| spkcvo_c |
|
Table of contents
Procedure
spkcvo_c ( SPK, constant velocity observer state )
void spkcvo_c ( ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * outref,
ConstSpiceChar * refloc,
ConstSpiceChar * abcorr,
ConstSpiceDouble obssta [6],
SpiceDouble obsepc,
ConstSpiceChar * obsctr,
ConstSpiceChar * obsref,
SpiceDouble state [6],
SpiceDouble * lt )
AbstractReturn the state of a specified target relative to an "observer," where the observer has constant velocity in a specified reference frame. The observer's state is provided by the calling program rather than by loaded SPK files. Required_ReadingFRAMES PCK SPK TIME KeywordsEPHEMERIS Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
target I Name of target ephemeris object.
et I Observation epoch.
outref I Reference frame of output state.
refloc I Output reference frame evaluation locus.
abcorr I Aberration correction.
obssta I Observer state relative to center of motion.
obsepc I Epoch of observer state.
obsctr I Center of motion of observer.
obsref I Frame of observer state.
state O State of target with respect to observer.
lt O One way light time between target and
observer.
Detailed_Input
target is the name of a target 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 target
is Earth.
Case and leading and trailing blanks are not
significant in the string `target'.
et is the ephemeris time at which the state of the
target relative to the observer is to be computed. `et'
is expressed as seconds past J2000 TDB. `et' refers to
time at the observer's location.
`et' is independent of the observer epoch `obsepc'.
outref is the name of the reference frame with respect to
which the output state is expressed.
When `outref' is time-dependent (non-inertial), its
orientation relative to the J2000 frame is evaluated
in the manner commanded by the input argument `refloc'
(see description below).
Case and leading and trailing blanks are not
significant in the string `outref'.
refloc is a string indicating the output reference frame
evaluation locus: this is the location associated
with the epoch at which this routine is to evaluate
the orientation, relative to the J2000 frame, of the
output frame `outref'. The values and meanings of
`refloc' are:
"OBSERVER" Evaluate `outref' at the observer's
epoch `et'.
Normally the locus "OBSERVER" should
be selected when `outref' is centered
at the observer.
"TARGET" Evaluate `outref' at the target epoch;
letting `lt' be the one-way light time
between the target and observer, the
target epoch is
et-lt if reception aberration
corrections are used
et+lt if transmission aberration
corrections are used
et if no aberration corrections
are used
Normally the locus "TARGET" should
be selected when `outref' is centered
at the target object.
"CENTER" Evaluate the frame `outref' at the epoch
associated its center. This epoch,
which we'll call `etctr', is determined
as follows:
Let `ltctr' be the one-way light time
between the observer and the center
of `outref'. Then `etctr' is
et-ltctr if reception
aberration corrections
are used
et+ltctr if transmission
aberration corrections
are used
et if no aberration
corrections are used
The locus "CENTER" should be selected
when the user intends to obtain
results compatible with those produced
by spkezr_c.
When `outref' is inertial, all choices of `refloc'
yield the same results.
Case and leading and trailing blanks are not
significant in the string `refloc'.
abcorr indicates the aberration corrections to be applied to
the observer-target state to account for one-way
light time and stellar aberration.
`abcorr' may be any of the following:
"NONE" Apply no correction. Return the
geometric state of the target
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. 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.
"CN+S" Converged Newtonian light time
and stellar aberration corrections.
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 and stellar
aberration corrections.
Neither special nor general relativistic effects are
accounted for in the aberration corrections applied
by this routine.
Case and leading and trailing blanks are not
significant in the string `abcorr'.
obssta is the geometric state of an observer moving at
constant velocity relative to its center of motion
`obsctr', expressed in the reference frame `obsref', at
the epoch `obsepc'.
`obssta' is a six-dimensional vector representing
position and velocity in cartesian coordinates: the
first three components represent the position of an
observer relative to its center of motion; the last
three components represent the velocity of the
observer.
Units are always km and km/sec.
obsepc is the epoch, expressed as seconds past J2000 TDB, at
which the observer state `obssta' is applicable. For
other epochs, the position of the observer relative
to its center of motion is linearly extrapolated
using the velocity component of `obssta'.
`obsepc' is independent of the epoch `et' at which the
state of the target relative to the observer is to be
computed.
obsctr is the name of the center of motion of `obssta'. The
ephemeris of `obsctr' is provided by loaded SPK files.
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 center of motion.
Case and leading and trailing blanks are not
significant in the string `obsctr'.
obsref is the name of the reference frame relative to which
the input state `obssta' is expressed. The observer has
constant velocity relative to its center of motion
in this reference frame.
Case and leading and trailing blanks are not
significant in the string `obsref'.
Detailed_Output
state is a Cartesian state vector representing the position
and velocity of the target relative to the specified
observer. `state' is corrected for the specified
aberrations and is expressed with respect to the
reference frame specified by `outref'. The first three
components of `state' 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 `state' 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 `state' is the derivative
with respect to time of the position component of
`state'.
Units are always km and km/sec.
When `state' is expressed in a time-dependent
(non-inertial) output frame, the orientation of that
frame relative to the J2000 frame is evaluated in the
manner indicated by the input argument `refloc' (see
description above).
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 either the name of the center of motion or the target
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 `outref' is unrecognized, the error
SPICE(UNKNOWNFRAME) is signaled by a routine in the call tree
of this routine.
3) If the reference frame `obsref' is unrecognized, an error is
signaled by a routine in the call tree of this routine.
4) If the frame evaluation locus `refloc' is not recognized, the
error SPICE(NOTSUPPORTED) is signaled by a routine in the call
tree of this routine.
5) 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.
6) 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.
7) If the aberration correction `abcorr' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
8) If any of the `target', `outref', `refloc', `abcorr',
`obsctr', `obsref' or `obssta' input string pointers is null,
the error SPICE(NULLPOINTER) is signaled.
9) If any of the `target', `outref', `refloc', `abcorr', `obsctr'
or `obsref' input strings has zero length, the error
SPICE(EMPTYSTRING) is signaled.
Files
Appropriate kernels must be loaded by the calling program before
this routine is called.
The following data are required:
- SPK data: ephemeris data for the observer center and target
must be loaded. If aberration corrections are used, the
states of the observer center and target relative to the solar
system barycenter must be calculable from the available
ephemeris data. Typically ephemeris data are made available
by loading one or more SPK files using furnsh_c.
The following data may be required:
- PCK data: if the target frame is a PCK frame, rotation data
for the target frame must be loaded. These may be provided
in a text or binary PCK file.
- Frame data: if a frame definition not built into SPICE is
required, for example to convert the observer-target state
to the output frame, that definition must be available in
the kernel pool. Typically frame definitions are supplied
by loading a frame kernel using furnsh_c.
- Additional kernels: if any frame used in this routine's
state computation is a CK frame, then at least one CK and
corresponding SCLK kernel is required. If dynamic frames
are used, additional SPK, PCK, CK, or SCLK kernels may be
required.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
Particulars
This routine computes observer-target states for observers whose
trajectories are not provided by SPK files.
Observers supported by this routine must have constant velocity
with respect to a specified center of motion, expressed in a
caller-specified reference frame. The state of the center of
motion relative to the target must be computable using
loaded SPK data.
For applications in which the observer has zero velocity
relative to its center of motion, the CSPICE routine
spkcpo_c { SPK, constant position observer }
can be used. spkcpo_c has a simpler interface than that of spkcvo_c.
This routine is suitable for computing states of target ephemeris
objects, as seen from landmarks on the surface of an extended
object, in cases where no SPK data are available for those
landmarks.
This routine's treatment of the output reference frame differs
from that of the principal SPK API routines
spkezr_c
spkez_c
spkpos_c
spkezp_c
which require both observer and target ephemerides to be provided
by loaded SPK files:
The SPK API routines listed above evaluate the orientation of
the output reference frame (with respect to the J2000 frame)
at an epoch corrected for one-way light time between the
observer and the center of the output frame. When the center
of the output frame is not the target (for example, when the
target is on the surface of Mars and the output frame is
centered at Mars' center), the epoch of evaluation may not
closely match the light-time corrected epoch associated with
the target itself. A similar problem may occur when the
observer is a surface point on an extended body and the
output frame is centered at the body center: the listed
routines will correct the orientation of the output frame for
one-way light time between the frame center and the observer.
This routine allows the caller to dictate how the orientation
of the output reference frame is to be evaluated. The caller
passes to this routine an input string called the output
frame's evaluation "locus." This string specifies the location
associated with the output frame's evaluation epoch. The three
possible values of the locus are
"TARGET"
"OBSERVER"
"CENTER"
The choice of locus has an effect when aberration corrections
are used and the output frame is non-inertial.
When the locus is "TARGET" and light time corrections are
used, the orientation of the output frame is evaluated at the
epoch obtained by correcting the observation epoch `et' for
one-way light time `lt'. The evaluation epoch will be either
et-lt or et+lt for reception or transmission corrections
respectively.
For remote sensing applications where the target is a surface
point on an extended object, and the orientation of that
object should be evaluated at the emission time, the locus
"TARGET" should be used.
When the output frame's orientation should be evaluated at
the observation epoch `et', which is the case when the
output frame is centered at the observer, the locus
"OBSERVER" should be used.
The locus option "CENTER" is provided for compatibility
with existing SPK state computation APIs such as spkezr_c.
Note that the output frame evaluation locus does not affect
the computation of light time between the target and
observer.
The SPK routines that compute observer-target states for
combinations of objects having ephemerides provided by the SPK
system and objects having constant position or constant velocity
are
spkcpo_c {SPK, Constant position observer}
spkcpt_c {SPK, Constant position target}
spkcvo_c {SPK, Constant velocity observer}
spkcvt_c {SPK, Constant velocity target}
Examples
The numerical results shown for these examples 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) Compute apparent solar azimuth and elevation as seen from a
specified surface point on the earth.
Task Description
================
In this example we'll use the location of the DSN station
DSS-14 as our surface point.
We'll perform the solar azimuth and elevation computation two
ways:
- Using a station frame kernel to provide the
specification of a topocentric reference frame
centered at DSS-14.
- Computing inline the transformation from the earth-fixed,
earth-centered frame ITRF93 to a topocentric frame
centered at DSS-14.
Note that results of the two computations will differ
slightly. This is due to differences in the orientations
of the topocentric frames. There are two sources of the
differences:
1) The station position is time-dependent due to tectonic
plate motion, and epochs of the station positions used
to specify the axes of the topocentric frame are
different in the two cases. This gives rise to different
orientations of the frame's axes relative to the frame
ITRF93.
2) The two computations use different earth radii; this
results in computation of different geodetic latitudes
of the station. This difference also affects the
topocentric frame orientation relative to ITRF93.
Kernels
=======
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkcvo_ex1.tm
This is the meta-kernel file for the header code example for
the subroutine spkcvo_c. These kernel files can be found on
the NAIF website.
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
--------- --------
de421.bsp Planetary ephemeris
pck00010.tpc Planet orientation and
radii
naif0010.tls Leapseconds
earth_720101_070426.bpc Earth historical
binary PCK
earthstns_itrf93_050714.bsp DSN station SPK
earth_topo_050714.tf DSN station FK
mgs_moc_v20.ti MGS MOC instrument
parameters
mgs_sclkscet_00061.tsc MGS SCLK coefficients
mgs_sc_ext12.bc MGS s/c bus attitude
mgs_ext12_ipng_mgs95j.bsp MGS ephemeris
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00010.tpc',
'naif0010.tls',
'earth_720101_070426.bpc',
'earthstns_itrf93_050714.bsp',
'earth_topo_050714.tf',
'mgs_moc_v20.ti',
'mgs_sclkscet_00061.tsc',
'mgs_sc_ext12.bc',
'mgs_ext12_ipng_mgs95j.bsp' )
\begintext
End of meta-kernel.
Example code begins here.
/.
Program spkcvo_ex1
This program uses spkcvo_c to compute solar azimuth
and elevation at a given surface point on the earth:
the DSN station DSS-14.
./
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "spkcvo_ex1.tm"
#define FRNMLN 33
#define SHAPLN 33
#define TIMFMT "YYYY MON DD HR:MN:SC.###### UTC"
#define TIMFM2 "YYYY MON DD HR:MN:SC.###### TDB ::TDB"
#define TIMLEN 41
/.
Local variables
./
SpiceChar * abcorr;
SpiceChar emitim [ TIMLEN ];
SpiceChar epcstr [ TIMLEN ];
SpiceChar * refloc;
SpiceChar * obsctr;
SpiceChar * obsref;
SpiceChar * obstim;
SpiceChar * outref;
SpiceChar * target;
SpiceDouble az;
SpiceDouble el;
SpiceDouble et;
SpiceDouble f;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble lt0;
SpiceDouble lt1;
SpiceDouble normal [ 3 ] ;
SpiceDouble obsalt;
SpiceDouble obslat;
SpiceDouble obslon;
SpiceDouble obsepc;
SpiceDouble obssta [ 6 ];
SpiceDouble r;
SpiceDouble radii [ 3 ];
SpiceDouble re;
SpiceDouble rp;
SpiceDouble state0 [ 6 ];
SpiceDouble state1 [ 6 ];
SpiceDouble topvec [ 3 ];
SpiceDouble xform [ 3 ][ 3 ];
SpiceDouble z [ 3 ] = { 0.0, 0.0, 1.0 };
SpiceInt n;
/.
Load SPICE kernels.
./
furnsh_c ( META );
/.
Convert the observation time to seconds past J2000 TDB.
./
obstim = "2003 OCT 13 06:00:00.000000 UTC";
str2et_c ( obstim, &et );
/.
Set the target, observer center, and observer frame.
./
target = "SUN";
obsctr = "EARTH";
obsref = "ITRF93";
/.
Set the state of DSS-14 relative to the earth's
center at the J2000 epoch, expressed in the
ITRF93 reference frame. Values come from the
earth station SPK specified in the meta-kernel.
The velocity is non-zero due to tectonic
plate motion.
./
obsepc = 0.0;
obssta[0] = -2353.6213656676991;
obssta[1] = -4641.3414911499403;
obssta[2] = 3677.0523293197439;
obssta[3] = -0.00000000000057086;
obssta[4] = 0.00000000000020549;
obssta[5] = -0.00000000000012171;
/.
Find the apparent state of the sun relative
to the station in the DSS-14_TOPO reference frame.
Evaluate the output frame's orientation, that is the
orientation of the DSS-14_TOPO frame relative to the
J2000 frame, at the observation epoch. This
correction is obtained by setting `refloc' to
"OBSERVER".
./
outref = "DSS-14_TOPO";
abcorr = "CN+S";
refloc = "OBSERVER";
/.
Compute the observer-target state.
./
spkcvo_c ( target, et, outref, refloc,
abcorr, obssta, obsepc, obsctr,
obsref, state0, <0 );
/.
Compute planetocentric coordinates of the
observer-target position in the local
topocentric reference frame DSS-14_TOPO.
./
reclat_c ( state0, &r, &lon, &lat );
/.
Compute solar azimuth. The latitude we've
already computed is the elevation. Express
both angles in degrees.
./
el = lat * dpr_c();
az = - lon * dpr_c();
if ( az < 0.0 )
{
az += 360.0;
}
/.
Display the computed state, light time, and angles.
./
timout_c ( et-lt0, TIMFMT, TIMLEN, emitim );
timout_c ( obsepc, TIMFM2, TIMLEN, epcstr );
printf ( "\n"
" Frame evaluation locus: %s\n"
"\n"
" Target: %s\n"
" Observation time: %s\n"
" Observer center: %s\n"
" Observer-center state time: %s\n"
" Observer frame: %s\n"
" Emission time: %s\n"
" Output reference frame: %s\n"
" Aberration correction: %s\n"
"\n"
" Observer-target position (km):\n"
" %20.8f %20.8f %20.8f\n"
" Observer-target velocity (km/s):\n"
" %20.8f %20.8f %20.8f\n"
" Light time (s): %20.8f\n",
refloc, target, obstim, obsctr,
epcstr, obsref, emitim, outref,
abcorr, state0[0], state0[1], state0[2],
state0[3], state0[4], state0[5], lt0 );
printf ( "\n"
" Solar azimuth (deg): %20.8f\n"
" Solar elevation (deg): %20.8f\n",
az, el );
/.
For an arbitrary surface point, we might not
have a frame kernel available. In this case
we can look up the state in the observer frame
using spkcvo_c and then convert the state to
the local topocentric frame. We'll first
create the transformation matrix for converting
vectors in the observer frame to the topocentric
frame.
First step: find the geodetic (planetodetic)
coordinates of the observer. We need the
equatorial radius and flattening coefficient
of the reference ellipsoid.
./
bodvrd_c ( "EARTH", "RADII", 3, &n, radii );
re = radii[0];
rp = radii[2];
f = ( re - rp ) / re;
recgeo_c ( obssta, re, f, &obslon, &obslat, &obsalt );
/.
Find the outward surface normal on the reference
ellipsoid at the observer's longitude and latitude.
./
latrec_c ( 1.0, obslon, obslat, normal );
/.
The topocentric frame has its +Z axis aligned
with NORMAL and its +X axis pointed north.
The north direction is aligned with the component
of the ITRF93 +Z axis orthogonal to the topocentric
+Z axis.
./
twovec_c ( normal, 3, z, 1, xform );
outref = "ITRF93";
abcorr = "CN+S";
refloc = "OBSERVER";
/.
Compute the observer-target state.
./
spkcvo_c ( target, et, outref, refloc,
abcorr, obssta, obsepc, obsctr,
obsref, state1, <1 );
/.
Convert the position to the topocentric frame.
./
mxv_c ( xform, state1, topvec );
/.
Compute azimuth and elevation.
./
reclat_c ( topvec, &r, &lon, &lat );
el = lat * dpr_c();
az = - lon * dpr_c();
if ( az < 0.0 )
{
az += 360.0;
}
printf ( "\n\n\n"
" AZ/EL computed without frame kernel:\n\n"
" Distance between last two "
"positions (km): %20.8f\n",
vdist_c( state0, topvec ) );
printf ( "\n"
" Solar azimuth (deg): %20.8f\n"
" Solar elevation (deg): %20.8f\n"
"\n",
az, el );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Frame evaluation locus: OBSERVER
Target: SUN
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Observer center: EARTH
Observer-center state time: 2000 JAN 01 12:00:00.000000 TDB
Observer frame: ITRF93
Emission time: 2003 OCT 13 05:51:42.068322 UTC
Output reference frame: DSS-14_TOPO
Aberration correction: CN+S
Observer-target position (km):
62512272.82076502 58967494.42506485 -122059095.46751761
Observer-target velocity (km/s):
2475.97326517 -9870.26706232 -3499.90809969
Light time (s): 497.93167797
Solar azimuth (deg): 316.67141599
Solar elevation (deg): -54.85253168
AZ/EL computed without frame kernel:
Distance between last two positions (km): 3.07056969
Solar azimuth (deg): 316.67141786
Solar elevation (deg): -54.85253216
2) Demonstrate applications of the output frame evaluation locus.
The following program is not necessarily realistic: for
brevity, it combines several unrelated computations.
Task Description
================
Find the state of the Mars Global Surveyor spacecraft, as seen
from a given surface point on earth, corrected for light time
and stellar aberration, expressed in the earth fixed reference
frame ITRF93. The surface point is the position of the DSN
station DSS-14.
Contrast the states computed by setting the output frame
evaluation locus to "OBSERVER" and to "CENTER". Show that the
latter choice produces results very close to those that
can be obtained using spkezr_c.
Also compute the central meridian longitude on Mars of DSS-14.
This computation performs aberration corrections for the center
of Mars.
Note that in general, the routine subpnt_c should be used for
sub-observer point computations when high-accuracy aberration
corrections are desired.
The observation epoch is 2003 OCT 13 06:00:00 UTC.
Kernels
=======
Use the meta-kernel of example 1 above.
Example code begins here.
/.
Program spkcvo_ex2
This program demonstrates the use of spkcvo_c.
Computations are performed using all three possible
values of the output frame evaluation locus `refloc':
"OBSERVER"
"CENTER"
"TARGET"
Several unrelated computations are performed in this
program. In particular, computation of the
central meridian longitude on Mars is included simply
to demonstrate use of the "TARGET" option.
./
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "spkcvo_ex1.tm"
#define FRNMLN 33
#define SHAPLN 33
#define TIMFMT "YYYY MON DD HR:MN:SC.###### UTC"
#define TIMFM2 "YYYY MON DD HR:MN:SC.###### TDB ::TDB"
#define TIMLEN 41
/.
Local variables
./
SpiceChar * abcorr;
SpiceChar emitim [ TIMLEN ];
SpiceChar epcstr [ TIMLEN ];
SpiceChar * refloc;
SpiceChar * obsctr;
SpiceChar * obsref;
SpiceChar * obsrvr;
SpiceChar * obstim;
SpiceChar * outref;
SpiceChar * target;
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble lt0;
SpiceDouble lt1;
SpiceDouble lt2;
SpiceDouble lt3;
SpiceDouble obsepc;
SpiceDouble obssta [ 6 ];
SpiceDouble obsvec [ 3 ];
SpiceDouble r;
SpiceDouble state0 [ 6 ];
SpiceDouble state1 [ 6 ];
SpiceDouble state2 [ 6 ];
SpiceDouble state3 [ 6 ];
/.
Load SPICE kernels.
./
furnsh_c ( META );
/.
Convert the observation time to seconds past J2000 TDB.
./
obstim = "2003 OCT 13 06:00:00.000000 UTC";
str2et_c ( obstim, &et );
/.
Set the target, observer center, and observer frame.
./
target = "MGS";
obsctr = "EARTH";
obsref = "ITRF93";
/.
Set the state of DSS-14 relative to the earth's
center at the J2000 epoch, expressed in the
ITRF93 reference frame. Values come from the
earth station SPK specified in the meta-kernel.
The velocity is non-zero due to tectonic
plate motion.
./
obsepc = 0.0;
obssta[0] = -2353.6213656676991;
obssta[1] = -4641.3414911499403;
obssta[2] = 3677.0523293197439;
obssta[3] = -0.00000000000057086;
obssta[4] = 0.00000000000020549;
obssta[5] = -0.00000000000012171;
/.
Find the apparent state of the spacecraft relative
to the station in the ITRF93 reference frame.
Evaluate the earth's orientation, that is the
orientation of the ITRF93 frame relative to the
J2000 frame, at the observation epoch. This
correction is obtained by setting `refloc' to
"OBSERVER".
./
outref = "ITRF93";
abcorr = "CN+S";
refloc = "OBSERVER";
/.
Compute the observer-target state.
./
spkcvo_c ( target, et, outref, refloc,
abcorr, obssta, obsepc, obsctr,
obsref, state0, <0 );
/.
Display the computed state and light time.
./
timout_c ( et-lt0, TIMFMT, TIMLEN, emitim );
timout_c ( obsepc, TIMFM2, TIMLEN, epcstr );
printf ( "\n"
" Frame evaluation locus: %s\n"
"\n"
" Target: %s\n"
" Observation time: %s\n"
" Observer center: %s\n"
" Observer-center state time: %s\n"
" Observer frame: %s\n"
" Emission time: %s\n"
" Output reference frame: %s\n"
" Aberration correction: %s\n"
"\n"
" Observer-target position (km):\n"
" %20.8f %20.8f %20.8f\n"
" Observer-target velocity (km/s):\n"
" %20.8f %20.8f %20.8f\n"
" Light time (s): %20.8f\n",
refloc, target, obstim, obsctr,
epcstr, obsref, emitim, outref,
abcorr, state0[0], state0[1], state0[2],
state0[3], state0[4], state0[5], lt0 );
/.
Repeat the computation, this time evaluating the
earth's orientation at the epoch obtained by
subtracting from the observation time the one way
light time from the earth's center.
This is equivalent to looking up the observer-target
state using spkezr_c.
./
refloc = "CENTER";
spkcvo_c ( target, et, outref, refloc,
abcorr, obssta, obsepc, obsctr,
obsref, state1, <1 );
/.
Display the computed state and light time.
./
timout_c ( et-lt1, TIMFMT, TIMLEN, emitim );
printf ( "\n\n"
" Frame evaluation locus: %s\n"
"\n"
" Target: %s\n"
" Observation time: %s\n"
" Observer center: %s\n"
" Observer-center state time: %s\n"
" Observer frame: %s\n"
" Emission time: %s\n"
" Output reference frame: %s\n"
" Aberration correction: %s\n"
"\n"
" Observer-target position (km):\n"
" %20.8f %20.8f %20.8f\n"
" Observer-target velocity (km/s):\n"
" %20.8f %20.8f %20.8f\n"
" Light time (s): %20.8f\n",
refloc, target, obstim, obsctr,
epcstr, obsref, emitim, outref,
abcorr, state1[0], state1[1], state1[2],
state1[3], state1[4], state1[5], lt1 );
printf ( "\n"
" Distance between above positions (km): "
" %20.8f\n"
" Velocity difference magnitude (km/s): "
" %20.8f\n",
vdist_c( state0, state1 ),
vdist_c( state0+3, state1+3 ) );
/.
Check: compare the state computed directly above
to one produced by spkezr_c:
./
obsrvr = "DSS-14";
spkezr_c ( target, et, outref, abcorr,
obsrvr, state2, <2 );
printf ( "\n\n"
" State computed using spkezr_c:\n"
"\n"
" Target: %s\n"
" Observation time: %s\n"
" Output reference frame: %s\n"
" Aberration correction: %s\n"
" Observer: %s\n"
"\n"
" Observer-target position (km):\n"
" %20.8f %20.8f %20.8f\n"
" Observer-target velocity (km/s):\n"
" %20.8f %20.8f %20.8f\n"
" Light time (s): %20.8f\n",
target, obstim, outref,
abcorr, obsrvr,
state2[0], state2[1], state2[2],
state2[3], state2[4], state2[5], lt2 );
printf ( "\n"
" Distance between last two "
"positions (km): %20.8f\n"
" Velocity difference magnitude "
" (km/s): %20.8f\n",
vdist_c( state1, state2 ),
vdist_c( state1+3, state2+3 ) );
/.
Finally, compute an observer-target state in
a frame centered at the target.
This state can be used to compute the sub-observer
longitude. The reference frame is the Mars-fixed
frame IAU_MARS.
./
target = "MARS";
outref = "IAU_MARS";
refloc = "TARGET";
spkcvo_c ( target, et, outref, refloc,
abcorr, obssta, obsepc, obsctr,
obsref, state3, <3 );
/.
Central meridian longitude is the longitude of the
observer relative to the target center, so we must
negate the position portion of the state we just
computed.
./
vminus_c ( state3, obsvec );
reclat_c ( obsvec, &r, &lon, &lat );
printf ( "\n\n"
" Frame evaluation locus: %s\n"
"\n"
" Target: %s\n"
" Observation time: %s\n"
" Observer center: %s\n"
" Observer-center state time: %s\n"
" Observer frame: %s\n"
" Emission time: %s\n"
" Output reference frame: %s\n"
" Aberration correction: %s\n"
"\n"
" Observer-target position (km):\n"
" %20.8f %20.8f %20.8f\n"
" Observer-target velocity (km/s):\n"
" %20.8f %20.8f %20.8f\n"
" Light time (s): %20.8f\n",
refloc, target, obstim, obsctr,
epcstr, obsref, emitim, outref,
abcorr, state3[0], state3[1], state3[2],
state3[3], state3[4], state3[5], lt3 );
printf ( "\n"
" Central meridian\n"
" longitude (deg): %20.8f\n\n\n",
lon * dpr_c() );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Frame evaluation locus: OBSERVER
Target: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Observer center: EARTH
Observer-center state time: 2000 JAN 01 12:00:00.000000 TDB
Observer frame: ITRF93
Emission time: 2003 OCT 13 05:55:44.201144 UTC
Output reference frame: ITRF93
Aberration correction: CN+S
Observer-target position (km):
-53720675.37940782 -51381249.05338467 -18838416.34716543
Observer-target velocity (km/s):
-3751.69274754 3911.73417167 -2.17503628
Light time (s): 255.79885530
Frame evaluation locus: CENTER
Target: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Observer center: EARTH
Observer-center state time: 2000 JAN 01 12:00:00.000000 TDB
Observer frame: ITRF93
Emission time: 2003 OCT 13 05:55:44.201144 UTC
Output reference frame: ITRF93
Aberration correction: CN+S
Observer-target position (km):
-53720595.74378239 -51381332.31467460 -18838416.34737090
Observer-target velocity (km/s):
-3751.69880992 3911.72835653 -2.17503628
Light time (s): 255.79885530
Distance between above positions (km): 115.21404098
Velocity difference magnitude (km/s): 0.00840050
State computed using spkezr_c:
Target: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Output reference frame: ITRF93
Aberration correction: CN+S
Observer: DSS-14
Observer-target position (km):
-53720595.74378239 -51381332.31467460 -18838416.34737090
Observer-target velocity (km/s):
-3751.69880992 3911.72835653 -2.17503628
Light time (s): 255.79885530
Distance between last two positions (km): 0.00000000
Velocity difference magnitude (km/s): 0.00000000
Frame evaluation locus: TARGET
Target: MARS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Observer center: EARTH
Observer-center state time: 2000 JAN 01 12:00:00.000000 TDB
Observer frame: ITRF93
Emission time: 2003 OCT 13 05:55:44.201144 UTC
Output reference frame: IAU_MARS
Aberration correction: CN+S
Observer-target position (km):
-71445232.12767348 2312773.74169024 27766441.52046534
Observer-target velocity (km/s):
155.65895286 5061.78618477 5.09447029
Light time (s): 255.79702283
Central meridian
longitude (deg): -1.85409037
Restrictions
1) This routine may not be suitable for work with stars or other
objects having large distances from the observer, due to loss
of precision in position vectors.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) S.C. Krening (JPL) B.V. Semenov (JPL) Version
-CSPICE Version 1.0.2, 05-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Corrected SPICE short error message in Exception #2.
Modified code examples output format for the solutions to fit
within the -Examples section without modifications.
-CSPICE Version 1.0.1, 09-SEP-2015 (NJB)
The -Exceptions section of the header was updated
to mention exceptions involving null pointers and
empty input strings.
-CSPICE Version 1.0.0, 27-MAR-2012 (NJB) (SCK) (BVS)
Index_Entriesstate relative to constant_velocity_observer state relative to constant_velocity surface_point state relative to surface_point on extended_object state relative to landmark on extended_object |
Fri Dec 31 18:41:12 2021