| spkpos_c |
|
Table of contents
Procedure
spkpos_c ( S/P Kernel, position )
void 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_ReadingABCORR SPK 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_Input
targ 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_Output
ptarg 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.
`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.
Units are always km.
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. Exceptions
1) 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 position 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 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_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. Examples
The 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 positions 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 spkpos_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
#define ABCORR "NONE"
#define FRAME "J2000"
#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 positions.
./
#define STEP 3600.0
#define MAXITR 4
#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;
spkpos_c ( TARGET, et, FRAME, ABCORR,
OBSERVER, pos, < );
printf( "\net = %20.10f\n\n", et );
printf( "J2000 x-position (km): %20.10f\n", pos[0] );
printf( "J2000 y-position (km): %20.10f\n", pos[1] );
printf( "J2000 z-position (km): %20.10f\n", pos[2] );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
et = 0.0000000000
J2000 x-position (km): -291608.3853096409
J2000 y-position (km): -266716.8329467875
J2000 z-position (km): -76102.4871467836
et = 3600.0000000000
J2000 x-position (km): -289279.8983133120
J2000 y-position (km): -269104.1084289378
J2000 z-position (km): -77184.2420729120
et = 7200.0000000000
J2000 x-position (km): -286928.0014055001
J2000 y-position (km): -271469.9902460162
J2000 z-position (km): -78259.9083077002
et = 10800.0000000000
J2000 x-position (km): -284552.9026554719
J2000 y-position (km): -273814.3097527430
J2000 z-position (km): -79329.4060465982
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionC.H. Acton (JPL) N.J. Bachman (JPL) J. Diaz del Rio (ODC Space) W.L. Taber (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 positions 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 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 |
Fri Dec 31 18:41:12 2021