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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:
Julian date + 0.5
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:
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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