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The Moon is the source of a significant perturbing gravitational force which must be included in the calculation of GPS satellite orbits. However, the prediction of the Earth's location relative to the Moon is a well determined, albeit non-trivial, calculation. For efficiency, arc reads a previously generated lunar ephemeris to be used a priori input file. Currently, the luntab. file is generated yearly by MIT and made available via anonymous FTP.
The luntab. 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:
Each line contains the PEP date - 2400000, and the components of the Moon - 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 0 hours of the given start time; subsequent entries are for every twelve hours.
Note: All vector components are in meters. When read with an f11.3 format, the vectors are effectively converted from meters to 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
Moon ephemeris for Nov 96 - Feb 98 rwk 18 Dec 96 (1x,i5,6i11) 0 2450361 2450920 3 -1 1.E-03 B1950 50361 -33335240 378889667 124105243 50361 -75223653 375088390 122337279 50362 -116233558 366903234 119139477 50362 -155906323 354467759 114561352 50363 -193803051 337952580 108664338 50363 -229505987 317563594 101521192 50364 -262619980 293540575 93215515 50364 -292774134 266156058 83841367 50365 -319623754 235714398 73502936 50365 -342852650 202550878 62314221 50366 -362175841 167030728 50398678 50366 -377342656 129547922 37888793 50367 -388140208 90523594 24925517 50367 -394397166 50403932 11657536 50368 -395987712 9657443 -1759678 50368 -392835543 -31228544 -15165054 50369 -384917735 -71752186 -28393389 50369 -372268270 -111402293 -41277085
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