; docformat = 'rst'
;+
;Retrieve constants associated with the WGS-84 ellipsoid, the project's 
;official model of the shape of the Earth.
;
;Notes: In the World Geodetic System of 1984 (WGS-84):
;
;'a' is the semi-major axis (distance from the center of the Earth to the
;equator) = 6378.1370 km.
;
;'b' is the semi-minor axis (distance from the center of the Earth to the
;pole) = 6356.7523142 km.
;
;'f' is the flattening factor = (a - b) / a.
;
;'mu' is the gravitational standard parameter for the Earth.
;
;'omega' is the angular speed of the Earth.
;
; :Examples:
;   get_wgs84_const(re=re, a=a, f=f, rp=rp, mu=mu, omega=omega)
;-
;+
; :Keywords:
;   re : in, optional, type=boolean
;     Return the equatorial radius, km.
;   a : in, optional, type=boolean
;     Return the equatorial radius, km.
;   f : in, optional, type=boolean
;     Return the flattening factor 1-(b/a).
;   rp : in, optional, type=boolean
;     Return the polar radius, km.
;   mu : in, optional, type=boolean
;     Return the gravitational parameter mu=G*M_earth, in km^3/s^2.
;   omega : in, optional, type=boolean
;     Return the inertial rotation rate of the Earth in rad/s.
;     
; :Returns:
;   The selected constant, in the units specified above.
;-
function get_wgs84_const,re=re,a=a,f=f,rp=rp,mu=mu,omega=omega
  ;WGS-84 constants
  wgs84_a = 6378.137d              ;km
  ;f_=1/298.257223563d; unitless
  wgs84_f = 0.0033528106647474805d ;computed constant, unitless
  wgs84_b = 6356.7523142d          ;km
  wgs84_mu = 398600.4418d          ;km^3/s^2
  wgs84_omega = 7.292115d-5        ;rad/sec
  if keyword_set(re) or keyword_set(a) then return, wgs84_a
  if keyword_set(f)                    then return, wgs84_f
  if keyword_set(rp)                   then return, wgs84_b
  if keyword_set(mu)                   then return, wgs84_mu
  if keyword_set(omega)                then return, wgs84_omega
end