1 /* spkpos.f -- translated by f2c (version 19980913).
2 You must link the resulting object file with the libraries:
3 -lf2c -lm (in that order)
4 */
5
6 #include "f2c.h"
7
8 /* $Procedure SPKPOS ( S/P Kernel, position ) */
spkpos_(char * targ,doublereal * et,char * ref,char * abcorr,char * obs,doublereal * ptarg,doublereal * lt,ftnlen targ_len,ftnlen ref_len,ftnlen abcorr_len,ftnlen obs_len)9 /* Subroutine */ int spkpos_(char *targ, doublereal *et, char *ref, char *
10 abcorr, char *obs, doublereal *ptarg, doublereal *lt, ftnlen targ_len,
11 ftnlen ref_len, ftnlen abcorr_len, ftnlen obs_len)
12 {
13 /* Initialized data */
14
15 static logical first = TRUE_;
16
17 extern /* Subroutine */ int zzbods2c_(integer *, char *, integer *,
18 logical *, char *, integer *, logical *, ftnlen, ftnlen),
19 zzctruin_(integer *), chkin_(char *, ftnlen);
20 integer obsid;
21 extern /* Subroutine */ int errch_(char *, char *, ftnlen, ftnlen);
22 logical found;
23 static logical svfnd1, svfnd2;
24 static integer svctr1[2], svctr2[2];
25 integer targid;
26 extern /* Subroutine */ int sigerr_(char *, ftnlen), chkout_(char *,
27 ftnlen);
28 static integer svtgid;
29 extern /* Subroutine */ int setmsg_(char *, ftnlen);
30 static integer svobsi;
31 static char svtarg[36], svobsn[36];
32 extern /* Subroutine */ int spkezp_(integer *, doublereal *, char *, char
33 *, integer *, doublereal *, doublereal *, ftnlen, ftnlen);
34 extern logical return_(void);
35
36 /* $ Abstract */
37
38 /* Return the position of a target body relative to an observing */
39 /* body, optionally corrected for light time (planetary aberration) */
40 /* and stellar aberration. */
41
42 /* $ Disclaimer */
43
44 /* THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE */
45 /* CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. */
46 /* GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE */
47 /* ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE */
48 /* PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" */
49 /* TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY */
50 /* WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A */
51 /* PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC */
52 /* SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE */
53 /* SOFTWARE AND RELATED MATERIALS, HOWEVER USED. */
54
55 /* IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA */
56 /* BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT */
57 /* LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, */
58 /* INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, */
59 /* REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE */
60 /* REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. */
61
62 /* RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF */
63 /* THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY */
64 /* CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE */
65 /* ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. */
66
67 /* $ Required_Reading */
68
69 /* SPK */
70 /* NAIF_IDS */
71 /* FRAMES */
72 /* TIME */
73
74 /* $ Keywords */
75
76 /* EPHEMERIS */
77
78 /* $ Declarations */
79 /* $ Abstract */
80
81 /* The parameters below form an enumerated list of the recognized */
82 /* frame types. They are: INERTL, PCK, CK, TK, DYN. The meanings */
83 /* are outlined below. */
84
85 /* $ Disclaimer */
86
87 /* THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE */
88 /* CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. */
89 /* GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE */
90 /* ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE */
91 /* PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" */
92 /* TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY */
93 /* WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A */
94 /* PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC */
95 /* SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE */
96 /* SOFTWARE AND RELATED MATERIALS, HOWEVER USED. */
97
98 /* IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA */
99 /* BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT */
100 /* LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, */
101 /* INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, */
102 /* REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE */
103 /* REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. */
104
105 /* RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF */
106 /* THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY */
107 /* CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE */
108 /* ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. */
109
110 /* $ Parameters */
111
112 /* INERTL an inertial frame that is listed in the routine */
113 /* CHGIRF and that requires no external file to */
114 /* compute the transformation from or to any other */
115 /* inertial frame. */
116
117 /* PCK is a frame that is specified relative to some */
118 /* INERTL frame and that has an IAU model that */
119 /* may be retrieved from the PCK system via a call */
120 /* to the routine TISBOD. */
121
122 /* CK is a frame defined by a C-kernel. */
123
124 /* TK is a "text kernel" frame. These frames are offset */
125 /* from their associated "relative" frames by a */
126 /* constant rotation. */
127
128 /* DYN is a "dynamic" frame. These currently are */
129 /* parameterized, built-in frames where the full frame */
130 /* definition depends on parameters supplied via a */
131 /* frame kernel. */
132
133 /* ALL indicates any of the above classes. This parameter */
134 /* is used in APIs that fetch information about frames */
135 /* of a specified class. */
136
137
138 /* $ Author_and_Institution */
139
140 /* N.J. Bachman (JPL) */
141 /* W.L. Taber (JPL) */
142
143 /* $ Literature_References */
144
145 /* None. */
146
147 /* $ Version */
148
149 /* - SPICELIB Version 4.0.0, 08-MAY-2012 (NJB) */
150
151 /* The parameter ALL was added to support frame fetch APIs. */
152
153 /* - SPICELIB Version 3.0.0, 28-MAY-2004 (NJB) */
154
155 /* The parameter DYN was added to support the dynamic frame class. */
156
157 /* - SPICELIB Version 2.0.0, 12-DEC-1996 (WLT) */
158
159 /* Various unused frames types were removed and the */
160 /* frame time TK was added. */
161
162 /* - SPICELIB Version 1.0.0, 10-DEC-1995 (WLT) */
163
164 /* -& */
165
166 /* End of INCLUDE file frmtyp.inc */
167
168 /* $ Abstract */
169
170 /* This include file defines the dimension of the counter */
171 /* array used by various SPICE subsystems to uniquely identify */
172 /* changes in their states. */
173
174 /* $ Disclaimer */
175
176 /* THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE */
177 /* CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. */
178 /* GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE */
179 /* ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE */
180 /* PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" */
181 /* TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY */
182 /* WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A */
183 /* PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC */
184 /* SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE */
185 /* SOFTWARE AND RELATED MATERIALS, HOWEVER USED. */
186
187 /* IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA */
188 /* BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT */
189 /* LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, */
190 /* INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, */
191 /* REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE */
192 /* REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. */
193
194 /* RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF */
195 /* THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY */
196 /* CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE */
197 /* ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. */
198
199 /* $ Parameters */
200
201 /* CTRSIZ is the dimension of the counter array used by */
202 /* various SPICE subsystems to uniquely identify */
203 /* changes in their states. */
204
205 /* $ Author_and_Institution */
206
207 /* B.V. Semenov (JPL) */
208
209 /* $ Literature_References */
210
211 /* None. */
212
213 /* $ Version */
214
215 /* - SPICELIB Version 1.0.0, 29-JUL-2013 (BVS) */
216
217 /* -& */
218
219 /* End of include file. */
220
221 /* $ Brief_I/O */
222
223 /* Variable I/O Description */
224 /* -------- --- -------------------------------------------------- */
225 /* TARG I Target body name. */
226 /* ET I Observer epoch. */
227 /* REF I Reference frame of output position vector. */
228 /* ABCORR I Aberration correction flag. */
229 /* OBS I Observing body name. */
230 /* PTARG O Position of target. */
231 /* LT O One way light time between observer and target. */
232
233 /* $ Detailed_Input */
234
235 /* TARG is the name of a target body. Optionally, you may */
236 /* supply the integer ID code for the object as an */
237 /* integer string. For example both 'MOON' and '301' */
238 /* are legitimate strings that indicate the moon is the */
239 /* target body. */
240
241 /* The target and observer define a position vector */
242 /* which points from the observer to the target. */
243
244 /* ET is the ephemeris time, expressed as seconds past */
245 /* J2000 TDB, at which the position of the target body */
246 /* relative to the observer is to be computed. ET */
247 /* refers to time at the observer's location. */
248
249 /* REF is the name of the reference frame relative to which */
250 /* the output position vector should be expressed. This */
251 /* may be any frame supported by the SPICE system, */
252 /* including built-in frames (documented in the Frames */
253 /* Required Reading) and frames defined by a loaded */
254 /* frame kernel (FK). */
255
256 /* When REF designates a non-inertial frame, the */
257 /* orientation of the frame is evaluated at an epoch */
258 /* dependent on the selected aberration correction. See */
259 /* the description of the output position vector PTARG */
260 /* for details. */
261
262 /* ABCORR indicates the aberration corrections to be applied to */
263 /* the position of the target body to account for */
264 /* one-way light time and stellar aberration. See the */
265 /* discussion in the Particulars section for */
266 /* recommendations on how to choose aberration */
267 /* corrections. */
268
269 /* ABCORR may be any of the following: */
270
271 /* 'NONE' Apply no correction. Return the */
272 /* geometric position of the target body */
273 /* relative to the observer. */
274
275 /* The following values of ABCORR apply to the */
276 /* "reception" case in which photons depart from the */
277 /* target's location at the light-time corrected epoch */
278 /* ET-LT and *arrive* at the observer's location at ET: */
279
280 /* 'LT' Correct for one-way light time (also */
281 /* called "planetary aberration") using a */
282 /* Newtonian formulation. This correction */
283 /* yields the position of the target at */
284 /* the moment it emitted photons arriving */
285 /* at the observer at ET. */
286
287 /* The light time correction uses an */
288 /* iterative solution of the light time */
289 /* equation (see Particulars for details). */
290 /* The solution invoked by the 'LT' option */
291 /* uses one iteration. */
292
293 /* 'LT+S' Correct for one-way light time and */
294 /* stellar aberration using a Newtonian */
295 /* formulation. This option modifies the */
296 /* position obtained with the 'LT' option */
297 /* to account for the observer's velocity */
298 /* relative to the solar system */
299 /* barycenter. The result is the apparent */
300 /* position of the target---the position */
301 /* as seen by the observer. */
302
303 /* 'CN' Converged Newtonian light time */
304 /* correction. In solving the light time */
305 /* equation, the 'CN' correction iterates */
306 /* until the solution converges (three */
307 /* iterations on all supported platforms). */
308 /* Whether the 'CN+S' solution is */
309 /* substantially more accurate than the */
310 /* 'LT' solution depends on the geometry */
311 /* of the participating objects and on the */
312 /* accuracy of the input data. In all */
313 /* cases this routine will execute more */
314 /* slowly when a converged solution is */
315 /* computed. See the Particulars section */
316 /* below for a discussion of precision of */
317 /* light time corrections. */
318
319 /* 'CN+S' Converged Newtonian light time */
320 /* correction and stellar aberration */
321 /* correction. */
322
323
324 /* The following values of ABCORR apply to the */
325 /* "transmission" case in which photons *depart* from */
326 /* the observer's location at ET and arrive at the */
327 /* target's location at the light-time corrected epoch */
328 /* ET+LT: */
329
330 /* 'XLT' "Transmission" case: correct for */
331 /* one-way light time using a Newtonian */
332 /* formulation. This correction yields the */
333 /* position of the target at the moment it */
334 /* receives photons emitted from the */
335 /* observer's location at ET. */
336
337 /* 'XLT+S' "Transmission" case: correct for */
338 /* one-way light time and stellar */
339 /* aberration using a Newtonian */
340 /* formulation. This option modifies the */
341 /* position obtained with the 'XLT' option */
342 /* to account for the observer's velocity */
343 /* relative to the solar system */
344 /* barycenter. The computed target */
345 /* position indicates the direction that */
346 /* photons emitted from the observer's */
347 /* location must be "aimed" to hit the */
348 /* target. */
349
350 /* 'XCN' "Transmission" case: converged */
351 /* Newtonian light time correction. */
352
353 /* 'XCN+S' "Transmission" case: converged */
354 /* Newtonian light time correction and */
355 /* stellar aberration correction. */
356
357
358 /* Neither special nor general relativistic effects are */
359 /* accounted for in the aberration corrections applied */
360 /* by this routine. */
361
362 /* Case and blanks are not significant in the string */
363 /* ABCORR. */
364
365 /* OBS is the name of an observing body. Optionally, you */
366 /* may supply the ID code of the object as an integer */
367 /* string. For example, both 'EARTH' and '399' are */
368 /* legitimate strings to supply to indicate the */
369 /* observer is Earth. */
370
371 /* $ Detailed_Output */
372
373 /* PTARG is a Cartesian 3-vector representing the position of */
374 /* the target body relative to the specified observer. */
375 /* PTARG is corrected for the specified aberrations, and */
376 /* is expressed with respect to the reference frame */
377 /* specified by REF. The three components of PTARG */
378 /* represent the x-, y- and z-components of the target's */
379 /* position. */
380
381 /* PTARG points from the observer's location at ET to */
382 /* the aberration-corrected location of the target. */
383 /* Note that the sense of this position vector is */
384 /* independent of the direction of radiation travel */
385 /* implied by the aberration correction. */
386
387 /* Units are always km. */
388
389 /* Non-inertial frames are treated as follows: letting */
390 /* LTCENT be the one-way light time between the observer */
391 /* and the central body associated with the frame, the */
392 /* orientation of the frame is evaluated at ET-LTCENT, */
393 /* ET+LTCENT, or ET depending on whether the requested */
394 /* aberration correction is, respectively, for received */
395 /* radiation, transmitted radiation, or is omitted. */
396 /* LTCENT is computed using the method indicated by */
397 /* ABCORR. */
398
399 /* LT is the one-way light time between the observer and */
400 /* target in seconds. If the target position is */
401 /* corrected for aberrations, then LT is the one-way */
402 /* light time between the observer and the light time */
403 /* corrected target location. */
404
405 /* $ Parameters */
406
407 /* None. */
408
409 /* $ Exceptions */
410
411 /* 1) If name of target or observer cannot be translated to its */
412 /* NAIF ID code, the error SPICE(IDCODENOTFOUND) is signaled. */
413
414 /* 2) If the reference frame REF is not a recognized reference */
415 /* frame the error 'SPICE(UNKNOWNFRAME)' is signaled. */
416
417 /* 3) If the loaded kernels provide insufficient data to */
418 /* compute the requested position vector, the deficiency will */
419 /* be diagnosed by a routine in the call tree of this routine. */
420
421 /* 4) If an error occurs while reading an SPK or other kernel file, */
422 /* the error will be diagnosed by a routine in the call tree */
423 /* of this routine. */
424
425 /* $ Files */
426
427 /* This routine computes positions using SPK files that have been */
428 /* loaded into the SPICE system, normally via the kernel loading */
429 /* interface routine FURNSH. See the routine FURNSH and the SPK */
430 /* and KERNEL Required Reading for further information on loading */
431 /* (and unloading) kernels. */
432
433 /* If the output position PTARG is to be expressed relative to a */
434 /* non-inertial frame, or if any of the ephemeris data used to */
435 /* compute PTARG are expressed relative to a non-inertial frame in */
436 /* the SPK files providing those data, additional kernels may be */
437 /* needed to enable the reference frame transformations required to */
438 /* compute the position. Normally these additional kernels are PCK */
439 /* files or frame kernels. Any such kernels must already be loaded */
440 /* at the time this routine is called. */
441
442 /* $ Particulars */
443
444 /* This routine is part of the user interface to the SPICE ephemeris */
445 /* system. It allows you to retrieve position information for any */
446 /* ephemeris object relative to any other in a reference frame that */
447 /* is convenient for further computations. */
448
449 /* This routine is identical in function to the routine SPKEZP */
450 /* except that it allows you to refer to ephemeris objects by name */
451 /* (via a character string). */
452
453
454 /* Aberration corrections */
455 /* ====================== */
456
457 /* In space science or engineering applications one frequently */
458 /* wishes to know where to point a remote sensing instrument, such */
459 /* as an optical camera or radio antenna, in order to observe or */
460 /* otherwise receive radiation from a target. This pointing problem */
461 /* is complicated by the finite speed of light: one needs to point */
462 /* to where the target appears to be as opposed to where it actually */
463 /* is at the epoch of observation. We use the adjectives */
464 /* "geometric," "uncorrected," or "true" to refer to an actual */
465 /* position or state of a target at a specified epoch. When a */
466 /* geometric position or state vector is modified to reflect how it */
467 /* appears to an observer, we describe that vector by any of the */
468 /* terms "apparent," "corrected," "aberration corrected," or "light */
469 /* time and stellar aberration corrected." The SPICE Toolkit can */
470 /* correct for two phenomena affecting the apparent location of an */
471 /* object: one-way light time (also called "planetary aberration") */
472 /* and stellar aberration. */
473
474 /* One-way light time */
475 /* ------------------ */
476
477 /* Correcting for one-way light time is done by computing, given an */
478 /* observer and observation epoch, where a target was when the */
479 /* observed photons departed the target's location. The vector from */
480 /* the observer to this computed target location is called a "light */
481 /* time corrected" vector. The light time correction depends on the */
482 /* motion of the target relative to the solar system barycenter, but */
483 /* it is independent of the velocity of the observer relative to the */
484 /* solar system barycenter. Relativistic effects such as light */
485 /* bending and gravitational delay are not accounted for in the */
486 /* light time correction performed by this routine. */
487
488 /* Stellar aberration */
489 /* ------------------ */
490
491 /* The velocity of the observer also affects the apparent location */
492 /* of a target: photons arriving at the observer are subject to a */
493 /* "raindrop effect" whereby their velocity relative to the observer */
494 /* is, using a Newtonian approximation, the photons' velocity */
495 /* relative to the solar system barycenter minus the velocity of the */
496 /* observer relative to the solar system barycenter. This effect is */
497 /* called "stellar aberration." Stellar aberration is independent */
498 /* of the velocity of the target. The stellar aberration formula */
499 /* used by this routine does not include (the much smaller) */
500 /* relativistic effects. */
501
502 /* Stellar aberration corrections are applied after light time */
503 /* corrections: the light time corrected target position vector is */
504 /* used as an input to the stellar aberration correction. */
505
506 /* When light time and stellar aberration corrections are both */
507 /* applied to a geometric position vector, the resulting position */
508 /* vector indicates where the target "appears to be" from the */
509 /* observer's location. */
510
511 /* As opposed to computing the apparent position of a target, one */
512 /* may wish to compute the pointing direction required for */
513 /* transmission of photons to the target. This also requires */
514 /* correction of the geometric target position for the effects of */
515 /* light time and stellar aberration, but in this case the */
516 /* corrections are computed for radiation traveling *from* the */
517 /* observer to the target. */
518
519 /* The "transmission" light time correction yields the target's */
520 /* location as it will be when photons emitted from the observer's */
521 /* location at ET arrive at the target. The transmission stellar */
522 /* aberration correction is the inverse of the traditional stellar */
523 /* aberration correction: it indicates the direction in which */
524 /* radiation should be emitted so that, using a Newtonian */
525 /* approximation, the sum of the velocity of the radiation relative */
526 /* to the observer and of the observer's velocity, relative to the */
527 /* solar system barycenter, yields a velocity vector that points in */
528 /* the direction of the light time corrected position of the target. */
529
530 /* One may object to using the term "observer" in the transmission */
531 /* case, in which radiation is emitted from the observer's location. */
532 /* The terminology was retained for consistency with earlier */
533 /* documentation. */
534
535 /* Below, we indicate the aberration corrections to use for some */
536 /* common applications: */
537
538 /* 1) Find the apparent direction of a target for a remote-sensing */
539 /* observation. */
540
541 /* Use 'LT+S' or 'CN+S: apply both light time and stellar */
542 /* aberration corrections. */
543
544 /* Note that using light time corrections alone ('LT' or 'CN') */
545 /* is generally not a good way to obtain an approximation to */
546 /* an apparent target vector: since light time and stellar */
547 /* aberration corrections often partially cancel each other, */
548 /* it may be more accurate to use no correction at all than to */
549 /* use light time alone. */
550
551
552 /* 2) Find the corrected pointing direction to radiate a signal */
553 /* to a target. This computation is often applicable for */
554 /* implementing communications sessions. */
555
556 /* Use 'XLT+S' or 'XCN+S: apply both light time and stellar */
557 /* aberration corrections for transmission. */
558
559
560 /* 3) Compute the apparent position of a target body relative */
561 /* to a star or other distant object. */
562
563 /* Use 'LT', 'CN', 'LT+S', or 'CN+S' as needed to match the */
564 /* correction applied to the position of the distant */
565 /* object. For example, if a star position is obtained from */
566 /* a catalog, the position vector may not be corrected for */
567 /* stellar aberration. In this case, to find the angular */
568 /* separation of the star and the limb of a planet, the */
569 /* vector from the observer to the planet should be */
570 /* corrected for light time but not stellar aberration. */
571
572
573 /* 4) Obtain an uncorrected position vector derived directly from */
574 /* data in an SPK file. */
575
576 /* Use 'NONE'. */
577
578
579 /* 5) Use a geometric position vector as a low-accuracy estimate */
580 /* of the apparent position for an application where execution */
581 /* speed is critical. */
582
583 /* Use 'NONE'. */
584
585
586 /* 6) While this routine cannot perform the relativistic */
587 /* aberration corrections required to compute positions */
588 /* with the highest possible accuracy, it can supply the */
589 /* geometric positions required as inputs to these */
590 /* computations. */
591
592 /* Use 'NONE', then apply high-accuracy aberration */
593 /* corrections (not available in the SPICE Toolkit). */
594
595
596 /* Below, we discuss in more detail how the aberration corrections */
597 /* applied by this routine are computed. */
598
599 /* Geometric case */
600 /* ============== */
601
602 /* SPKPOS begins by computing the geometric position T(ET) of the */
603 /* target body relative to the solar system barycenter (SSB). */
604 /* Subtracting the geometric position of the observer O(ET) gives */
605 /* the geometric position of the target body relative to the */
606 /* observer. The one-way light time, LT, is given by */
607
608 /* | T(ET) - O(ET) | */
609 /* LT = ------------------- */
610 /* c */
611
612 /* The geometric relationship between the observer, target, and */
613 /* solar system barycenter is as shown: */
614
615
616 /* SSB ---> O(ET) */
617 /* | / */
618 /* | / */
619 /* | / */
620 /* | / T(ET) - O(ET) */
621 /* V V */
622 /* T(ET) */
623
624
625 /* The returned position vector is */
626
627 /* T(ET) - O(ET) */
628
629
630
631 /* Reception case */
632 /* ============== */
633
634 /* When any of the options 'LT', 'CN', 'LT+S', 'CN+S' is selected */
635 /* for ABCORR, SPKPOS computes the position of the target body at */
636 /* epoch ET-LT, where LT is the one-way light time. Let T(t) and */
637 /* O(t) represent the positions of the target and observer */
638 /* relative to the solar system barycenter at time t; then LT is */
639 /* the solution of the light-time equation */
640
641 /* | T(ET-LT) - O(ET) | */
642 /* LT = ------------------------ (1) */
643 /* c */
644
645 /* The ratio */
646
647 /* | T(ET) - O(ET) | */
648 /* --------------------- (2) */
649 /* c */
650
651 /* is used as a first approximation to LT; inserting (2) into the */
652 /* right hand side of the light-time equation (1) yields the */
653 /* "one-iteration" estimate of the one-way light time ("LT"). */
654 /* Repeating the process until the estimates of LT converge */
655 /* yields the "converged Newtonian" light time estimate ("CN"). */
656
657 /* Subtracting the geometric position of the observer O(ET) gives */
658 /* the position of the target body relative to the observer: */
659 /* T(ET-LT) - O(ET). */
660
661 /* SSB ---> O(ET) */
662 /* | \ | */
663 /* | \ | */
664 /* | \ | T(ET-LT) - O(ET) */
665 /* | \ | */
666 /* V V V */
667 /* T(ET) T(ET-LT) */
668
669 /* The light time corrected position vector is */
670
671 /* T(ET-LT) - O(ET) */
672
673 /* If correction for stellar aberration is requested, the target */
674 /* position is rotated toward the solar system barycenter- */
675 /* relative velocity vector of the observer. The rotation is */
676 /* computed as follows: */
677
678 /* Let r be the light time corrected vector from the observer */
679 /* to the object, and v be the velocity of the observer with */
680 /* respect to the solar system barycenter. Let w be the angle */
681 /* between them. The aberration angle phi is given by */
682
683 /* sin(phi) = v sin(w) / c */
684
685 /* Let h be the vector given by the cross product */
686
687 /* h = r X v */
688
689 /* Rotate r by phi radians about h to obtain the apparent */
690 /* position of the object. */
691
692
693 /* Transmission case */
694 /* ================== */
695
696 /* When any of the options 'XLT', 'XCN', 'XLT+S', 'XCN+S' is */
697 /* selected, SPKPOS computes the position of the target body T at */
698 /* epoch ET+LT, where LT is the one-way light time. LT is the */
699 /* solution of the light-time equation */
700
701 /* | T(ET+LT) - O(ET) | */
702 /* LT = ------------------------ (3) */
703 /* c */
704
705 /* Subtracting the geometric position of the observer, O(ET), */
706 /* gives the position of the target body relative to the */
707 /* observer: T(ET-LT) - O(ET). */
708
709 /* SSB --> O(ET) */
710 /* / | * */
711 /* / | * T(ET+LT) - O(ET) */
712 /* / |* */
713 /* / *| */
714 /* V V V */
715 /* T(ET+LT) T(ET) */
716
717 /* The light-time corrected position vector is */
718
719 /* T(ET+LT) - O(ET) */
720
721 /* If correction for stellar aberration is requested, the target */
722 /* position is rotated away from the solar system barycenter- */
723 /* relative velocity vector of the observer. The rotation is */
724 /* computed as in the reception case, but the sign of the */
725 /* rotation angle is negated. */
726
727
728 /* Precision of light time corrections */
729 /* =================================== */
730
731 /* Corrections using one iteration of the light time solution */
732 /* ---------------------------------------------------------- */
733
734 /* When the requested aberration correction is 'LT', 'LT+S', */
735 /* 'XLT', or 'XLT+S', only one iteration is performed in the */
736 /* algorithm used to compute LT. */
737
738 /* The relative error in this computation */
739
740 /* | LT_ACTUAL - LT_COMPUTED | / LT_ACTUAL */
741
742 /* is at most */
743
744 /* (V/C)**2 */
745 /* ---------- */
746 /* 1 - (V/C) */
747
748 /* which is well approximated by (V/C)**2, where V is the */
749 /* velocity of the target relative to an inertial frame and C is */
750 /* the speed of light. */
751
752 /* For nearly all objects in the solar system V is less than 60 */
753 /* km/sec. The value of C is ~300000 km/sec. Thus the */
754 /* one-iteration solution for LT has a potential relative error */
755 /* of not more than 4e-8. This is a potential light time error of */
756 /* approximately 2e-5 seconds per astronomical unit of distance */
757 /* separating the observer and target. Given the bound on V cited */
758 /* above: */
759
760 /* As long as the observer and target are separated by less */
761 /* than 50 astronomical units, the error in the light time */
762 /* returned using the one-iteration light time corrections is */
763 /* less than 1 millisecond. */
764
765 /* The magnitude of the corresponding position error, given */
766 /* the above assumptions, may be as large as (V/C)**2 * the */
767 /* distance between the observer and the uncorrected target */
768 /* position: 300 km or equivalently 6 km/AU. */
769
770 /* In practice, the difference between positions obtained using */
771 /* one-iteration and converged light time is usually much smaller */
772 /* than the value computed above and can be insignificant. For */
773 /* example, for the spacecraft Mars Reconnaissance Orbiter and */
774 /* Mars Express, the position error for the one-iteration light */
775 /* time correction, applied to the spacecraft-to-Mars center */
776 /* vector, is at the 1 cm level. */
777
778 /* Comparison of results obtained using the one-iteration and */
779 /* converged light time solutions is recommended when adequacy of */
780 /* the one-iteration solution is in doubt. */
781
782
783 /* Converged corrections */
784 /* --------------------- */
785
786 /* When the requested aberration correction is 'CN', 'CN+S', */
787 /* 'XCN', or 'XCN+S', as many iterations as are required for */
788 /* convergence are performed in the computation of LT. Usually */
789 /* the solution is found after three iterations. The relative */
790 /* error present in this case is at most */
791
792 /* (V/C)**4 */
793 /* ---------- */
794 /* 1 - (V/C) */
795
796 /* which is well approximated by (V/C)**4. */
797
798 /* The precision of this computation (ignoring round-off */
799 /* error) is better than 4e-11 seconds for any pair of objects */
800 /* less than 50 AU apart, and having speed relative to the */
801 /* solar system barycenter less than 60 km/s. */
802
803 /* The magnitude of the corresponding position error, given */
804 /* the above assumptions, may be as large as (V/C)**4 * the */
805 /* distance between the observer and the uncorrected target */
806 /* position: 1.2 cm at 50 AU or equivalently 0.24 mm/AU. */
807
808 /* However, to very accurately model the light time between */
809 /* target and observer one must take into account effects due to */
810 /* general relativity. These may be as high as a few hundredths */
811 /* of a millisecond for some objects. */
812
813
814 /* Relativistic Corrections */
815 /* ========================= */
816
817 /* This routine does not attempt to perform either general or */
818 /* special relativistic corrections in computing the various */
819 /* aberration corrections. For many applications relativistic */
820 /* corrections are not worth the expense of added computation */
821 /* cycles. If however, your application requires these additional */
822 /* corrections we suggest you consult the astronomical almanac (page */
823 /* B36) for a discussion of how to carry out these corrections. */
824
825
826 /* $ Examples */
827
828 /* 1) Load a planetary ephemeris SPK, then look up a series of */
829 /* geometric positions of the moon relative to the earth, */
830 /* referenced to the J2000 frame. */
831
832
833 /* IMPLICIT NONE */
834 /* C */
835 /* C Local constants */
836 /* C */
837 /* CHARACTER*(*) FRAME */
838 /* PARAMETER ( FRAME = 'J2000' ) */
839
840 /* CHARACTER*(*) ABCORR */
841 /* PARAMETER ( ABCORR = 'NONE' ) */
842
843 /* C */
844 /* C The name of the SPK file shown here is fictitious; */
845 /* C you must supply the name of an SPK file available */
846 /* C on your own computer system. */
847 /* C */
848 /* CHARACTER*(*) SPK */
849 /* PARAMETER ( SPK = 'planet.bsp' ) */
850
851 /* C */
852 /* C ET0 represents the date 2000 Jan 1 12:00:00 TDB. */
853 /* C */
854 /* DOUBLE PRECISION ET0 */
855 /* PARAMETER ( ET0 = 0.0D0 ) */
856
857 /* C */
858 /* C Use a time step of 1 hour; look up 100 positions. */
859 /* C */
860 /* DOUBLE PRECISION STEP */
861 /* PARAMETER ( STEP = 3600.0D0 ) */
862
863 /* INTEGER MAXITR */
864 /* PARAMETER ( MAXITR = 100 ) */
865
866 /* CHARACTER*(*) OBSRVR */
867 /* PARAMETER ( OBSRVR = 'Earth' ) */
868
869 /* CHARACTER*(*) TARGET */
870 /* PARAMETER ( TARGET = 'Moon' ) */
871
872 /* C */
873 /* C Local variables */
874 /* C */
875 /* DOUBLE PRECISION ET */
876 /* DOUBLE PRECISION LT */
877 /* DOUBLE PRECISION POS ( 3 ) */
878
879 /* INTEGER I */
880
881 /* C */
882 /* C Load the SPK file. */
883 /* C */
884 /* CALL FURNSH ( SPK ) */
885
886 /* C */
887 /* C Step through a series of epochs, looking up a */
888 /* C position vector at each one. */
889 /* C */
890 /* DO I = 1, MAXITR */
891
892 /* ET = ET0 + (I-1)*STEP */
893
894 /* CALL SPKPOS ( TARGET, ET, FRAME, ABCORR, OBSRVR, */
895 /* . POS, LT ) */
896
897 /* WRITE (*,*) 'ET = ', ET */
898 /* WRITE (*,*) 'J2000 x-position (km): ', POS(1) */
899 /* WRITE (*,*) 'J2000 y-position (km): ', POS(2) */
900 /* WRITE (*,*) 'J2000 z-position (km): ', POS(3) */
901 /* WRITE (*,*) ' ' */
902
903 /* END DO */
904
905 /* END */
906
907
908 /* $ Restrictions */
909
910 /* None. */
911
912 /* $ Literature_References */
913
914 /* SPK Required Reading. */
915
916 /* $ Author_and_Institution */
917
918 /* C.H. Acton (JPL) */
919 /* B.V. Semenov (JPL) */
920 /* N.J. Bachman (JPL) */
921 /* W.L. Taber (JPL) */
922
923 /* $ Version */
924
925 /* - SPICELIB Version 3.1.0, 03-JUL-2014 (NJB) (BVS) */
926
927 /* Discussion of light time corrections was updated. Assertions */
928 /* that converged light time corrections are unlikely to be */
929 /* useful were removed. */
930
931 /* Last update was 19-SEP-2013 (BVS) */
932
933 /* Updated to save the input body names and ZZBODTRN state */
934 /* counters and to do name-ID conversions only if the counters */
935 /* have changed. */
936
937 /* - SPICELIB Version 3.0.3, 04-APR-2008 (NJB) */
938
939 /* Corrected minor error in description of XLT+S aberration */
940 /* correction. */
941
942 /* - SPICELIB Version 3.0.2, 20-OCT-2003 (EDW) */
943
944 /* Added mention that LT returns in seconds. */
945
946 /* - SPICELIB Version 3.0.1, 29-JUL-2003 (NJB) (CHA) */
947
948 /* Various minor header changes were made to improve clarity. */
949
950 /* - SPICELIB Version 3.0.0, 31-DEC-2001 (NJB) */
951
952 /* Updated to handle aberration corrections for transmission */
953 /* of radiation. Formerly, only the reception case was */
954 /* supported. The header was revised and expanded to explain */
955 /* the functionality of this routine in more detail. */
956
957 /* - SPICELIB Version 1.0.0, 03-MAR-1999 (WLT) */
958
959 /* -& */
960 /* $ Index_Entries */
961
962 /* using body names get position relative to an observer */
963 /* get position relative observer corrected for aberrations */
964 /* read ephemeris data */
965 /* read trajectory data */
966
967 /* -& */
968 /* $ Revisions */
969
970 /* None. */
971
972 /* -& */
973
974 /* SPICELIB functions */
975
976
977 /* Saved body name length. */
978
979
980 /* Local variables */
981
982
983 /* Saved name/ID item declarations. */
984
985
986 /* Saved name/ID items. */
987
988
989 /* Initial values. */
990
991
992 /* Standard SPICE error handling. */
993
994 if (return_()) {
995 return 0;
996 } else {
997 chkin_("SPKPOS", (ftnlen)6);
998 }
999
1000 /* Initialization. */
1001
1002 if (first) {
1003
1004 /* Initialize counters. */
1005
1006 zzctruin_(svctr1);
1007 zzctruin_(svctr2);
1008 first = FALSE_;
1009 }
1010
1011 /* Starting from translation of target name to its code */
1012
1013 zzbods2c_(svctr1, svtarg, &svtgid, &svfnd1, targ, &targid, &found, (
1014 ftnlen)36, targ_len);
1015 if (! found) {
1016 setmsg_("The target, '#', is not a recognized name for an ephemeris "
1017 "object. The cause of this problem may be that you need an up"
1018 "dated version of the SPICE toolkit. Alternatively you may ca"
1019 "ll SPKEZP directly if you know the SPICE id-codes for both '"
1020 "#' and '#' ", (ftnlen)250);
1021 errch_("#", targ, (ftnlen)1, targ_len);
1022 errch_("#", targ, (ftnlen)1, targ_len);
1023 errch_("#", obs, (ftnlen)1, obs_len);
1024 sigerr_("SPICE(IDCODENOTFOUND)", (ftnlen)21);
1025 chkout_("SPKPOS", (ftnlen)6);
1026 return 0;
1027 }
1028
1029 /* Now do the same for observer. */
1030
1031 zzbods2c_(svctr2, svobsn, &svobsi, &svfnd2, obs, &obsid, &found, (ftnlen)
1032 36, obs_len);
1033 if (! found) {
1034 setmsg_("The observer, '#', is not a recognized name for an ephemeri"
1035 "s object. The cause of this problem may be that you need an "
1036 "updated version of the SPICE toolkit. Alternatively you may "
1037 "call SPKEZP directly if you know the SPICE id-codes for both"
1038 " '#' and '#' ", (ftnlen)252);
1039 errch_("#", obs, (ftnlen)1, obs_len);
1040 errch_("#", targ, (ftnlen)1, targ_len);
1041 errch_("#", obs, (ftnlen)1, obs_len);
1042 sigerr_("SPICE(IDCODENOTFOUND)", (ftnlen)21);
1043 chkout_("SPKPOS", (ftnlen)6);
1044 return 0;
1045 }
1046
1047 /* After all translations are done we can call SPKEZP. */
1048
1049 spkezp_(&targid, et, ref, abcorr, &obsid, ptarg, lt, ref_len, abcorr_len);
1050 chkout_("SPKPOS", (ftnlen)6);
1051 return 0;
1052 } /* spkpos_ */
1053
1054