An equation that expresses the radar horizon range (RHR,) given by where RHRs is the radar horizon range in statute miles, h is the antenna height in feet, and a is the object critical altitude, i.e., the target altitude in feet, below which the radar cannot illuminate the object. Note: The RHR is also given by where RHRk is the radar horizon range in kilometers when h and a are in meters. The effective Earth radius, namely 4/3 times the actual Earth radius, is used in deriving these formulas. The effective Earth radius for LOS varies with carrier frequency. Second-order differentials are neglected. They contribute less than 0. 1%.