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soltab.

A table of the Sun's coordinates

Overview

The Sun is the source of two perturbing forces significant to the calculation of GPS satellite orbits, gravitational and radiation pressure. However, the prediction of the Earth's location relative to the Sun is a well determined, albeit non-trivial, calculation. For efficiency, arc reads a previously generated solar ephemeris to be used a priori input file. Currently, the soltab. file is generated yearly by MIT and made available via anonymous FTP.

The soltab. file has a two line header which describes the contents of the file followed by the ephemeris. The format of this file should be considered fixed. The first line of the header is purely descriptive and is ignored. The second describes the ephemeris and contains the following information:

  1. the format for reading/writing the ephemeris,
  2. an unknown parameter (initialization flag for ephemeris interpolator?),
  3. the start time in PEP days,
  4. the stop time in PEP days,
  5. the maximum number of values on any line,
  6. the interval, in days, between entries in this ephemeris,
  7. a scaling factor for the values in the table,
  8. and the inertial frame.
Note: internally, arc keeps the date in a variant of the Julian date called a PEP day to better distinguish it from a true Julian date. The PEP day equals
Julian date + 0.5
or
modified Julian date + 2400001.0.

In the current version of arc, only the start and stop time, and frame are read and used. The format for reading these values is: (31x, f7.0, 1x, f7.0, 7x, a5). The other parameters are "hardwired" to:

  1. format = (6x, 3f11.0),
  2. parameter = 0,
  3. the start time is read from the header,
  4. the stop time is read from the header,
  5. values per line = 3, i.e. the three vector components,
  6. interval = 4 days,
  7. scaling factor = 1.0,
  8. and the inertial frame is read from the header.

The ephemeris values are no longer read using the format and scaling factor given in the header. Each line contains the PEP date - 2400000, and the components of the Sun - Earth vector for that epoch. Remember that this vector is expressed in the inertial frame shown in the file header. The first entry is for the given start time; subsequent entries are for every fourth day.

Note: All vector components are in kilometers. These values are no longer read using the format and scaling factor given in the header and entries must be consistent with this format.

Example 


Earth ephemeris for Dec 95 - 22 Dec 96   rwk 23 Sept 95                         
(1x,i5,6i11)                 0 2450041 2450457  3  4          1.E+00 B1950
 50041   83681276  111843292   48490507
 50045   74824094  116879737   50673791
 50049   65597565  121338440   52606847
 50053   56050151  125196384   54279830
 50057   46229796  128436239   55685003
 50061   36183139  131043395   56815729
 50065   25956869  133004627   57666095
 50069   15599019  134308035   58230967
 50073    5160096  134943503   58506199
 50077   -5305659  134904527   58489318
 50081  -15742332  134191613   58180611
 50085  -26096996  132811201   57582597
 50089  -36319897  130772283   56698890
 50093  -46362881  128085349   55533907
 50097  -56178220  124762345   54092903
 50101  -65717484  120817212   52382193
 50105  -74930704  116267745   50409803
 50109  -83768748  111138440   48186350


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soltab.html
January 14, 2000
Steve Hilla