Bernese GPS Software Version 5.0 - Bernese GNSS Software

7.7.1.2 Processing LEO Data
7.7 Special Applications
GPSEST allows the computation of dynamic, reduced-dynamic, and kinematic orbits for
LEOs in both, zero-difference and double-difference mode [Bock, 2004], [Jäggi, 2006]. For
most applications we recommend a PPP-style processing. Just like for ground stations zerodifference
observation files are processed introducing precise high-rate GPS clock corrections
which may, e.g., be downloaded from CODE (see Section 4.12.1). These clocks have to be
consistent with the GNSS satellite orbits and the used Earth orientation parameters. PPP
for ground stations is described in Section 10.5.
Sensor offsets are applied using the satellite’s attitude information which may stem from an
attitude file (see Sections 22.7.13 and 7.7.1.3), or they are computed based on the attitude
flag specified in the satellite information file. Since each LEO mission provides the satellite
attitude information in a different file format we will describe the procedure how these files
may be used within the Bernese GPS Software, Version 5.0 in Section 7.7.1.3.
Some programs have an option to explicitly activate the LEO data processing. In general,
this option can be found in the top panel of the corresponding programs (e.g., “GPSEST 1.1:
Input Files 1”, “LEO data processing”). Tropospheric refraction is automatically disabled for
a spaceborne receiver in all processing programs. At LEO altitudes, the ionosphere has in
general a still significant effect on the signal propagation. This effect needs to be eliminated
by analyzing the ionosphere-free linear combination. Usually the elevation cutoff angle can
be set to a very low value or even to zero.
An elevation-dependent weighting may be appropriate if significant multipath is present.
The processing steps for a LEO orbit determination are the following:
(1) The import of a LEO RINEX file is done with program RXOBV3 (see Section 4.2.3).
(2) An initial orbit may be generated based on code observations using program CODSPP
in kinematic mode (see Section 6.3.3). Within this CODSPP run the receiver clock is
synchronized to GPS system time, too. Although no files are available to be selected
for “LEO standard orbit” in panel “CODSPP 1.2: LEO Files”, the checkbox “LEO files” has
to be activated in panel “CODSPP 1: Filenames”.
(3) The resulting kinematic coordinate file (description in Section 22.8.9) has to be converted
to a precise orbit file (description in Section 4.4.1) with program KINPRE. This
program is not accessible through the menu and has to be executed using the RUNGPS
command (see Section 18.8).
(4) From the precise file a standard orbit is computed using programs PRETAB and
ORBGEN, following the instructions in Section 5.4. The orbit integration has to be
based on a recent gravity field model (e.g., ${X}/GEN/EIGEN2.) with a degree and
order exceeding 70. Select an integration step size of at least 0.05 hours (3 minutes)
for the equation of motion in panel “ORBGEN 3.2: Options”, option “EQUATION OF MO-
TION: Length of interval”. You have to pay attention to set the integration step size to
the same value for all subsequent ORBGEN runs for the Low Earth Orbiter. In order
to have the possibility for an orbit improvement, a “Radiation pressure coeff.” file has to
be specified in panel “ORBGEN 2: Result and Output Files”). You may set up all dynamical
parameters in panel “ORBGEN 4: Parameter Selection”. The dynamical parameters
Bernese GPS Software Version 5.0 Page 157