subroutine geo_ll(geog,plat,plon)
C  given a vector geog in the geo (geographic) coordinate system,
C  returns as plat and plon the latitude and longitude of the 
C  corresponding point.
C
C  The geo system is a 3-D Cartesian coordinate system, with origin
C  at the Center of the Earth, with the x_3 axis pointing to the North
C  Pole, the x_1 axis pointing to the Equator at the Greenwich Meridian,
C  and the x_2 axis by the right hand rule pointing to the Equator
C  at 90 E.
      parameter (pi=3.14159265358979323846,radpdg=pi/180.)
      parameter (dgprad=180./pi)
      dimension geog(3)
      fact = geog(1)*geog(1) + geog(2)*geog(2)
      if (fact .le. 0.) then
        plon = 0.
        plat = sign(90.,geog(3))
      else
        fact = sqrt(fact)
        plat = DGPRAD * atan2(geog(3),fact)
        plon = DGPRAD * atan2(geog(2),geog(1))
      endif
      return
      end