| spkezr_c |
|
Table of contents
Procedure
spkezr_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_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 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_Output
starg 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. 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 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. 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 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;
spkezr_c ( TARGET, et, FRAME, ABCORR,
OBSERVER, state, < );
printf( "\net = %20.10f\n\n", et );
printf( "J2000 x-position (km): %20.10f\n", state[0] );
printf( "J2000 y-position (km): %20.10f\n", state[1] );
printf( "J2000 z-position (km): %20.10f\n", state[2] );
printf( "J2000 x-velocity (km/s): %20.10f\n", state[3] );
printf( "J2000 y-velocity (km/s): %20.10f\n", state[4] );
printf( "J2000 z-velocity (km/s): %20.10f\n", state[5] );
}
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
J2000 x-velocity (km/s): 0.6435313868
J2000 y-velocity (km/s): -0.6660876862
J2000 z-velocity (km/s): -0.3013257043
et = 3600.0000000000
J2000 x-position (km): -289279.8983133120
J2000 y-position (km): -269104.1084289378
J2000 z-position (km): -77184.2420729120
J2000 x-velocity (km/s): 0.6500629244
J2000 y-velocity (km/s): -0.6601685834
J2000 z-velocity (km/s): -0.2996455351
et = 7200.0000000000
J2000 x-position (km): -286928.0014055001
J2000 y-position (km): -271469.9902460162
J2000 z-position (km): -78259.9083077002
J2000 x-velocity (km/s): 0.6565368360
J2000 y-velocity (km/s): -0.6542023962
J2000 z-velocity (km/s): -0.2979431229
et = 10800.0000000000
J2000 x-position (km): -284552.9026554719
J2000 y-position (km): -273814.3097527430
J2000 z-position (km): -79329.4060465982
J2000 x-velocity (km/s): 0.6629527800
J2000 y-velocity (km/s): -0.6481896017
J2000 z-velocity (km/s): -0.2962186180
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 spkezr_c.
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