Table of contents
CSPICE_SPKCPT returns the state, relative to a specified observer, of a
target having constant position in a specified reference frame. The
target's position is provided by the calling program rather than by
loaded SPK files.
Given:
trgpos the fixed (constant) geometric position of a target relative to
its "center of motion" `trgctr', expressed in the reference
frame `trgref'.
help, trgpos
DOUBLE = Array[3]
Units are always km.
trgctr the name of the center of motion of `trgpos'.
help, trgctr
STRING = Scalar
The ephemeris of `trgctr' 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 `trgctr'.
trgref the name of the reference frame relative to which the input
position `trgpos' is expressed.
help, trgref
STRING = Scalar
The target has constant position relative to its center of
motion in this reference frame.
Case and leading and trailing blanks are not
significant in the string `trgref'.
et the ephemeris time at which the state of the target relative to
the observer is to be computed.
help, et
DOUBLE = Scalar
`et' is expressed as seconds past J2000 TDB. `et' refers to
time at the observer's location.
outref the name of the reference frame with respect to which the output
state is expressed.
help, outref
STRING = Scalar
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 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'.
help, refloc
STRING = Scalar
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 `ltime' be the one-way light time
between the target and observer, the
target epoch is
et-ltime if reception aberration
corrections are used
et+ltime if transmission aberration
corrections are used
et if no aberration corrections
are used
Normally the locus 'TARGET' should
be selected when `outref' is `trgref',
the frame in which the target position
is specified.
'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 cspice_spkezr.
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.
help, abcorr
STRING = Scalar
`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-ltime 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+ltime:
'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'.
obsrvr the name of an observing body.
help, obsrvr
STRING = Scalar
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.
Case and leading and trailing blanks are not
significant in the string `obsrvr'.
the call:
cspice_spkcpt, trgpos, trgctr, trgref, et, outref, $
refloc, abcorr, obsrvr, state, ltime
returns:
state a Cartesian state vector representing the position and velocity
of the target relative to the specified observer.
help, state
DOUBLE = Array[6]
`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).
ltime the one-way light time between the observer and target in
seconds.
help, ltime
DOUBLE = Scalar
If the target state is corrected for aberrations, then `ltime'
is the one-way light time between the observer and the light
time corrected target location.
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) Demonstrate use of this routine; in particular 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 a given surface point on earth, corrected
for light time and stellar aberration, relative to the Mars
Global Surveyor spacecraft, expressed in the earth fixed
reference frame ITRF93. The selected point is the position
of the DSN station DSS-14.
Contrast the states computed by setting the output frame
evaluation locus to 'TARGET' and to 'CENTER'. Show that the
latter choice produces results very close to those that
can be obtained using cspice_spkezr.
Also compute the state of a selected Mars surface point as
seen from MGS. The point we'll use is the narrow angle MOC
boresight surface intercept corresponding to the chosen
observation time. Express the state in a spacecraft-centered
reference frame. Use the output frame evaluation locus
'OBSERVER' for this computation.
The observation epoch is 2003 OCT 13 06:00:00 UTC.
Kernels
=======
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkcpt_ex1.tm
This is the meta-kernel file for the header code example for
the subroutine cspice_spkcpt. The kernel files referenced by this
meta-kernel 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
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',
'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.
;;
;; This program demonstrates the use of cspice_spkcpt.
;; Computations are performed using all three possible
;; values of the output frame evaluation locus `refloc':
;;
;; 'TARGET'
;; 'OBSERVER'
;; 'CENTER'
;;
;; Several unrelated computations are performed in this
;; program. In particular, computations involving a surface
;; point on Mars are included simply to demonstrate use of
;; the 'OBSERVER' option.
;;
PRO spkcpt_ex1
;;
;; Local constants
;;
CAMERA = 'MGS_MOC_NA'
MAXBND = 100
META = 'spkcpt_ex1.tm'
TIMFMT = 'YYYY MON DD HR:MN:SC.###### UTC'
TIMLEN = 41
;;
;; Load SPICE kernels.
;;
cspice_furnsh, META
;;
;; Convert the observation time to seconds past J2000 TDB.
;;
obstim = '2003 OCT 13 06:00:00.000000 UTC'
cspice_str2et, obstim, et
;;
;; Set the observer, target center, and target frame.
;;
obsrvr = 'MGS'
trgctr = 'EARTH'
trgref = '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 actual station velocity is non-zero due
;; to tectonic plate motion; we ignore the motion
;; in this example. See the routine cspice_spkcvt for an
;; example in which the plate motion is accounted for.
;;
trgpos = [ -2353.6213656676991D, $
-4641.3414911499403D, $
3677.0523293197439D ]
;;
;; Find the apparent state of the station relative
;; to the spacecraft 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 epoch obtained by correcting
;; the observation time for one-way light time. This
;; correction is obtained by setting `refloc' to 'TARGET'.
;;
outref = 'ITRF93'
abcorr = 'CN+S'
refloc = 'TARGET'
;;
;; Compute the observer-target state.
;;
cspice_spkcpt, trgpos, trgctr, trgref, $
et, outref, refloc, abcorr, $
obsrvr, state0, lt0
;;
;; Display the computed state and light time.
;;
cspice_timout, et-lt0, TIMFMT, TIMLEN, emitim
print, ' Frame evaluation locus: ', refloc
print, ' '
print, ' Observer: ', obsrvr
print, ' Observation time: ', obstim
print, ' Target center: ', trgctr
print, ' Target frame: ', trgref
print, ' Emission time: ', emitim
print, ' Output reference frame: ', outref
print, ' Aberration correction: ', abcorr
print, ' '
print, ' Observer-target position (km):'
print, format = '(3F20.8)', state0[0:2]
print, ' Observer-target velocity (km/s):'
print, format = '(3F20.8)', state0[3:5]
print, format='(" Light time (s): ",F20.8)', 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 cspice_spkezr.
;;
refloc = 'CENTER'
cspice_spkcpt, trgpos, trgctr, trgref, $
et, outref, refloc, abcorr, $
obsrvr, state1, lt1
;;
;; Display the computed state and light time.
;;
cspice_timout, et-lt1, TIMFMT, TIMLEN, emitim
print, ' '
print, ' Frame evaluation locus: ', refloc
print, ' '
print, ' Observer: ', obsrvr
print, ' Observation time: ', obstim
print, ' Target center: ', trgctr
print, ' Target frame: ', trgref
print, ' Emission time: ', emitim
print, ' Output reference frame: ', outref
print, ' Aberration correction: ', abcorr
print, ' '
print, ' Observer-target position (km):'
print, format = '(3F20.8)', state1[0:2]
print, ' Observer-target velocity (km/s):'
print, format = '(3F20.8)', state1[3:5]
print, format='(" Light time (s): ",F20.8)', lt1
print, ' '
print, $
format = '(" Distance between above positions (km): ", F20.8)', $
cspice_vdist( state0[0:2], state1[0:2] )
print, $
format = '(" Velocity difference magnitude (km/s): ", F20.8)', $
cspice_vdist( state0[3:5], state1[3:5] )
;;
;; Check: compare the state computed directly above
;; to one produced by cspice_spkezr:
;;
target = 'DSS-14'
cspice_spkezr, target, et, outref, abcorr, $
obsrvr, state2, lt2
print, ' '
print, ' State computed using cspice_spkezr:'
print, ' '
print, ' Observer: ', obsrvr
print, ' Observation time: ', obstim
print, ' Target: ', target
print, ' Output reference frame: ', outref
print, ' Aberration correction: ', abcorr
print, ' '
print, ' Observer-target position (km):'
print, format = '(3F20.8)', state2[0:2]
print, ' Observer-target velocity (km/s):'
print, format = '(3F20.8)', state2[3:5]
print, format='(" Light time (s): ",F20.8)', lt2
print, ' '
print, ' Distance between last two '
print, format='(" positions (km): ", F20.8)', $
cspice_vdist( state1[0:2], state2[0:2])
print, ' Velocity difference magnitude '
print, format='(" (km/s): ", F20.8)', $
cspice_vdist( state1[3:5], state2[3:5])
;;
;; Finally, compute an observer-target state in
;; a frame centered at the observer.
;; The reference frame will be that of the
;; MGS MOC NA camera.
;;
;; In this case we'll use as the target the surface
;; intercept on Mars of the camera boresight. This
;; allows us to easily verify the correctness of
;; the results returned by cspice_spkcpt.
;;
;; Get camera frame and FOV parameters. We'll need
;; the camera ID code first.
;;
cspice_bodn2c, CAMERA, camid, found
if ( not found ) then begin
message, 'Camera name could not be mapped to an ID code.'
endif
;;
;; cspice_getfov will return the name of the camera-fixed frame
;; in the string `camref', the camera boresight vector in
;; the array `bsight', and the FOV corner vectors in the
;; array `bounds'. All we're going to use are the camera
;; frame name and camera boresight.
;;
cspice_getfov, camid, MAXBND, $
shape, camref, bsight, bounds
;;
;; Find the camera boresight surface intercept.
;;
trgctr = 'MARS'
trgref = 'IAU_MARS'
cspice_sincpt, 'Ellipsoid', trgctr, et, trgref, $
abcorr, obsrvr, camref, bsight, $
spoint, trgepc, srfvec, found
outref = camref
refloc = 'OBSERVER'
cspice_spkcpt, spoint, trgctr, trgref, $
et, outref, refloc, abcorr, $
obsrvr, state3, lt3
;;
;; Convert the emission time and the target state
;; evaluation epoch to strings for output.
;;
cspice_timout, et-lt3, TIMFMT, TIMLEN, emitim
print, ' '
print, ' Frame evaluation locus: ', refloc
print, ' '
print, ' Observer: ', obsrvr
print, ' Observation time: ', obstim
print, ' Target center: ', trgctr
print, ' Target frame: ', trgref
print, ' Emission time: ', emitim
print, ' Output reference frame: ', outref
print, ' Aberration correction: ', abcorr
print, ' '
print, ' Observer-target position (km):'
print, format = '(3F20.8)', state3[0:2]
print, ' Observer-target velocity (km/s):'
print, format = '(3F20.8)', state3[3:5]
print, format='(" Light time (s): ",F20.8)', lt3
print, $
format='(" Target range from cspice_sincpt (km): ", F20.8)', $
cspice_vnorm( srfvec )
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
Frame evaluation locus: TARGET
Observer: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Target center: EARTH
Target frame: ITRF93
Emission time: 2003 OCT 13 05:55:44.232914 UTC
Output reference frame: ITRF93
Aberration correction: CN+S
Observer-target position (km):
52746468.84243592 52367725.79653772 18836142.68957234
Observer-target velocity (km/s):
3823.39593314 -3840.60002121 2.21337692
Light time (s): 255.76708533
Frame evaluation locus: CENTER
Observer: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Target center: EARTH
Target frame: ITRF93
Emission time: 2003 OCT 13 05:55:44.232914 UTC
Output reference frame: ITRF93
Aberration correction: CN+S
Observer-target position (km):
52746419.34648802 52367775.65036674 18836142.68969753
Observer-target velocity (km/s):
3823.40103499 -3840.59789000 2.21337692
Light time (s): 255.76708533
Distance between above positions (km): 70.25135676
Velocity difference magnitude (km/s): 0.00552910
State computed using cspice_spkezr:
Observer: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Target: DSS-14
Output reference frame: ITRF93
Aberration correction: CN+S
Observer-target position (km):
52746419.34641990 52367775.65039122 18836142.68968301
Observer-target velocity (km/s):
3823.40103499 -3840.59789000 2.21337692
Light time (s): 255.76708533
Distance between last two
positions (km): 0.00007383
Velocity difference magnitude
(km/s): 0.00000000
Frame evaluation locus: OBSERVER
Observer: MGS
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Target center: MARS
Target frame: IAU_MARS
Emission time: 2003 OCT 13 05:59:59.998702 UTC
Output reference frame: MGS_MOC_NA
Aberration correction: CN+S
Observer-target position (km):
0.00000001 -0.00000001 388.97573572
Observer-target velocity (km/s):
2.91968665 0.15140014 0.92363513
Light time (s): 0.00129748
Target range from cspice_sincpt (km): 388.97573572
This routine computes observer-target states for targets whose
trajectories are not provided by SPK files.
Targets supported by this routine must have constant position
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 observer must be computable using
loaded SPK data.
For applications in which the target has non-zero, constant velocity
relative to its center of motion, the Icy routine
cspice_spkcvt { SPK, constant velocity target }
can be used.
This routine is suitable for computing states of landmarks on the
surface of an extended object, as seen by a specified observer,
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
cspice_spkezr
cspice_spkez
cspice_spkpos
cspice_spkezp
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.
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
observer-target light time `ltime'. The evaluation epoch will be
either et-ltime or et+ltime 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 cspice_spkezr.
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 SPK files and
objects having constant position or constant velocity are
cspice_spkcpo {SPK, Constant position observer}
cspice_spkcpt {SPK, Constant position target}
cspice_spkcvo {SPK, Constant velocity observer}
cspice_spkcvt {SPK, Constant velocity target}
1) If either the name of the center of motion or the observer
cannot be translated to its NAIF ID code, an error is signaled
by a routine in the call tree of this routine.
2) If the reference frame `outref' is unrecognized, an error
is signaled by a routine in the call tree of this
routine.
3) If the reference frame `trgref' 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, an
error 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 input arguments, `trgpos', `trgctr', `trgref',
`et', `outref', `refloc', `abcorr' or `obsrvr', is undefined,
an error is signaled by the IDL error handling system.
9) If any of the input arguments, `trgpos', `trgctr', `trgref',
`et', `outref', `refloc', `abcorr' or `obsrvr', is not of the
expected type, or it does not have the expected dimensions and
size, an error is signaled by the Icy interface.
10) If any of the output arguments, `state' or `ltime', is not a
named variable, an error is signaled by the Icy interface.
Appropriate kernels must be loaded by the calling program before
this routine is called.
The following data are required:
- SPK data: ephemeris data for target center and observer
must be loaded. If aberration corrections are used, the
states of target center and observer 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 cspice_furnsh.
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 cspice_furnsh.
- 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.
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.
FRAMES.REQ
ICY.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.0.1, 10-AUG-2021 (JDR)
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Edited to the header to comply with NAIF standard. Added
example's task description. Updated -Index_Entries.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.0, 09-APR-2012 (EDW)
state of constant_position_target
state of fixed_position_target
state of surface_point on extended_object
state of landmark on extended_object
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