Equations
- Maidenhead locator → coordinates: the first letter pair is the field (20° longitude × 10° latitude), the digit pair is the square (2° × 1°), the second letter pair is the subsquare (5′ × 2.5′). I take the centre of the cell.
- Distance (orthodrome): the haversine formula on a sphere of radius
R = 6371 km— the shortest route over the Earth’s surface (great circle). - Initial azimuth:
θ = atan2( sin Δλ · cos φ₂ , cos φ₁ · sin φ₂ − sin φ₁ · cos φ₂ · cos Δλ ), normalised to 0–360°. That’s the direction your antenna points at the start of the path. - Back azimuth I compute separately from the other point (on a sphere it is not simply +180°).
- Long path: the opposite way round the globe — azimuth +180°, distance =
40,030 km − short path. Sometimes propagation goes the “long way round”.