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luntab.

A table of the Moon's coordinates

Overview

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:

  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, 3f11.3),
  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 = 0.5 days (although positive numbers are used directly, negative numbers are interpreted as 2**interval days, therefore the header value corresponding to 0.5 days would be -1),
  7. scaling factor = 1.0,
  8. and the inertial frame is read from the header.

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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luntab.html
January 14, 2000
Steve Hilla