Bernese GPS Software Version 5.0 - Bernese GNSS Software

5.4.1 Conversion of Precise Orbit Information to Tabular Format
5.4 Preparation of Precise Orbit Information
To generate a standard orbit the program ORBGEN may read a so-called tabular orbit file or
directly precise orbit files (see next section). The program PRETAB (”Menu>Orbits/EOP>Create
tabular orbits”) may be used to generate the intermediate tabular orbit files (default extension
TAB) but it allows in addition to extract clock information from the precise orbit files and
write it to a Bernese satellite clock file (see Section 5.5).
Program PRETAB converts the satellite positions from the Earth-fixed system to the inertial
system J2000.0. For this transformation the Earth orientation parameters (EOPs, or Earth
rotation parameters, ERPs) delivered in the form of a pole file together with the precise file
should be used in order to guarantee maximum consistency. Using a different pole file will
in general result in a orbital fit of lower quality.
In any program reading orbit information it is mandatory to always use the standard orbit
(based on a tabular file) together with the pole file and the nutation and subdaily Earth
rotation information which you used in PRETAB to create the tabular orbit. Only in this
way you assure that the transformation from Earth-fixed to inertial system (to generate
tabular and standard orbit files) is the same as the transformation back to the Earth-fixed
system (e.g., in program GPSEST).
5.4.2 Generation of Standard Orbit Information
The orbit information in standard orbit format is generated by program ORBGEN based on
one of the following input (see panel “ORBGEN 1: Input Files”):
• Tabular orbit files generated by program PRETAB or BRDTAB.
• Precise orbit files. In this case the Earth orientation parameter files accompanying the
orbits must have the same name as the precise orbit files (but different extension). The
same pole files have necessarily to be used in all cases where the resulting standard
orbits are used by any program.
• Element files. This input is only required if orbits are improved (see Chapter 15 for
details).
It is important to know that in the second option listed above, the Earth orientation information
for transforming the satellite positions to inertial space is not taken from the file
specified in option “Pole file”, but from an ERP-file with the same name as the precise orbit
file. To avoid confusion it is safer to work with the first of the above options (i.e., using
program PRETAB for orbit transformation). Note that satellite clock information can only
be extracted from precise orbit files by program PRETAB, an additional argument to work
with intermediate tabular orbit files.
For the standard application, the program is executed once for each input precise or tabular
orbit file. The time window specified in panel “ORBGEN 5: Orbital Arc Definition” covers the
time interval for which the input satellite positions are defined (in general one day). Program
ORBGEN may, however, also be used to extrapolate satellite orbits beyond the time interval
covered by the input tabular positions by specifying the time window accordingly. Care has
to be taken if extrapolating backwards. In this case the number of iterations may have to
be increased.
Bernese GPS Software Version 5.0 Page 91