function tan_path_len,zt,dz,re

;---------------------------------------------------------------
;  Routine computes the path length of the tangent point sample
;  volume for a limb view of the earths atmosphere from space.
;  Path length is along the limb.  This calculation does not
;  account for refraction.
;
;  Input:
;    zt.....tangent point altitude, km
;    dz.....vertical resolution at tangent, km
;    re.....earth's radius, km, if zero then 6370.0 is used 
;
;  Output:
;    pl.....path length, km
;
;  Source: Mark Hervig
;---------------------------------------------------------------

   if re le 0.0 then re = 6370.0   ; earth radius (km)

;- calculate the tangent path length

   rt = re + zt

   pl = 2.* sqrt((rt+dz)^2 - (rt)^2)

   return,pl

   end