r. goldstein     april, 1976
interpolations are now done by calls to mntrp,emtrp,
pltrp, and sbtrp.
polynomial extrapolations are now done by dguess

sept. 1976..a derivative of sbdldp (sbbg)
this routine handles planetary landers only
ncodf.eq.2       nspot.gt.0
oct. 1976....large sections of code replaced by calls to
subroutines propco, phadop, and radrel
june 1978...converted to implicit real*8,
fixed interface to trp routines

xslcns(1-6,j) = position and velocity (not used) of the center
of mass of the solar system relative to the sun
at receive time (j=1), reflection time (j=2)
and send time (j=3)

correct for propagation effects by calling propco
set ipct as follows:
-2: first leg of phase delay doppler
-1: second leg of phase delay doppler
1: time delay

there are three possible doppler observables:
lopler=-1  observable is phase
lopler= 0  observable is averaged integrated frequency (jpl)
lopler= 1  observable is instantaneous frequency
lopler=0 has been removed from the code and has been gone
since before 1975.