Convert ephemeris seconds past J2000 (ET) to integral encoded spacecraft clock (`ticks'). For conversion to fractional ticks, (required for C-kernel production), see the routine sce2c_c..
Variable I/O Description -------- --- -------------------------------------------------- sc I NAIF spacecraft ID code. et I Ephemeris time, seconds past J2000. sclkdp O SCLK, encoded as ticks since spacecraft clock start.
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 integral spacecraft clock value. `sclkdp' is an encoded representation of the total count 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' is rounded to the nearest integral double precision number.
This function outputs discrete, encoded SCLK values. Since continuous SCLK values are generally more useful, the newer function sce2c_c (ET to continuous ticks) should normally be used in place of this one. However, the functionality of this routine is needed for converting ET to SCLK strings. This function outputs encoded SCLK values. 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 function sce2s_c.
1) Convert ET directly to an 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: ./ furnsh_c ( "leapseconds.ker" ) furnsh_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. sce2t_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: ./ furnsh_c ( "leapseconds.ker" ); furnsh_c ( "vgr2sclk.ker" ); /. Find the encoded Voyager 2 SCLK value sclkdp corresponding to the given UTC time. ./ utc2et ( utc, &et ); sce2t_c ( -32, et, &sclkdp );
1) An SCLK kernel appropriate to the spacecraft clock identified by SC must be loaded at the time this function is called. 2) If the SCLK kernel used with this function does not map SCLK directly to barycentric dynamical time, a leapseconds kernel must be loaded at the time this function is called.
1) This function 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 functions called by this function. 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 function is called. If a leapseconds kernel is required for conversion between SCLK and ET but is not loaded, the error will be diagnosed by functions called by this function. 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 function, the error SPICE(NOTSUPPORTED) is signaled. 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 functions called by this function. The output argument sclkdp will not be modified.
N.J. Bachman (JPL)
 CK Required Reading  SPK Required Reading
-CSPICE Version 1.0.2, 27-JAN-2004 (NJB) Header was updated to remove comments indicating this routine was deprecated. Minor changes were made to clarify both the functionality of this routine and the difference between this routine and sce2c_c. Examples were updated to use furnsh_c. -CSPICE Version 1.0.1, 09-MAR-1999 (NJB) Updated to reflect the introduction of continuous ticks and the function sce2c_c. Occurrences of "routine" were changed to "function" in the header. -CSPICE Version 1.0.0, 08-FEB-1998 (NJB) Based on SPICELIB Version 1.0.2, 10-APR-1992 (NJB) (WLT)
ephemeris time to spacecraft_clock ticks