Table of contents
CSPICE_SPKGEO computes the geometric state (position and velocity) of a
target body relative to an observing body.
Given:
targ the standard NAIF ID code for a target body.
[1,1] = size(targ); int32 = class(targ)
et the epoch (ephemeris time) at which the state of the target
body is to be computed.
[1,1] = size(et); double = class(et)
ref the name of the reference frame to which the state vector
returned by the routine should be rotated.
[1,c1] = size(ref); char = class(ref)
or
[1,1] = size(ref); cell = class(ref)
This may be any frame supported by the SPICELIB function
FRMCHG. See also the Frames Required Reading for a list of
supported frames.
obs the standard NAIF ID code for an observing body.
[1,1] = size(obs); int32 = class(obs)
the call:
[state, lt] = cspice_spkgeo( targ, et, ref, obs )
returns:
state contains the geometric position and velocity of the target
body, relative to the observing body, at epoch `et'.
[6,1] = size(state); double = class(state)
`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 the one-way light time from the observing body to the
geometric position of the target body in seconds at the
specified epoch.
[1,1] = size(lt); double = class(lt)
None.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
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.
function spkgeo_ex1()
%
% Load a set of kernels. Use a meta
% kernel for convenience.
%
cspice_furnsh( '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.
%
[et] = cspice_str2et( epoch );
%
% Look-up the state for the defined parameters.
%
[state, lt] = cspice_spkgeo( target, et, reffrm, obsrvr );
%
% Output...
%
fprintf( 'The position of : %2d\n', target )
fprintf( 'As observed from : %2d\n', obsrvr )
fprintf( 'In reference frame : %s\n', reffrm )
fprintf( 'At epoch : %s\n', epoch )
fprintf( ' \n' )
%
% The first three entries of state contain the
% X, Y, Z position components. The final three contain
% the Vx, Vy, Vz velocity components.
%
fprintf( 'R (km): %17.5f %17.5f %17.5f\n', ...
state(1), state(2), state(3) )
fprintf( 'V (km/s): %17.5f %17.5f %17.5f\n', ...
state(4), state(5), state(6) )
fprintf( ' \n' )
fprintf( [ 'Light time (s) between observer and target: ', ...
' %18.13f\n' ], lt )
%
% It's always good form to unload kernels after use,
% particularly in Matlab due to data persistence.
%
cspice_kclear
When this program was executed on a Mac/Intel/Octave5.x/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
cspice_spkgeo 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. cspice_spkgeo reads as many segments
as necessary, from as many files as necessary, using files
that have been loaded by previous calls to cspice_furnsh or
cspice_spklef (load ephemeris file).
cspice_spkgeo is similar to cspice_spkez but returns geometric states
only, with no option to make planetary (light-time) nor
stellar aberration corrections. The geometric states
returned by cspice_spkez and cspice_spkgeo are the same.
1) 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 any of the input arguments, `targ', `et', `ref' or `obs',
is undefined, an error is signaled by the Matlab error
handling system.
3) If any of the input arguments, `targ', `et', `ref' or `obs',
is not of the expected type, or it does not have the expected
dimensions and size, an error is signaled by the Mice
interface.
See -Restrictions.
1) The ephemeris files to be used by cspice_spkgeo must be loaded
by cspice_furnsh or cspice_spklef before cspice_spkgeo is called.
MICE.REQ
SPK.REQ
None.
J. Diaz del Rio (ODC Space)
-Mice Version 1.0.0, 09-AUG-2021 (JDR)
geometric state of one body relative to another
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