[Spice_discussion] discrepancy SPICE vs horizons

[Nat Bachman]

Dear Hans-Peter,

Your use of CSPICE code is correct. But you have
a difference in input time systems used for your
SPICE and Horizons computations: you used UTC for
SPICE and, based on your results, Horizons considered
your time input to be TDB.

   -Nat Bachman (JPL/NAIF)

On 02/05/13 15:13, Hans-Peter Doerr wrote:
> 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 suggested.
>
> 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-> 2013-APR-27
> 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?
>
>
> Thanks!
>
> Hans-Peter
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