Table of contents
CSPICE_SPKCPO returns the state of a specified target relative to
an "observer," where the observer has constant position in a
specified reference frame. The observer's position is provided
by the calling program rather than by loaded SPK files.
Given:
target scalar string name of a target body.
help, target
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 target is earth.
Case and leading and trailing blanks are not significant
in the string `target'.
et scalar double precision 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 scalar string 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 scalar string name 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 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 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 scalar string name indicating 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'.
obspos double precision 3-vector defining the fixed (constant)
geometric position of an observer relative to its center of
motion `obsctr', expressed in the reference frame `obsref'.
help, obspos
DOUBLE = Array[3]
Units are always km.
obsctr scalar string name of the center of motion of `obspos'.
help, obsctr
STRING = Scalar
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 scalar string name of the reference frame relative to which the
input position `obspos' is expressed.
help, obsref
STRING = Scalar
The observer 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 `obsref'.
the call:
cspice_spkcpo, target, et, outref, refloc, $
abcorr, obspos, obsctr, $
obsref, state, ltime
returns:
state a double precision Cartesian 6-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 scalar double precision 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.
Please note, CSPICE documentation and source code
uniformly uses the variable name `lt' to designate
the light-time between an observer and target. IDL
uses "lt" as the less-than numeric comparison
operator and so does not allow "lt" as a variable name.
Therefore, Icy documentation uses the name `ltime'
for the light-time value.
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) 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. There are three 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.
3) The station movement between `et' and the epoch at which
the DSS-14_TOPO frame is specified contributes a very
small offset---on the order of 10 cm---to the station-sun
position vector, expressed in the ITRF93 frame.
Kernels
=======
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: spkcpo_ex1.tm
This is the meta-kernel file for the header code example for
the subroutine cspice_spkcpo. 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.
;;
;; This program uses cspice_spkcpo to compute solar azimuth
;; and elevation at a given surface point on the earth.
;;
PRO spkcpo_ex1
;;
;; Local parameters.
;;
META = 'spkcpo_ex1.tm'
TIMFMT = 'YYYY MON DD HR:MN:SC.###### UTC'
TIMFM2 = 'YYYY MON DD HR:MN:SC.###### TDB ::TDB'
TIMLEN = 41
;;
;; Initial values
;;
z = [ 0.0D, 0.0, 1.0 ]
;;
;; 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 target, observer center, observer frame, and
;; observer position relative to its center.
;;
target = 'SUN'
obsctr = 'EARTH'
obsref = 'ITRF93'
;;
;; Set the position 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_spkcvo for an
;; example in which the plate motion is accounted for.
;;
obspos = [ -2353.6213656676991D0, $
-4641.3414911499403D0, $
3677.0523293197439D0 ]
;;
;; 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.
;;
cspice_spkcpo, target, et, outref, refloc, abcorr, $
obspos, obsctr, obsref, state0, lt0
;;
;; Compute planetocentric coordinates of the
;; observer-target position in the local
;; topocentric reference frame DSS-14_TOPO.
;;
cspice_reclat, state0[0:2], r, lon, lat
;;
;; Compute solar azimuth. The latitude we've
;; already computed is the elevation. Express
;; both angles in degrees.
;;
el = lat * cspice_dpr()
az = - lon * cspice_dpr()
if ( az lt 0.d0 ) then begin
az += 360.0d0
endif
;;
;; Display the computed state, light time, and angles.
;;
cspice_timout, et-lt0, TIMFMT, TIMLEN, emitim
print, ' Frame evaluation locus: ', refloc
print, ' '
print, ' Target: ', target
print, ' Observation time: ', obstim
print, ' Observer center: ', obsctr
print, ' Observer frame: ', obsref
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
print, ' '
print, format = '(" Solar azimuth (deg): ", F20.8)', az
print, format = '(" Solar elevation (deg): ", F20.8)', 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 cspice_spkcpo 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.
;;
cspice_bodvrd, 'EARTH', 'RADII', 3, radii
re = radii[0]
rp = radii[2]
f = ( re - rp ) / re
cspice_recgeo, obspos, re, f, obslon, obslat, obsalt
;;
;; Find the outward surface normal on the reference
;; ellipsoid at the observer's longitude and latitude.
;;
cspice_latrec, 1.D0, 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.
;;
cspice_twovec, normal, 3, z, 1, xform
outref = 'ITRF93'
abcorr = 'CN+S'
refloc = 'OBSERVER'
;;
;; Compute the observer-target state.
;;
cspice_spkcpo, target, et, outref, refloc, abcorr, $
obspos, obsctr, obsref, state1, lt1
;;
;; Convert the position to the topocentric frame.
;;
cspice_mxv, xform, state1[0:2], topvec
;;
;; Compute azimuth and elevation.
;;
cspice_reclat, topvec, r, lon, lat
el = lat * cspice_dpr()
az = - lon * cspice_dpr()
if ( az lt 0.d0 ) then begin
az += 360.d0
endif
print, ' '
print, ' AZ/EL computed without frame kernel:'
print, ' Distance between last two '
print, format='(" positions (km): ", F20.8)', $
cspice_vdist( state0[0:2], topvec )
print, ' '
print, format = '(" Solar azimuth (deg): ", F20.8)', az
print, format = '(" Solar elevation (deg): ", F20.8)', el
;;
;; 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: OBSERVER
Target: SUN
Observation time: 2003 OCT 13 06:00:00.000000 UTC
Observer center: EARTH
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):
62512272.82074845 58967494.42513601 -122059095.46751881
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): 3.07056970
Solar azimuth (deg): 316.67141786
Solar elevation (deg): -54.85253216
This routine computes observer-target states for observers whose
trajectories are not provided by SPK files.
Observers 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 target must be computable using
loaded SPK data.
For applications in which the observer has constant, non-zero velocity
relative to its center of motion, the Icy routine
cspice_spkcvo { SPK, constant velocity observer state }
can be used.
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
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. 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 `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 the SPK
system 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 target
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 `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, 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, `target', `et', `outref',
`refloc', `abcorr', `obspos', `obsctr' or `obsref', is
undefined, an error is signaled by the IDL error handling
system.
9) If any of the input arguments, `target', `et', `outref',
`refloc', `abcorr', `obspos', `obsctr' or `obsref', 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 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 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, 01-JUN-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.
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 relative to constant_position_observer
state relative to constant_position surface_point
state relative to surface_point on extended_object
state relative to landmark on extended_object
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