 and 6,

(3) Auxiliary Data Words.

(c) The period during which erroneous guidance information is radiated must not exceed the periods specified in §171.315(a). If the fault is not cleared within the time allowed, the ground equipment must be shut down. After shutdown, no attempt must be made to restore service until a period of 20 seconds has elapsed.

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§171.317   Approach elevation performance requirements.

This section prescribes the performance requirements for the elevation equipment components of the MLS as follows:

(a) Elevation coverage requirements. The approach elevation facility must provide proportional guidance information in at least the following volume of space (see Figure 13):

(1) Laterally within a sector originating at the datum point which is at least equal to the proportional guidance sector provided by the approach azimuth ground equipment.

(2) Longitudinally from 75 meters (250 feet) from the datum point to 20 nautical miles from threshold in the direction of the approach.

(3) Vertically within the sector bounded by:

(i) A surface which is the locus of points 2.5 meters (8 feet) above the runway surface;

(ii) A conical surface originating at the datum point and inclined 0.9 degree above the horizontal and,

(iii) A conical surface originating at the datum point and inclined at 15.0 degrees above the horizontal up to a height of 6000 meters (20,000 feet).

eCFR graphic ec15se91.020.gif

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Where the physical characteristics of the approach region prevent theachievement of the standards under paragraphs (a) (1), (2), and (3) of this section, guidance need not be provided below a conical surface originating at the elevation antenna and inclined 0.9 degree above the line of sight.

(4) Within the elevation coverage sector defined in paragraphs (a) (1), (2) and (3) of this section, the power densities must not be less than those shown in Table 9, but the equipment design must also allow for:

(i) Transmitter power degradation from normal by −1.5 dB.

(ii) Rain loss of −2.2 dB at the coverage extremes.

(b) Elevation siting requirements. The Elevation Antenna System must:

(1) Be located as close to runway centerline as possible (without violating obstacle clearance criteria).

(2) Be located near runway threshold such that the asymptote of the minimum glidepath crosses the threshold of the runway at the Approach Reference Datum height. Normally, the minimum glidepath should be 3 degrees and the Approach Reference Datum height should be 50 feet. However, there are circumstances where other glideslopes and reference datum heights are appropriate. Some of these instances are discussed in FAA Order 8260.34 (Glide Slope Threshold Crossing Height Requirements) and Order 8260.3 (IFR Approval of MLS.)

(3) Be located such that the MLS Approach Reference Datum and ILS Reference Datum heights are coincident within a tolerance of 3 feet when MLS is installed on a runway already served by an ILS. This requirement applies only if the ILS glide slope is sited such that the height of the reference datum meets the requirements of FAA Order 8260.34.

(c) Antenna coordinates. The scanning beams transmitted by the elevation subsystem must be conical.

(d) Elevation accuracy. (1) The accuracies shown in Table 13 are required at the approach reference datum. From the approach reference datum to the coverage limit, the PFE, PFN and CMN limits shall be allowed to linearly increase as follows:

(i) With distance along the runway centerline extended at the minimum glide path angle, by a factor of 1.2 for the PFE and PFN limits and to ±0.10 degree for the CMN limits;

(ii) With azimuth angle, from runway centerline extended to the coverage extreme, by a factor of 1.2 for the PFE and PFN limits and by a factor of 2.0 for the CMN limits;

(iii) With increasing elevation angles from + 3 degrees to + 15 degrees, by a factor of 2.0 for the PFE and PFN limits;

Table 13—Elevation Accuracies at the Approach Reference Datum

Error type System Angular error (degrees)
Ground subsystem Airborne subsystem4
PFE1 2 ±0.133(3)±0.017
CMN1±0.050±0.020±0.010

Notes:

1Includes errors due to ground and airborne equipment and propagation effects.

2The system PFN component must not exceed ±0.087 degree.

3The mean (bias) error component contributed by the ground equipment should not exceed ±0.067 degree.

4The airborne subsystem angular errors are provided for information only.

(iv) With decreasing elevation angle from + 3 degrees (or 60% of the minimum glide path angle, whichever is less) to the coverage extreme, by a factor of 3 for the PFE, PFN and CMN limits; and

(v) Maximum angular limits. the CMN limits shall not exceed ±0.10 degree in any coverage region within ±10 degrees laterally of runway centerline extended which is above the elevation angle specified in (iv) above.

Note: It is desirable that the CMN not exceed ±0.10 degree throughout the coverage region above the elevation angle specified in paragraph (d)(1)(iv) of this section.

(2) The system and ground subsystem accuracies shown in Table 13 are to be demonstrated at commissioning as maximum error limits. Subsequent to commissioning, the accuracies are to be considered at 95% probability limits.

(e) Elevation antenna characteristics are as follows:

(1) Drift. Any elevation angle as encoded by the scanning beam at any point within the coverage sector must not vary more than 0.04 degree over the range of service conditions specified in §171.309(d) without the use of internal environmental controls. Multipath effects are excluded from this requirement.

(2) Beam pointing errors. The elevation angle as encoded by the scanning beam at any point within the coverage sector must not deviate from the true elevation angle at that point by more than ±0.04 degree for elevation angles from 2.5° to 3.5°. Above 3.5° these errors may linearly increase to ±0.1 degree at 7.5°. Multipath and drift effects are excluded from this requirement.

(3) Antenna alignment. The antenna must be equipped with suitable optical, electrical, or mechanical means or any combination of the three, to align the lowest operationally required glidepath to the true glidepath angle with a maximum error of 0.01 degree. Additionally, the elevation antenna bias adjustment must be electronically steerable at least to the monitor limits in steps not greater than 0.005 degrees.

(4) Antenna far field patterns in the plane of scan. On the lowest operationally required glidepath, the antenna mainlobe pattern must conform to Figure 10, and the beamwidth must be such that in the installed environment, no significant ground reflections of the mainlobe exist. In any case, the beamwidth must not exceed 2 degrees. The antenna mainlobe may be allowed to broaden from the value at boresight by a factor of 1/cosθ, where θ is the angle of boresight. Anywhere within coverage, the −3 dB width of the antenna mainlobe, while scanning normally, must not be less than 25 microseconds (0.5 degrees) or greater than 250 microseconds (5 degrees). The sidelobe levels must be as follows:

(i) Dynamic sidelobe levels. With the antenna scanning normally, the dynamic sidelobe level that is detected by a receiver at any point within the proportional coverage sector must be down at least 10 dB from the peak of the mainlobe. Outside the proportional coverage sector, the radiation from the scanning beam antenna must be of such a nature that receiver warnings will not be removed or a suitable OCI signal must be provided.

(ii) Effective sidelobe levels. With the antenna scanning normally, the sidelobe levels in the plane of scan must be such that, when reflected from the ground, the resultant PFE along any glidepath does not exceed 0.083 degrees.

(5) Antenna far field pattern in the horizontal plane. The horizontal pattern of the antenna must gradually de-emphasize the signal away from antenna boresight. Typically, the horizontal pattern should be reduced by at least 3 dB at 20 degrees off boresight and by at least 6 dB at 40 degrees off boresight. Depending on the actual multipath conditions, the horizontal radiation patterns may require more or less de-emphasis.

(6) Data antenna. The data antenna must have horizontal and vertical patterns as required for its function.

(f) False guidance. False courses which can be acquired and tracked by an aircraft shall not exist anywhere either inside or outside of the MLS coverage sector. False courses which exist outside of the minimum coverage sector may be suppressed by the use of OCI.

Note: False courses may be due to (but not limited to) MLS airborne receiver acquisition of the following types of false guidance: reflections of the scanning beam and scanning beam antenna sidelobes and grating lobes.

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§171.319   Approach elevation monitor system requirements.

(a) The monitor system must act to ensure that any of the following conditions do not persist for longer than the periods specified when:

(1) There is a change in the ground component contribution to the mean glidepath error component such that the path following error on any glidepath exceeds the limits specified in §171.317(d) for a period of more than one second.

Note: The above requirement and the requirement to limit the ground equipment mean error to ±0.067 degree can be satisfied by the following procedure. The integral monitor alarm limit should be set to ±0.067 degree. This will limit the electrical component of mean glidepath error to ±0.067 degree. The field monitor alarm limit should be set such that with the mean glidepath error at the alarm limit the total allowed PFE is not exceeded on any commissioned glidepath from the limit of coverage to an altitude of 100 feet.

(2) There is a reduction in the radiated power to a level not less than that specified in §171.317(a)(4) for a period of more than one second.

(3) There is an error in the preamble DPSK transmission which occurs more than once in any one second period.

(4) There is an error in the time division multiplex synchronization of a particular elevation function such that the requirement specified in §171.311(e) is not satisfied and this condition persists for more than one second.

(5) A failure of the monitor is detected.

(b) The period during which erroneous guidance information is radiated must not exceed the periods specified in §171.319(a). If the fault is not cleared within the time allowed, radiation shall cease. After shutdown, no attempt must be made to restore service until a period of 20 seconds has elapsed.

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§171.321   DME and marker beacon performance requirements.

(a) The DME equipment must meet the performance requirements prescribed in subpart G of the part. This subpart imposes requirements that performance features must comply with International Standards and Recommended Practices, Aeronautical Telecommunications, Vol. I of Annex 10 to ICAO. It is available from ICAO, Aviation Building, 1080 University Street, Montreal 101, Quebec, Canada, Attention: Distribution Officer and also available for inspection at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal__register/code__of__federal__regulations/ibr__locations.html.

(b) MLS marker beacon equipment must meet the performance requirements prescribed in subpart H of this part. This subpart imposes requirements that performance features must comply with International Standards and Recommended Practices, Aeronautical Telecommuncations, Vol. I of Annex 10 to ICAO.

[Doc. No. 5034, 29 FR 11337, Aug. 6, 1964, as amended at 69 FR 18803, Apr. 9, 2004]

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§171.323   Fabrication and installation requirements.

(a) The MLS facility must be permanent and must be located, constructed, and installed in accordance with best commercial engineering practices, using applicable electric and safety codes and Federal Communications Commission (FCC) licensing requirements and siting requirements of §§171.313(b) and 171.317(b).

(b) The MLS facility components must utilize solid state technology except that traveling wave tube amplifiers (TWTA) may be used. A maximum level of common modularity must be provided along with diagnostics to facilitate maintenance and troubleshooting.

(c) An approved monitoring capability must be provided which indicates the status of the equipment at the site and at a remotely located maintenance area, with monitor capability that provides pre-alarm of impending system failures. This monitoring feature must be capable of transmitting the status and pre-alarm over standard phone lines to a remote section. In the event the sponsor requests the FAA to assume ownership of the facility, the monitoring feature must also be capable of interfacing with FAA remote monitoring requirements. This requirement may be complied with by the addition of optional software and/or hardware in space provided in the original equipment.

(d) The mean corrective maintenance time of the MLS equipment must be equal to or less than 0.5 hours with a maximum corrective maintenance time not to exceed 1.5 hours. This measure applies to correction of unscheduled failures of the monitor, transmitter and associated antenna assemblies, limited to unscheduled outage and out of tolerance conditions.

(e) The mean-time-between-failures of the MLS angle system must not be less than 1,500 hours. This measure applies to unscheduled outage, out-of-tolerance conditions, and failures of the monitor, transmitter, and associated antenna assemblies.

(f) The MLS facility must have a reliable source of suitable primary power, either from a power distribution system or locally generated. Adequate power capacity must be provided for the operation of the MLS as well as the test and working equipment of the MLS.

(g) The MLS facility must have a continuously engaged or floating battery power source for the continued normal operation of the ground station operation if the primary power fails. A trickle charge must be supplied to recharge the batteries during the period of available primary power. Upon loss and subsequent restoration of power, the battery must be restored to full charge within 24 hours. When primary power is applied, the state of the battery charge must not affect the operation of the MLS ground station. The battery must allow continuation of normal operation of the MLS facility for at least 2 hours without the use of additional sources of power. When the system is operating from the battery supply without prime power, the radome deicers and the environmental system need not operate. The equipment must meet all specification requirements with or without batteries installed.

(h) There must be a means for determining, from the ground, the performance of the system including antenna, both initially and periodically.

(i) The facility must have, or be supplemented by, ground, air, or landline communications services. At facilities within or immediately adjacent to controlled airspace, that are intended for use as instrument approach aids for an airport, there must be ground air communications or reliable communications (at least a landline telephone) from the airport to the nearest FAA air traffic control or communication facility. Compliance with this paragraph need not be shown at airports where an adjacent FAA facility can communicate with aircraft on the ground at the airport and during the entire proposed instrument approach procedure. In addition, at low traffic density airports within or immediately adjacent to controlled airspace, and where extensive delays are not a factor, the requirements of this paragraph may be reduced to reliable communications from the airport to the nearest FAA air traffic control or communications facility. If the adjacent FAA facility can communicate with aircraft during the proposed instrument approach procedure down to the airport surface or at least down to the minimum en route altitude, this would require at least a landline telephone.

(j) The location of the phase center for all antennas must be clearly marked on the antenna enclosures.

(k) The latitude, longitude and mean sea level elevation of all MLS antennas, runway threshold and runway stop end must be determined by survey with an accuracy of ±3 meters (±10 feet) laterally and ±0.3 meter (±1.0 foot) vertically. The relative lateral and vertical offsets of all antenna phase centers, and both runway ends must be determined with an accuracy of ±0.3 meter (±1.0 foot) laterally and ±0.03 meter (±0.1 foot) vertically. The owner must bear all costs of the survey. The results of this survey must be included in the “operations and maintenance” manual required by section 171.325 of this subpart and will be noted on FAA Form 198 required by §171.327.

[Doc. No. 20669, 51 FR 33177, Sept. 18, 1986, as amended by Amdt. 171-16, 56 FR 65665, Dec. 17, 1991]

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§171.325   Maintenance and operations requirements.

(a) The owner of the facility must establish an adequate maintenance system and provide MLS qualified maintenance personnel to maintain the facility at the level attained at the time it was commissioned. Each person who maintains a facility must meet the FCC licensing requirements and demonstrate that he has the special knowledge and skills needed to maintain an MLS facility, including proficiency in maintenance procedures and the use of specialized test equipment.

(b) In the event of out-of-tolerance conditions or malfunctions, as evidenced by receiving two successive pilot reports, the owner must close the facility by encasing radiation, and issue a “Notice to Airmen” (NOTAM) that the facility is out of service.

(c) The owner must prepare, and obtain approval of, an operations and maintenance manual that sets forth mandatory procedures for operations, periodic maintenance, and emergency maintenance, including instructions on each of the following:

(1) Physical security of the facility.

(2) Maintenance and operations by authorized persons.

(3) FCC licensing requirements for operations and maintenance personnel.

(4) Posting of licenses and signs.

(5) Relations between the facility and FAA air traffic control facilities, with a description of the boundaries of controlled airspace over or near the facility, instructions for relaying air traffic control instructions and information, if applicable, and instructions for the operation of an air traffic advisory service if the facility is located outside of controlled airspace.

(6) Notice to the Administrator of any suspension of service.

(7) Detailed and specific maintenance procedures and servicing guides stating the frequency of servicing.

(8) Air-ground communications, if provided, expressly written or incorporating appropriate sections of FAA manuals by reference.

(9) Keeping the station logs and other technical reports, and the submission of reports required by §171.327.

(10) Monitoring of the MLS facility.

(11) Inspections by United States personnel.

(12) Names, addresses, and telephone numbers of persons to be notified in an emergency.

(13) Shutdowns for periodic maintenance and issuing of NOTAM for routine or emergency shutdowns.

(14) Commissioning of the MLS facility.

(15) An acceptable procedure for amending or revising the manual.

(16) An explanation of the kinds of activities (such as construction or grading) in the vicinity of the MLS facility that may require shutdown or recertification of the MLS facility by FAA flight check.

(17) Procedures for conducting a ground check of the azimuth and elevation alignment.

(18) The following information concerning the MLS facility:

(i) Facility component locations with respect to airport layout, instrument runways, and similar areas.

(ii) The type, make and model of the basic radio equipment that provides the service including required test equipment.

(iii) The station power emission, channel, and frequency of the azimuth, elevation, DME, marker beacon, and associated compass locators, if any.

(iv) The hours of operation.

(v) Station identification call letters and method of station identification and the time spacing of the identification.

(vi) A description of the critical parts that may not be changed, adjusted, or repaired without an FAA flight check to confirm published operations.

(d) The owner or his maintenance representative must make a ground check of the MLS facility periodically in accordance with procedures approved by the FAA at the time of commissioning, and must report the results of the checks as provided in §171.327.

(e) The only modifications permitted are those that are submitted to FAA for approval by the MLS equipment manufacturer. The owner or sponsor of the facility must incorporate these modifications in the MLS equipment. Associated changes must also be made to the operations and maintenance manual required in paragraph (c) of this section. This and all other corrections and additions to this operations and maintenance manual must also be submitted to FAA for approval.

(f) The owner or the owner's maintenance representative must participate in inspections made by the FAA.

(g) The owner must ensure the availability of a sufficient stock of spare parts, including solid state components, or modules to make possible the prompt replacement of components or modules that fail or deteriorate in service.

(h) FAA approved test instruments must be used for maintenance of the MLS facility.

(i) Inspection consists of an examination of the MLS equipment to ensure that unsafe operating conditions do not exist.

(j) Monitoring of the MLS radiated signal must ensure a high degree of integrity and minimize the requirements for ground and flight inspection. The monitor must be checked daily during the in-service test evaluation period (96 hour burn in) for calibration and stability. These tests and ground checks or azimuth, elevation, DME, and marker beacon radiation characteristics must be conducted in accordance with the maintenance requirements of this section.

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§171.327   Operational records.

The owner of the MLS facility or his maintenance representative must submit the following operational records at the indicated time to the appropriate FAA regional office where the facility is located.

(a) Facility Equipment Performance & Adjustment Data (FAA Form 198). The FAA Form 198 shall be filled out by the owner or his maintenance representative with the equipment adjustments and meter readings as of the time of facility commissioning. One copy must be kept in the permanent records of the facility and two copies must be sent to the appropriate FAA regional office. The owner or his maintenance representative must revise the FAA Form 198 data after any major repair, modernization, or retuning to reflect an accurate record of facility operation and adjustment.

(b) Facility Maintenance Log (FAA Form 6030-1). FAA Form 6030-1 is permanent record of all the activities required to maintain the MLS facility. The entries must include all malfunctions met in maintaining the facility including information on the kind of work and adjustments made, equipment failures, causes (if determined) and corrective action taken. In addition, the entries must include completion of periodic maintenance required to maintain the facility. The owner or his maintenance representative must keep the original of each form at the facility and send a copy to the appropriate FAA regional office at the end of each month in which it is prepared. However, where an FAA approved remote monitoring system is installed which precludes the need for periodic maintenance visits to the facility, monthly reports from the remote monitoring system control point must be forwarded to the appropriate FAA regional office, and a hard copy retained at the control point.

(c) Technical Performance Record (FAA Form 6830 (formerly FAA Form 418)). This form contains a record of system parameters as specified in the manufacturer's equipment manual. This data will be recorded on each scheduled visit to the facility. The owner or his maintenance representative shall keep the original of each record at the facility and send a copy of the form to the appropriate FAA regional office.

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