| SUBPT User's Guide |
Table of ContentsSUBPT User's Guide Abstract Summary SUBPT User's Guide
Abstract
Summary
The ``apparent sub-observer point'' is defined in this program to be the point on the target body that appears to be closest to the observer. The apparent sub-observer point may also be defined as the intercept on the target's surface of the ray emanating from the observer and passing through the apparent target body's center, but we don't demonstrate use of that definition here. See the header of the function subpnt_c for details. In order to compute the apparent location of the sub-observer point, we correct the position of the sub-observer point for both light time and stellar aberration, and we correct the orientation of the target body for light time. We consider ``light time'' to be the time it takes a photon to travel from the sub-observer point to the observer. If the light time is given the name LT, then the apparent position of the sub-observer point relative to the observer is defined by the vector from the sub-observer point's location (relative to the solar system barycenter) at ET-LT, minus the observer's location (again, relative to the solar system barycenter) at ET, where this difference vector is corrected for stellar aberration. See the header of the CSPICE function spkezr_c for more information on light time and stellar aberration corrections; see the header of the CSPICE routine subpnt_c for an explanation of how it applies aberration corrections. SUBPT demonstrates the use of the following high-level CSPICE functions:
SUBPT prompts you for the NAIF IDs or string name of a target body and observing body, the name of the body-fixed reference frame associated with the target body, the UTC end-points of a time interval, and the number of evaluations to perform over the assigned time interval. The program then computes the planetocentric coordinates of the apparent sub-observer point on the target body, printing to the terminal screen for each time in the interval. Below, find a sample session using SUBPT to calculate the latitude and longitude of the nearest point on the Earth to the Sun through a single day. SUBPT can be used with any SPK file containing appropriate data. Please note: FORTRAN and C versions of the program can output numerical values in slightly different formats. It is assumed the kernel files used by SUBPT exist in the current directory (i.e. the directory from which your execute SUBPT). This particular session was run on an Intel box using the LINUX operating system. First, create the binary SPK kernel cook_01.bsp by running the CSPICE Toolkit TOBIN application on the transfer format file cook_01.tsp located in the CSPICE data directory. The program also requires a leapseconds kernel to run; an example leapseconds kernel, cook_01.tls exists within the same directory. Now, execute SUBPT:
Welcome to SUBPT
This program demonstrates the use of CSPICE in computing
the apparent sub-observer point on a target body. The
computations use light time and stellar aberration
corrections.
Enter the name of leapseconds kernel file: cook_01.tls
Enter the name of a planetary constants kernel: cook_01.tpc
Enter the name of a binary SPK file: cook_01.bsp
Working ... Please wait.
Enter the name for the observing body: sun
Enter the name for a target body: earth
Enter the name of the target body-fixed frame: iau_earth
Enter the number of points to calculate: 24
Enter the beginning UTC time: jul 1 1990
Enter the ending UTC time: jul 2 1990
Planetocentric coordinates for the nearest point
on the target body to the observing body (deg).
Target body: earth Observing body: sun
UTC Time Lat Lon
----------------------------------------------
1990 JUL 01 00:00:00 23.00157 -176.92004
1990 JUL 01 01:02:36 22.99879 167.42991
1990 JUL 01 02:05:13 22.99600 151.77986
1990 JUL 01 03:07:49 22.99320 136.12981
1990 JUL 01 04:10:26 22.99039 120.47976
1990 JUL 01 05:13:02 22.98757 104.82970
1990 JUL 01 06:15:39 22.98473 89.17964
1990 JUL 01 07:18:15 22.98188 73.52958
1990 JUL 01 08:20:52 22.97902 57.87952
1990 JUL 01 09:23:28 22.97614 42.22946
1990 JUL 01 10:26:05 22.97325 26.57939
1990 JUL 01 11:28:41 22.97035 10.92932
1990 JUL 01 12:31:18 22.96744 -4.72075
1990 JUL 01 13:33:54 22.96451 -20.37082
1990 JUL 01 14:36:31 22.96157 -36.02090
1990 JUL 01 15:39:07 22.95862 -51.67097
1990 JUL 01 16:41:44 22.95566 -67.32105
1990 JUL 01 17:44:20 22.95268 -82.97114
1990 JUL 01 18:46:57 22.94969 -98.62122
1990 JUL 01 19:49:33 22.94669 -114.27131
1990 JUL 01 20:52:10 22.94368 -129.92140
1990 JUL 01 21:54:46 22.94065 -145.57149
1990 JUL 01 22:57:23 22.93761 -161.22158
1990 JUL 02 00:00:00 22.93456 -176.87168
Continue? (Enter Y or N): N
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