radar xxline-of-sight (LOS) equation
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%. [From Weik ’89]