Empirical function to compute the distance to the horizon from a given altitude. The earth is assumed to be smooth, i.e. mountains and other obstacles are ignored.
Usage
horizon(h, r=6378137)
Arguments
h
altitude, numeric >= 0. Should have the same unit as r
r
radius of the earth; default value is 6378137 m
Value
Distance in units of h (default is meters)
References
http://williams.best.vwh.net/avform.htm#Horizon
Bowditch, 1995. American Practical Navigator. Table 12.