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Дата изменения: Sat Dec 17 06:09:38 2005 Дата индексирования: Sun Apr 10 23:27:20 2016 Кодировка: |
Convert ephemeris seconds past j2000_c (ET) to continuous encoded spacecraft clock (`ticks'). Non-integral tick values may be returned.
SCLK TIME
Variable I/O Description -------- --- -------------------------------------------------- sc I NAIF spacecraft ID code. et I Ephemeris time, seconds past j2000_c. sclkdp O SCLK, encoded as ticks since spacecraft clock start. sclkdp need not be integral.
sc is a NAIF integer code for a spacecraft whose encoded SCLK value at the epoch specified by et is desired. et is an epoch, specified as ephemeris seconds past J2000.
sclkdp is an encoded spacecraft clock value. sclkdp is an encoded representation of the total number of spacecraft clock ticks measured from the time the spacecraft clock started to the epoch et: partition information IS reflected in the encoded value. sclkdp may be non-integral: sclkdp is NOT rounded to the nearest whole tick.
None.
This routine outputs continuous encoded SCLK values; unlike the routine sce2t_c, the values output by this routine need not be integral. This routine supports use of non-integral encoded clock values in C-kernels: non-integral clock values may be stored as pointing time tags when a C-kernel is created, and they may be supplied as request times to the C-kernel readers. The advantage of encoded SCLK, as opposed to character string representations of SCLK, is that encoded SCLK values are easy to perform arithmetic operations on. Also, working with encoded SCLK reduces the overhead of repeated conversion of character strings to integers or double precision numbers. To convert ET to a string representation of an SCLK value, use the CSPICE routine sce2s_c. See the SCLK Required Reading for a list of the entire set of SCLK conversion routines.
1) Convert ET directly to a continuous, encoded SCLK value. Use both of these time values to look up both C-kernel (pointing) and SPK (position and velocity) data for an epoch specified by an ephemeris time. During program initialization, load the leapseconds and SCLK kernels. We will pretend that these files are named "leapseconds.ker" and "gllsclk.ker". To use this code fragment, you must substitute the actual names of these kernel files for the names used here. /. Load leapseconds and SCLK kernels: ./ ldpool_c ( "leapseconds.ker" ) ldpool_c ( "gllsclk.ker" ) The mission is Galileo, which has spacecraft ID -77. Let ET be the epoch, specified in ephemeris seconds past J2000, at which both position and pointing data is desired. Find the encoded SCLK value corresponding to ET. sce2c_c ( -77, et, &sclkdp ); Now you're ready to call both ckgp_c, which expects the input epoch to be specified by an encoded SCLK string, and spkez_c, which expects the epoch to be specified as an ephemeris time. /. Find scan platform pointing cmat and s/c--target vector (first 3 components of state) at epoch. We assume that CK and SPK kernels have been loaded already, via cklpf_c and spklef_c respectively. ./ ckgp_c ( scanpl, sclkdp, tol, refsys, cmat, &clkout, &found ); spkez_c ( target, et, refsys, corr, -77, state, < ); 2) Convert UTC to an encoded Voyager 2 SCLK value. Again, your initialization code must load the leapseconds and SCLK kernels. /. Load leapseconds and SCLK kernels: ./ ldpool_c ( "leapseconds.ker" ); ldpool_c ( "vgr2sclk.ker" ); /. Find the encoded Voyager 2 SCLK value sclkdp corresponding to the given UTC time. ./ utc2et ( utc, &et ); sce2c_c ( -32, et, &sclkdp );
1) An SCLK kernel appropriate to the spacecraft clock identified by SC must be loaded at the time this routine is called. 2) If the SCLK kernel used with this routine does not map SCLK directly to barycentric dynamical time, a leapseconds kernel must be loaded at the time this routine is called.
1) This routine assumes that that an SCLK kernel appropriate to the spacecraft clock identified by the input argument sc has been loaded. If an SCLK kernel has not been loaded, does not contain all of the required data, or contains invalid data, error diagnoses will be performed by routines called by this routine. The output argument sclkdp will not be modified. 2) When using SCLK kernels that map SCLK to a time system other than ET (also called barycentric dynamical time---`TDB'), it is necessary to have a leapseconds kernel loaded at the time this routine is called. If a leapseconds kernel is required for conversion between SCLK and ET but is not loaded, the error will be diagnosed by routines called by this routine. The output argument sclkdp will not be modified. The time system that an SCLK kernel maps SCLK to is indicated by the variable SCLK_TIME_SYSTEM_nn in the kernel, where nn is the negative of the NAIF integer code for the spacecraft. The time system used in a kernel is TDB if and only if the variable is assigned the value 1. 3) If the clock type for the spacecraft clock identified by SC is not supported by this routine, the error SPICE(NOTSUPPORTED) is signalled. The output argument sclkdp will not be modified. 4) If the input ET value is not representable as an encoded spacecraft clock value for the spacecraft clock identified by sc, the error will be diagnosed by routines called by this routine. The output argument sclkdp will not be modified.
None.
N.J. Bachman (JPL)
[1] CK Required Reading [2] SPK Required Reading
-CSPICE Version 1.0.0, 18-JUN-1999 (NJB)
ephemeris time to continuous spacecraft_clock ticks