;far is a function which calculates the distance between two global
;locations. 
;Use of far is as follows:
;
;                d=far(lat1,lon1,lat2,lon2)
;    Where
;                  lat1 = latitude of location 1 in deg
;                  lon1 = longitude of location 1 in deg
;
;                  lat2 = latitude of location 2 in deg
;                  lon2 = longitude of location 2 in deg
;
;                     d = distance in kilometers between location 1 & 2
;BWB 1/16/92
;
;Converted to an idl code 5/5/00

function far,lat1,lon1,lat2,lon2
r=double(3959.)*double(1.609)

pi=2.*asin(double(1))

lat1=lat1*pi/180.
lon1=lon1*pi/180.
lat2=lat2*pi/180.
lon2=lon2*pi/180.
r1x=cos(lon1)*cos(lat1)*r
r1y=sin(lon1)*cos(lat1)*r
r1z=sin(lat1)*r
r2x=cos(lon2)*cos(lat2)*r
r2y=sin(lon2)*cos(lat2)*r
r2z=sin(lat2)*r
d=acos((r1x*r2x+r1y*r2y+r1z*r2z)/r^2)*r


return,d
end