--
Hidden Height (m) = R - sqrt(R^2 - d^2) - (observerHeight) - (targetHeight?), where R = 6371 km. Simplified: h = d^2 / (2*R) * 1000, then subtract observer and target heights. For target height, we assume line of sight: if hidden height <= 0, target is visible.
Note: This calculator uses the standard approximation for small distances.