FUNCTION Seg_Len,h,zeta,del_h=del_h

; This function returns the length (in km) of the geometric segment 
; between the observer, at altitude h, and the altitude h + del_h (km) 
; along a ray at an angle of zeta degrees from the local zenith, where 
; zeta < 90 degrees.

; Note: observer's altitude h is assumed to be in km
;       zenith angle zeta is assumed to be in degrees
;       del_h defaults to 1 km

r=6371.D0	; Earth radius, in km

IF N_Elements(del_h) EQ 0 THEN del_h=1.D0

hd=Double(h)
zd=Double(zeta)
h_tan=(r+hd)*Sin(!DtoR*zd)-r
Return,Float(SQRT((r+hd+del_h)^2-(r+h_tan)^2)-SQRT((r+hd)^2-(r+h_tan)^2))
END