spkpos_c |

## Procedurevoid spkpos_c ( ConstSpiceChar * targ, SpiceDouble et, ConstSpiceChar * ref, ConstSpiceChar * abcorr, ConstSpiceChar * obs, SpiceDouble ptarg[3], SpiceDouble * lt ) ## AbstractReturn the position of a target body relative to an observing body, optionally corrected for light time (planetary aberration) and stellar aberration. ## Required_ReadingSPK NAIF_IDS FRAMES TIME ## KeywordsEPHEMERIS ## Brief_I/OVariable I/O Description -------- --- -------------------------------------------------- targ I Target body name. et I Observer epoch. ref I Reference frame of output position vector. abcorr I Aberration correction flag. obs I Observing body name. ptarg O Position 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 position vector which points from the observer to the target. et is the ephemeris time, expressed as seconds past J2000 TDB, at which the position 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 position 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 position vector `ptarg' for details. abcorr indicates the aberration corrections to be applied to the position 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 position 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 position 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 position obtained with the "LT" option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent position of the target---the position 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 position 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 position obtained with the "XLT" option to account for the observer's velocity relative to the solar system barycenter. The computed target position 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_Outputptarg is a Cartesian 3-vector representing the position of the target body relative to the specified observer. `ptarg' is corrected for the specified aberrations, and is expressed with respect to the reference frame specified by `ref'. The three components of `ptarg' represent the x-, y- and z-components of the target's position. Units are always km. `ptarg' points from the observer's location at `et' to the aberration-corrected location of the target. Note that the sense of this position vector is independent of the direction of radiation travel implied by the aberration correction. 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 position 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. 2) If the reference frame `ref' is not a recognized reference frame the error SPICE(UNKNOWNFRAME) is signaled. 3) If the loaded kernels provide insufficient data to compute the requested position vector, the deficiency will be diagnosed 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 will be diagnosed by a routine in the call tree of this routine. ## FilesThis routine computes positions 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 position `ptarg' is to be expressed relative to a non-inertial frame, or if any of the ephemeris data used to compute `ptarg' 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 position. 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 position 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 SPKEZP except that it allows you to refer to ephemeris objects by name (via a character string). Aberration corrections ====================== In space science or engineering applications one frequently wishes to know where to point a remote sensing instrument, such as an optical camera or radio antenna, in order to observe or otherwise receive radiation from a target. This pointing problem is complicated by the finite speed of light: one needs to point to where the target appears to be as opposed to where it actually is at the epoch of observation. We use the adjectives "geometric," "uncorrected," or "true" to refer to an actual position or state of a target at a specified epoch. When a geometric position or state vector is modified to reflect how it appears to an observer, we describe that vector by any of the terms "apparent," "corrected," "aberration corrected," or "light time and stellar aberration corrected." The SPICE Toolkit can correct for two phenomena affecting the apparent location of an object: one-way light time (also called "planetary aberration") and stellar aberration. One-way light time ------------------ Correcting for one-way light time is done by computing, given an observer and observation epoch, where a target was when the observed photons departed the target's location. The vector from the observer to this computed target location is called a "light time corrected" vector. The light time correction depends on the motion of the target relative to the solar system barycenter, but it is independent of the velocity of the observer relative to the solar system barycenter. Relativistic effects such as light bending and gravitational delay are not accounted for in the light time correction performed by this routine. Stellar aberration ------------------ The velocity of the observer also affects the apparent location of a target: photons arriving at the observer are subject to a "raindrop effect" whereby their velocity relative to the observer is, using a Newtonian approximation, the photons' velocity relative to the solar system barycenter minus the velocity of the observer relative to the solar system barycenter. This effect is called "stellar aberration." Stellar aberration is independent of the velocity of the target. The stellar aberration formula used by this routine does not include (the much smaller) relativistic effects. Stellar aberration corrections are applied after light time corrections: the light time corrected target position vector is used as an input to the stellar aberration correction. When light time and stellar aberration corrections are both applied to a geometric position vector, the resulting position vector indicates where the target "appears to be" from the observer's location. As opposed to computing the apparent position of a target, one may wish to compute the pointing direction required for transmission of photons to the target. This also requires correction of the geometric target position for the effects of light time and stellar aberration, but in this case the corrections are computed for radiation traveling *from* the observer to the target. We will refer to this situation as the "transmission" case. The "transmission" light time correction yields the target's location as it will be when photons emitted from the observer's location at `et' arrive at the target. The transmission stellar aberration correction is the inverse of the traditional stellar aberration correction: it indicates the direction in which radiation should be emitted so that, using a Newtonian approximation, the sum of the velocity of the radiation relative to the observer and of the observer's velocity, relative to the solar system barycenter, yields a velocity vector that points in the direction of the light time corrected position of the target. One may object to using the term "observer" in the transmission case, in which radiation is emitted from the observer's location. The terminology was retained for consistency with earlier documentation. Below, we indicate the aberration corrections to use for some common applications: 1) Find the apparent direction of a target. This is the most common case for a remote-sensing observation. Use "LT+S" or "CN+S": apply both light time and stellar aberration corrections. Note that using light time corrections alone ("LT") is generally not a good way to obtain an approximation to an apparent target vector: since light time and stellar aberration corrections often partially cancel each other, it may be more accurate to use no correction at all than to use light time alone. 2) Find the corrected pointing direction to radiate a signal to a target. This computation is often applicable for implementing communications sessions. Use "XLT+S" or "XCN+S": apply both light time and stellar aberration corrections for transmission. 3) Compute the apparent position of a target body relative to a star or other distant object. Use one of "LT", "CN", "LT+S", or "CN+S" as needed to match the correction applied to the position of the distant object. For example, if a star position is obtained from a catalog, the position vector may not be corrected for stellar aberration. In this case, to find the angular separation of the star and the limb of a planet, the vector from the observer to the planet should be corrected for light time but not stellar aberration. 4) Obtain an uncorrected position vector derived directly from data in an SPK file. Use "NONE". 5) Use a geometric position vector as a low-accuracy estimate of the apparent position for an application where execution speed is critical: Use "NONE". 6) While this routine cannot perform the relativistic aberration corrections required to compute positions with the highest possible accuracy, it can supply the geometric positions required as inputs to these computations: Use "NONE", then apply relativistic aberration corrections (not available in the SPICE Toolkit). Below, we discuss in more detail how the aberration corrections applied by this routine are computed. Geometric case ============== ## Examples1) Load a planetary ephemeris SPK, then look up a series of geometric positions of the moon relative to the earth, referenced to the J2000 frame. #include <stdio.h> #include "SpiceUsr.h" void 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 "planetary_spk.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 100 states. ./ #define STEP 3600.0 #define MAXITR 100 #define OBSERVER "earth" #define TARGET "moon" /. Local variables ./ SpiceInt i; SpiceDouble et; SpiceDouble lt; SpiceDouble pos [3]; /. Load the spk file. ./ furnsh_c ( SPK ); /. Step through a series of epochs, looking up a position vector at each one. ./ for ( i = 0; i < MAXITR; i++ ) { et = ET0 + i*STEP; ## RestrictionsNone. ## Literature_ReferencesSPK Required Reading. ## Author_and_InstitutionC.H. Acton (JPL) B.V. Semenov (JPL) N.J. Bachman (JPL) W.L. Taber (JPL) ## Version-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 2.0.4, 04-APR-2008 (NJB) Corrected minor error in description of XLT+S aberration correction. -CSPICE Version 2.0.3, 17-APR-2005 (NJB) Error was corrected in example program: variable name `state' was changed to `pos' in printf calls. -CSPICE Version 2.0.2, 13-OCT-2003 (EDW) Various minor header changes were made to improve clarity. Added mention that 'lt' returns a value in seconds. -CSPICE Version 2.0.1, 27-JUL-2003 (NJB) (CHA) Various header corrections were made. -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 1.0.0, 29-MAY-1999 (NJB) (WLT) ## Index_Entriesusing names get target position relative to an observer position relative to observer corrected for aberrations read ephemeris data read trajectory data |

Wed Apr 5 17:54:44 2017