function ang2lin_fov,Zsc,Zt,Re,ang

;-------------------------------------------------------------------
;  relate an angular "sweep",  or field of view (FOV)
;  at spacecraft altitude to a linear distance subtended at tangent
;
;  input:
;    Zsc........spacecraft altitude, km
;    Zt.........tangent altitude, km
;    Re.........Earth's radius, km, if zero then mean value is used
;    ang........angular sweep (FOV) in arc minutes
;
;  output:
;    dz_t.......distance subtended at tangent, km
;
;  source: Mark Hervig
;-------------------------------------------------------------------

   if Re lt 5000. then Re  = 6378.0    ; mean earth radius, km

   ang_r = ang * (!pi / (180. * 60.) )

;- the distance subtended at tangent

   dz_t = ang_r * sqrt( (Zsc + Re)^2 - (Zt + Re)^2 )

   return,dz_t

   end