[Spice_discussion] discrepancy SPICE vs horizons

Hans-Peter Doerr doerr at kis.uni-freiburg.de
Tue Feb 5 15:13:54 PST 2013

Dear Navigators,

while testing a SPICE-based code, I saw a significant discrepancy
between values obtained via horizons and those computed via CSPICE. I
boiled it down to the following code snippet:

   SpiceDouble et, lt, r, rr, relstate[6], los[3];
   furnsh_c ("kernels/pck00010.tpc");
   furnsh_c ("kernels/naif0010.tls");
   furnsh_c ("kernels/de405.bsp");
   furnsh_c ("kernels/earth_latest_high_prec.bpc");
   furnsh_c ("kernels//earthstns_fx_050714.bsp");
   furnsh_c ("kernels/earth_fixed_itrf93.tf");

   str2et_c ("2011-Aug-01 06:00:00.0000", &et);
   spkezr_c ("Sun", et, "J2000", "NONE", "DSS-12", relstate, &lt);
   unorm_c (relstate, los, &r);
   rr = vdot_c (los, &relstate[3]);

   printf ("dist: %.16E  v_los: % .16E\n", r, rr);

this gives:
   dist: 1.5185605413773715E+08  v_los: -4.6934776127220079E-02

the 'earth_fixex_itrf93' text kernels binds EARTH_FIXED to ITRF93 as 

whith these horizons settings:

   Target body name: Sun (10)                 {source: DE405}
   Center body name: Earth (399)              {source: DE405}
   Center-site name: DSS 12
   EOP file        : eop.130204.p130428
   EOP coverage    : DATA-BASED 1962-JAN-20 TO 2013-FEB-04. PREDICTS-> 
   Output type     : GEOMETRIC cartesian states
   Coordinate systm: Earth Mean Equator and Equinox of Reference Epoch

I get (jd, range, range-rate):

   2455774.750000000, 1.518560571902532E+08, -4.540414938447372E-02

these values are off by about 3 km and 1.5 m/s. I don't see what could 
be the issue here.

Any Ideas?



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