ash / amuchastegui - october 1969 - subroutine ertorb
computations for forces on earth satellite motion due to
earth shadow, radiation pressure (direct and reflected),
air drag, charge drag, satellite thrustors and sensors, etc.

ncall=-2 setup once per iteration of a given step for partials
ncall=-1 setup once per iteration of a given step for motion
ncall= 0 setup once per step
ncall= k evaluate acceleration for equation k.gt.0

kp(81)= kreflt*10+ kdirct
kdirct=0 no radiation pressure
=1 direct radiation,exact shadow
=2 direct radiation,approx. shadow.(not coded)
=3 direct radiation, no shadow