47 C.F.R. § 73.190
Formula 1. Skywave field strength, 50% of the time (at SS + 6):
The skywave field strength, Fc(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, [PHI]M, is given by:
[PHI]M = arcsin[sin aM sin 78.5° + cos aM cos 78.5° cos(69 + bM)]degrees (Eq. 3)
The short great-circle path distance, d, is given by:
Where:
d° = arccos[sin aT sin aR + cos aT cos aR cos(bR - bT)]degrees (Eq.5)
Where:
aT is the geographic latitude of the transmitting terminal (degrees)
aR is the geographic latitude of the receiving terminal (degrees)
bT is the geographic longitude of the transmitting terminal (degrees)
bR is the geographic longitude of the receiving terminal (degrees)
aM is the geographic latitude of the midpoint of the great-circle path (degrees) and is given by:
bM is the geographic longitude of the midpoint of the great-circle path (degrees) and is given by:
Note (1): If |FM| is greater than 60 degrees, equation (1) is evaluated for | FM| = 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., bR [GREATER THAN]bT), otherwise k = 1.
Formula 2. Skywave field strength, 10% of the time (at SS + 6):
The skywave field strength, Fc(10), is given by:
Fc(10) = Fc(50) + [DELTA] dB([MICRO]V/m)
Where:
[DELTA] = 6 when | FM| [LESS THAN]40
[DELTA] = 0.2 | FM| - 2 when 40 [LESS THAN EQUAL TO]| FM| [LESS THAN EQUAL TO]60
[DELTA] = 10 when | FM| [GREATER THAN]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
Note: Computations using these formulas should not be carried beyond 0.1 degree.
47 C.F.R. §73.190