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e-CFR data is current as of February 25, 2021

Title 14Chapter ISubchapter JPart 171Subpart J → §171.311


Title 14: Aeronautics and Space
PART 171—NON-FEDERAL NAVIGATION FACILITIES
Subpart J—Microwave Landing System (MLS)


§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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