spkezr_c |

Table of contents## Procedurespkezr_c ( S/P Kernel, easier reader ) void spkezr_c ( ConstSpiceChar *targ, SpiceDouble et, ConstSpiceChar *ref, ConstSpiceChar *abcorr, ConstSpiceChar *obs, SpiceDouble starg[6], SpiceDouble *lt ) ## AbstractReturn the state (position and velocity) of a target body relative to an observing body, optionally corrected for light time (planetary aberration) and stellar aberration. ## Required_ReadingABCORR FRAMES NAIF_IDS SPK TIME ## KeywordsEPHEMERIS ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- targ I Target body name. et I Observer epoch. ref I Reference frame of output state vector. abcorr I Aberration correction flag. obs I Observing body name. starg O State of target. lt O One way light time between observer and target. ## Detailed_Inputtarg is the name of a target body. Optionally, you may supply the integer ID code for the object as an integer string. For example both "MOON" and "301" are legitimate strings that indicate the moon is the target body. The target and observer define a state vector whose position component points from the observer to the target. et is the ephemeris time, expressed as seconds past J2000 TDB, at which the state of the target body relative to the observer is to be computed. `et' refers to time at the observer's location. ref is the name of the reference frame relative to which the output state vector should be expressed. This may be any frame supported by the SPICE system, including built-in frames (documented in the Frames Required Reading) and frames defined by a loaded frame kernel (FK). When `ref' designates a non-inertial frame, the orientation of the frame is evaluated at an epoch dependent on the selected aberration correction. See the description of the output state vector `starg' for details. abcorr indicates the aberration corrections to be applied to the state of the target body to account for one-way light time and stellar aberration. See the discussion in the -Particulars section for recommendations on how to choose aberration corrections. `abcorr' may be any of the following: "NONE" Apply no correction. Return the geometric state of the target body relative to the observer. The following values of `abcorr' apply to the "reception" case in which photons depart from the target's location at the light-time corrected epoch et-lt and *arrive* at the observer's location at `et': "LT" Correct for one-way light time (also called "planetary aberration") using a Newtonian formulation. This correction yields the state of the target at the moment it emitted photons arriving at the observer at `et'. The light time correction uses an iterative solution of the light time equation (see -Particulars for details). The solution invoked by the "LT" option uses one iteration. "LT+S" Correct for one-way light time and stellar aberration using a Newtonian formulation. This option modifies the state obtained with the "LT" option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent state of the target---the position and velocity of the target as seen by the observer. "CN" Converged Newtonian light time correction. In solving the light time equation, the "CN" correction iterates until the solution converges (three iterations on all supported platforms). Whether the "CN+S" solution is substantially more accurate than the "LT" solution depends on the geometry of the participating objects and on the accuracy of the input data. In all cases this routine will execute more slowly when a converged solution is computed. See the -Particulars section below for a discussion of precision of light time corrections. "CN+S" Converged Newtonian light time correction and stellar aberration correction. The following values of `abcorr' apply to the "transmission" case in which photons *depart* from the observer's location at `et' and arrive at the target's location at the light-time corrected epoch et+lt: "XLT" "Transmission" case: correct for one-way light time using a Newtonian formulation. This correction yields the state of the target at the moment it receives photons emitted from the observer's location at `et'. "XLT+S" "Transmission" case: correct for one-way light time and stellar aberration using a Newtonian formulation This option modifies the state obtained with the "XLT" option to account for the observer's velocity relative to the solar system barycenter. The position component of the computed target state indicates the direction that photons emitted from the observer's location must be "aimed" to hit the target. "XCN" "Transmission" case: converged Newtonian light time correction. "XCN+S" "Transmission" case: converged Newtonian light time correction and stellar aberration correction. Neither special nor general relativistic effects are accounted for in the aberration corrections applied by this routine. Case and blanks are not significant in the string `abcorr'. obs is the name of an observing body. Optionally, you may supply the ID code of the object as an integer string. For example, both "EARTH" and "399" are legitimate strings to supply to indicate the observer is Earth. ## Detailed_Outputstarg is a Cartesian state vector representing the position and velocity of the target body relative to the specified observer. `starg' is corrected for the specified aberrations, and is expressed with respect to the reference frame specified by `ref'. The first three components of `starg' represent the x-, y- and z-components of the target's position; the last three components form the corresponding velocity vector. The position component of `starg' points from the observer's location at `et' to the aberration-corrected location of the target. Note that the sense of the position vector is independent of the direction of radiation travel implied by the aberration correction. The velocity component of `starg' is the derivative with respect to time of the position component of `starg'. Units are always km and km/sec. Non-inertial frames are treated as follows: letting `ltcent' be the one-way light time between the observer and the central body associated with the frame, the orientation of the frame is evaluated at et-ltcent, et+ltcent, or `et' depending on whether the requested aberration correction is, respectively, for received radiation, transmitted radiation, or is omitted. `ltcent' is computed using the method indicated by `abcorr'. lt is the one-way light time between the observer and target in seconds. If the target state is corrected for aberrations, then `lt' is the one-way light time between the observer and the light time corrected target location. ## ParametersNone. ## Exceptions1) If name of target or observer cannot be translated to its NAIF ID code, the error SPICE(IDCODENOTFOUND) is signaled by a routine in the call tree of this routine. 2) If the reference frame `ref' is not a recognized reference frame, the error SPICE(UNKNOWNFRAME) is signaled by a routine in the call tree of this routine. 3) If the loaded kernels provide insufficient data to compute the requested state vector, an error is signaled by a routine in the call tree of this routine. 4) If an error occurs while reading an SPK or other kernel file, the error is signaled by a routine in the call tree of this routine. 5) If any of the `targ', `ref', `abcorr' or `obs' input string pointers is null, the error SPICE(NULLPOINTER) is signaled. 6) If any of the `targ', `ref', `abcorr' or `obs' input strings has zero length, the error SPICE(EMPTYSTRING) is signaled. ## FilesThis routine computes states using SPK files that have been loaded into the SPICE system, normally via the kernel loading interface routine furnsh_c. See the routine furnsh_c and the SPK and KERNEL Required Reading for further information on loading (and unloading) kernels. If the output state `starg' is to be expressed relative to a non-inertial frame, or if any of the ephemeris data used to compute `starg' are expressed relative to a non-inertial frame in the SPK files providing those data, additional kernels may be needed to enable the reference frame transformations required to compute the state. These additional kernels may be C-kernels, PCK files or frame kernels. Any such kernels must already be loaded at the time this routine is called. ## ParticularsThis routine is part of the user interface to the SPICE ephemeris system. It allows you to retrieve state information for any ephemeris object relative to any other in a reference frame that is convenient for further computations. This routine is identical in function to the routine spkez_c except that it allows you to refer to ephemeris objects by name (via a character string). Please refer to the Aberration Corrections Required Reading abcorr.req for detailed information describing the nature and calculation of the applied corrections. ## 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) Load a planetary ephemeris SPK, then look up a series of geometric states of the Moon relative to the Earth, referenced to the J2000 frame. Use the SPK kernel below to load the required Earth and Moon ephemeris data. de421.bsp Example code begins here. /. Program spkezr_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main() { #define ABCORR "NONE" #define FRAME "J2000" /. The name of the SPK file shown here is fictitious; you must supply the name of an SPK file available on your own computer system. ./ #define SPK "de421.bsp" /. ET0 represents the date 2000 Jan 1 12:00:00 TDB. ./ #define ET0 0.0 /. Use a time step of 1 hour; look up 4 states. ./ #define STEP 3600.0 #define MAXITR 4 #define OBSERVER "earth" #define TARGET "moon" /. Local variables ./ SpiceInt i; SpiceDouble et; SpiceDouble lt; SpiceDouble state [6]; /. Load the spk file. ./ furnsh_c ( SPK ); /. Step through a series of epochs, looking up a state vector at each one. ./ for ( i = 0; i < MAXITR; i++ ) { et = ET0 + i*STEP; ## RestrictionsNone. ## Literature_ReferencesNone. ## Author_and_InstitutionC.H. Acton (JPL) N.J. Bachman (JPL) J. Diaz del Rio (ODC Space) B.V. Semenov (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 3.0.2, 01-NOV-2021 (JDR) Edited the header to comply with NAIF standard. Added reference to the required SPK for the example code. Reduced the number of states to be computed. Moved SPK required reading from -Literature_References to -Required_Reading section. Added entries #5 and #6 to -Exceptions section. Updated -Particulars to refer to Aberration Corrections Required Reading document, which was added to -Required_Reading list. -CSPICE Version 3.0.1, 07-JUL-2014 (NJB) Discussion of light time corrections was updated. Assertions that converged light time corrections are unlikely to be useful were removed. -CSPICE Version 3.0.0, 27-DEC-2007 (NJB) This routine was upgraded to more accurately compute aberration-corrected velocity, and in particular, make it more consistent with observer-target positions. When light time corrections are used, the derivative of light time with respect to time is now accounted for in the computation of observer-target velocities. When the reference frame associated with the output state is time-dependent, the derivative of light time with respect to time is now accounted for in the computation of the rate of change of orientation of the reference frame. When stellar aberration corrections are used, velocities now reflect the rate of range of the stellar aberration correction. -CSPICE Version 2.0.2, 13-OCT-2003 (EDW) Added mention that 'lt' returns a value in seconds. -CSPICE Version 2.0.1, 29-JUL-2003 (NJB) (CHA) Various minor header changes were made to improve clarity. -CSPICE Version 2.0.0, 31-DEC-2001 (NJB) Updated to handle aberration corrections for transmission of radiation. Formerly, only the reception case was supported. The header was revised and expanded to explain the functionality of this routine in more detail. -CSPICE Version 1.2.0, 29-MAY-1999 (NJB) (BVS) Comment correction: the name spkez_c was changed to ## Index_Entriesusing body names get target state relative to an observer get state relative to observer corrected for aberrations read ephemeris data read trajectory data |

Fri Dec 31 18:41:12 2021