spkgeo_c |
Table of contents
Procedurespkgeo_c ( S/P Kernel, geometric state ) void spkgeo_c ( SpiceInt targ, SpiceDouble et, ConstSpiceChar * ref, SpiceInt obs, SpiceDouble state[6], SpiceDouble * lt ) AbstractCompute the geometric state (position and velocity) of a target body relative to an observing body. Required_ReadingSPK KeywordsEPHEMERIS Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- targ I Target body. et I Target epoch. ref I Target reference frame. obs I Observing body. state O State of target. lt O Light time. Detailed_Inputtarg is the standard NAIF ID code for a target body. et is the epoch (ephemeris time) at which the state of the target body is to be computed. ref is the name of the reference frame to which the state vector returned by the routine should be rotated. This may be any frame supported by the SPICELIB subroutine FRMCHG. See also the Frames Required Reading for a list of supported frames. obs is the standard NAIF ID code for an observing body. Detailed_Outputstate contains the geometric position and velocity of the target body, relative to the observing body, at epoch `et'. `state' has six elements: the first three contain the target's position; the last three contain the target's velocity. These vectors are transformed into the specified reference frame. Units are always km and km/sec. lt is the one-way light time from the observing body to the geometric position of the target body in seconds at the specified epoch. ParametersNone. Exceptions1) If insufficient ephemeris data has been loaded to compute the necessary states, the error SPICE(SPKINSUFFDATA) is signaled by a routine in the call tree of this routine. 2) If the `ref' input string pointer is null, the error SPICE(NULLPOINTER) is signaled. 3) If the `ref' input string has zero length, the error SPICE(EMPTYSTRING) is signaled. FilesSee -Restrictions. Particularsspkgeo_c computes the geometric state, targ(t), of the target body and the geometric state, obs(t), of the observing body relative to the first common center of motion. Subtracting obs(t) from targ(t) gives the geometric state of the target body relative to the observer. center ----- obs(t) | / | / | / | / targ(t) - obs(t) | / targ(t) The one-way light time, tau, is given by | targ(t) - obs(t) | tau = ---------------------- C For example, if the observing body is -94, the Mars Observer spacecraft, and the target body is 401, Phobos, then the first common center is probably 4, the Mars Barycenter. obs(t) is the state of -94 relative to 4 and targ(t) is the state of 401 relative to 4. The center could also be the Solar System Barycenter, body 0. For example, if the observer is 399, Earth, and the target is 299, Venus, then obs(t) would be the state of 399 relative to 0 and targ(t) would be the state of 299 relative to 0. Ephemeris data from more than one segment may be required to determine the states of the target body and observer relative to a common center. spkgeo_c reads as many segments as necessary, from as many files as necessary, using files that have been loaded by previous calls to furnsh_c or spklef_c (load ephemeris file). spkgeo_c is similar to spkez_c but returns geometric states only, with no option to make planetary (light-time) nor stellar aberration corrections. The geometric states returned by spkez_c and spkgeo_c are the same. ExamplesThe numerical results shown for this example may differ across platforms. The results depend on the SPICE kernels used as input, the compiler and supporting libraries, and the machine specific arithmetic implementation. 1) Return the geometric state vector of Mars (499) as seen from Earth (399) in the J2000 frame and the one-way light time between them at the epoch July 4, 2003 11:00 AM PST. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File: spkgeo_ex1.tm This meta-kernel is intended to support operation of SPICE example programs. The kernels shown here should not be assumed to contain adequate or correct versions of data required by SPICE-based user applications. In order for an application to use this meta-kernel, the kernels referenced here must be present in the user's current working directory. The names and contents of the kernels referenced by this meta-kernel are as follows: File name Contents --------- -------- de430.bsp Planetary ephemeris mar097.bsp Mars satellite ephemeris naif0011.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'de430.bsp', 'mar097.bsp', 'naif0011.tls' ) \begintext End of meta-kernel Example code begins here. /. Program spkgeo_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main( ) { /. Local variables. ./ SpiceChar * epoch; SpiceChar * reffrm; SpiceDouble et; SpiceDouble lt; SpiceDouble state [6]; SpiceInt obsrvr; SpiceInt target; /. Load a set of kernels. Use a meta kernel for convenience. ./ furnsh_c ( "spkgeo_ex1.tm" ); /. Define parameters for a state lookup. ./ target = 499; epoch = "July 4, 2003 11:00 AM PST"; reffrm = "J2000"; obsrvr = 399; /. Convert the epoch to ephemeris time. ./ str2et_c ( epoch, &et ); /. Look-up the state for the defined parameters. ./ spkgeo_c ( target, et, reffrm, obsrvr, state, < ); /. Output... ./ printf( "The position of : %2d\n", target ); printf( "As observed from : %2d\n", obsrvr ); printf( "In reference frame : %s\n", reffrm ); printf( "At epoch : %s\n", epoch ); printf( " \n" ); /. The first three entries of state contain the X, Y, Z position components. The final three contain the Vx, Vy, Vz velocity components. ./ printf( "R (km): %17.5f %17.5f %17.5f\n", state[0], state[1], state[2] ); printf( "V (km/s): %17.5f %17.5f %17.5f\n", state[3], state[4], state[5] ); printf( " \n" ); printf( "Light time (s) between observer and target:" " %18.13f\n", lt ); return ( 0 ); } When this program was executed on a Mac/Intel/cc/64-bit platform, the output was: The position of : 499 As observed from : 399 In reference frame : J2000 At epoch : July 4, 2003 11:00 AM PST R (km): 73826216.43529 -27128030.73241 -18741973.86829 V (km/s): -6.80950 7.51381 3.00129 Light time (s) between observer and target: 269.7026477631753 Restrictions1) The ephemeris files to be used by spkgeo_c must be loaded by furnsh_c or spklef_c before spkgeo_c is called. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) J.E. McLean (JPL) B.V. Semenov (JPL) W.L. Taber (JPL) E.D. Wright (JPL) Version-CSPICE Version 1.1.3, 10-AUG-2021 (JDR) Edited the header to comply with NAIF standard. Added complete code example to -Examples section. Added reference to furnsh_c in -Particulars and -Restrictions sections. Added entries #2 and #3 in -Exceptions section. -CSPICE Version 1.1.2, 08-JAN-2014 (BVS) Fixed description of "state" in -Detailed_Output. Replaced spklef_c with furnsh_c and fixed errors in -Examples. -CSPICE Version 1.1.1, 13-OCT-2003 (EDW) Various minor header changes were made to improve clarity. Added mention that 'lt' returns a value in seconds. -CSPICE Version 1.1.0, 08-FEB-1998 (NJB) References to C2F_CreateStr_Sig were removed; code was cleaned up accordingly. String checks are now done using the macro CHKFSTR. -CSPICE Version 1.0.0, 25-OCT-1997 (NJB) (JEM) (WLT) Based on SPICELIB Version 2.2.0, 11-APR-1997 (WLT) Index_Entriesgeometric state of one body relative to another |
Fri Dec 31 18:41:12 2021