The GOA-II-specific  jpl.txt format represents satellite positions in an
earth-fixed  reference  frame as a  function  of time.  An example  of a
record (one line with 175 columns) is

11 -23553.38205 -11660.92636   2333.45344  -.32517466   .06044521 -3.49080214 1993  4 18 12  0  0.00   .12864E+01   .34824E+01   .72475E+01   .5721E+00   .2981E+00   .8688E-01

The fields are as follows:

 1  prn number
 2  x position (km)
 3  y    "      "     
 4  z    "      "
 5  x velocity (km/s)
 6  y    "       "
 7  z    "       "
 8  year (time is GPS time)
 9  month (1-12)
10  day
11  hour
12  minute
13  second
14  x formal error (m) (optional)
15  y   "      "    "      "
16  z   "      "    "      "
17  x velocity formal error (mm/s) (optional)
18  y    "       "      "     "        "
19  z    "       "      "     "        "

GPS satellite (0 < prn < 98) records are written with format

   (I2,3F13.5,3F12.8,I5,4I3,F6.2,3e13.5,3e12.4)

For c, perl, awk format as:
"%2d%13.5f%13.5f%13.5f%12.8f%12.8f%12.8f%5d%3d%3d%3d%3d\n"

while those  corresponding to Topex (prn = 0) and Explorer (prn = 98, 99
for the two antennae) are written with format

   (I2,3F13.6,3F12.7,I5,4I3,F6.2,3e13.5,3e12.4)

The sort hierarchy of the fields is not part of the specification.

Note that jpl.txt files created prior to 1993 August 2 use f format (not
e) in the formal error  fields.  This can cause  asterisks to be written
for satellites  whose data may have been  deweighted,  especially in the
velocity  formal  error  field.  (No  problems  of this  sort have  been
observed in the position formal error field, although  extensive  checks
have not been made.)

The program rsmsig is the primary  creator of  jpl.txt-formatted  files.
Others include sp32t, which converts  sp3-formatted files to the jpl.txt
format,  and  an   undocumented   program   called   ecef,  which  reads
broadcast-ephemeris  records and writes jpl.txt records.  These last two
do _not_ include the formal error fields.  Furthermore,  sp32t  includes
phony velocities (since the sp3 format does not necessarily  include any
velocity information).

Additionally, the coordinates may refer to the spacecraft center of mass
or the antenna  phase  center of the  spacecraft.  The center of mass is
more  common.  (Up  to  an   including   GPS  week  665,   however,  the
JPL-produced Flinn orbits referred to the antenna phase center.)