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

sept. 1976....derivative of deldop (dldpbg)
this routine handles radar from a planet
ncodf.eq.1  klanb.eq.0    nspot.ge.0
arguments

dstf(1) =ctrecf=coordinate time at reception (fraction of day)
dstf(2) = longitude of subradar point (deg)
dstf(3) = latitude of subradar point (deg)
dstf(4) = time delay from receive to reflection (sec)
dstf(5) = longitude of observed spot away from subradar point
if spot name='&&&&' and nspot=0 (deg)
dstf(6) = latitude of observed spot away from subradar point
if spot name='&&&&' and nspot=0 (deg)
dstf(8) = round-trip topography at observed spot (light sec)
dstf(10)= fdev =1+fractional frequency offset from atomic
time for unit of time delay measurement

relativistic correction.  since radrel is used, xsbsun must be
set up. however, since vectors to the subradar point are not
calculated in the above code, we must use vectors to the c.m.
of the planet. this produces negligable errors.
for logspt, the calculations are done correctly.