Index Page SUBPT User's Guide

## Table of Contents

       SUBPT User's Guide
          Abstract
          Summary

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# SUBPT User's Guide

Last revised on 2008 FEB 07 by N. J. Bachman.

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## Abstract

SUBPT is a cookbook program that demonstrates how to use CSPICE Toolkit routines to compute a sub-observer point.

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## Summary

The SUBPT ``cookbook'' program illustrates the use of CSPICE Toolkit software for solving a typical geometrical problem --- computing the apparent sub-observer point on a target body using light time and stellar aberration corrections.

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:

prompt_c
  Interactively prompt user for a string input

furnsh_c
  Load SPICE kernels

str2et_c
  Convert time string to ephemeris time

subpnt_c
  Calculate the sub-point

et2utc_c
  Convert ephemeris time to UTC string

To run SUBPT, you need a binary SPK ephemeris file and knowledge of the bodies and the corresponding time intervals contained in that file. The utility program named BRIEF summarizes the contents and time coverage of a binary SPK file. Refer to NAIF IDs Required Reading (naif_ids.req) for a list of body names and integer codes. In addition to an SPK file, you also require access to leapsecond (LSK) and planetary constants (PCK) kernels.

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