Formula 1. Skywave field strength, 50% of the time (at SS + 6):

The skywave field strength, F_c(50), for a characteristic field strength of 100 mV/m at 1 km is given by:

The slant distance, D, is given by:

The geomagnetic latitude of the midpoint of the path, Φ_M, is given by:

Φ_M = arcsin[sin a_M sin 78.5° + cos a_M cos 78.5° cos(69 + b_M)]degrees (Eq. 3)

The short great-circle path distance, d, is given by:

Where:

d° = arccos[sin a_T sin a_R + cos a_T cos a_R cos(b_R − b_T)]degrees (Eq.5)

Where:

a_T is the geographic latitude of the transmitting terminal (degrees)

a_R is the geographic latitude of the receiving terminal (degrees)

b_T is the geographic longitude of the transmitting terminal (degrees)

b_R is the geographic longitude of the receiving terminal (degrees)

a_M is the geographic latitude of the midpoint of the great-circle path (degrees) and is given by:

b_M is the geographic longitude of the midpoint of the great-circle path (degrees) and is given by:

Note (1): If |F_M| is greater than 60 degrees, equation (1) is evaluated for | F_M| = 60 degrees.

Note (2): North and east are considered positive; south and west negative.

Note (3): In equation (7), k = −1 for west to east paths (i.e., b_R >b_T), otherwise k = 1.

Formula 2. Skywave field strength, 10% of the time (at SS + 6):

The skywave field strength, F_c(10), is given by:

F_c(10) = F_c(50) + Δ dB(µV/m)

Where:

Δ = 6 when | F_M| <40

Δ = 0.2 | F_M| − 2 when 40 ≤| F_M| ≤60

Δ = 10 when | F_M| >60

Where:

d = distance in kilometers

n = 1 for 50% field strength values

n = 2 or 3 for 10% field strength values

and where

K₁ = 0.00752

K₂ = 0.00938

K₃ = 0.00565