data pp,tc,tk,pw,an/984.,9.92,283.,7.79,306.123/
pp =absolute pressure in millibars
tc =temperature in degrees centigrade
tk =temperature in degrees kelvin   =tc+273.18
pw =water vapor partial pressure in millibars
an =surface refractivity = (refractive index-1.)*1.e6
= 77.6*(pp+4810.*pw/tk)/tk

entries a (mdeg) and b (mdeg/n-unit) correspond to elevation
intervals of 0.5 deg between 0 deg and 6 deg elevation,
intervals of 2.0 deg between 6 deg and 20 deg elevation,
intervals of 10. deg between 20 deg and 90 deg elevation.
model fails if unrealistic refractivity is specified.