;
; NAME: 
;   PixRadius
; 
; PURPOSE:
;   Calculate the radius of the Earth at a given latitude
;
; CATEGORY: 
;   SPT Tool
;
; CALLING SEQUENCE:
;    PixRadius, Latitude, Radius
;
; INPUT
;   Latitude: Latitude
;
; OUTPUT
;   Radius: radius of the Eath at the current latitude
;
; KEYWORD PARAMETERS:
;
; MODIFICATION HISTORY:
;   Written by:     Marco Clerici, 27.02.04
;

PRO PixRadius, Latitude, Radius
                            
  EARTH_RADIUS_M    =     6378.214d0
  RPO               =     6356.829d0
  
  Exc     = sqrt((EARTH_RADIUS_M * EARTH_RADIUS_M) - $
                   (RPO * RPO))/(EARTH_RADIUS_M)

  Exc2    = Exc*Exc           
  Exc4    = Exc2*Exc2
  
  LatRad  = Latitude * !DTOR
  Sp1     = sin(LatRad)
  Sp12    = Sp1*Sp1
  Sp14    = Sp12*Sp12
  
  Radius = EARTH_RADIUS_M * (1. - $ 
             (Exc2*Sp12)/2. + (Exc4*Sp12)/2. - (5.*Sp14*Exc4)/8.)
   
END