Bernese GPS Software Version 5.0 - Bernese GNSS Software

5. Preparation of Earth Orientation, GNSS Orbit, and Satellite Clock Information
The output of the program ORBGEN is a standard orbit file (default extension STD) containing
the satellite orbit information and a so-called radiation pressure file (default extension
RPR) containing the derivatives of the satellite positions with respect to the initial conditions
and the dynamical parameters. The second file is only needed if orbits are to be improved
with program GPSEST (or ADDNEQ2). For details see Chapter 15.
The standard orbit may be composed of one or more arcs, each of which characterized by a
start and an end time. Each standard arc is a solution of the equations of motion, Eqn. (2.8),
characterized by six initial conditions and a user-specified number of dynamical parameters.
A maximum of nine deterministic parameters per satellite are possible. Standard orbits
generated from an orbital element file may be characterized additionally by stochastic pulses.
The multi-arc option of the program is not recommended (panel “ORBGEN 5”) and not
necessary because a multi-day solution may be produced using ADDNEQ2. In case a multiday
solution with GPSEST is indispensable several arcs may be generated each covering one
day. It is recommended to always generate standard orbits covering one day (or a multiple
thereof) even if the data to be processed covers a shorter time interval. The only exception
is the Precise Point Positioning (PPP) where shortening the length of the orbital arcs
improves the capability to recover the orbits from software packages using slightly different
orbit models than ORBGEN.
All standard orbits and all partial derivatives (variational equations) are computed by numerical
integration of the equations of motion in program ORBGEN (see Section 2.2). No
other program in Version 5.0 performs numerical integration. If tabular or precise files are
selected as input the program uses the satellite positions as pseudo-observations in an orbit
determination process (one such process per arc and satellite). The orbit always refers to
the center of mass of the satellite. The satellite antenna phase center correction is applied
in every program that reads the orbit file. No antenna offset has, therefore, to be applied
when generating the standard orbit (see panel “ORBGEN 3.1: Options”, option “Apply antenna
offset”). The corresponding option is available only for test purposes.
The program supports several force models.The models may be selected in panel “ORB-
GEN 3.1: Options”, see Figure 5.4. Details on the models may be found in Table 5.2. For
retaining backward compatibility old orbit models are still available. New models are included
for testing purposes. It is recommended, however, to use either model “B” or model
“D” for the GNSS orbit generation which are compliant with the IERS conventions.
Apart from the radiation pressure parameterization (see Section 2.2.2), the new models
use the modern gravity models (JGM3, EGM96, EIGEN2, TEG4), the DE200 development
ephemerides from JPL [Standish, 1990], account for the gravitational attraction of Sun,
Moon and the major planets (Jupiter, Venus, and Mars), and consider general relativistic
corrections. They apply elastic Earth tidal corrections according to IERS 1996 conventions
[McCarthy, 1996] applying all step 1 corrections and the step 2 corrections for the seven
largest frequency dependent corrections of C21 and S21, pole tide, and permanent tide
corrections (subroutine TIDPT2), and ocean tide corrections up to the four terms larger
than 0.05 cm (subroutine OTIDES).
Program ORBGEN checks the consistency of options and input files in order to guarantee the
use of a well defined orbit model. Table 5.3 summarizes the settings for the old orbit model
(“0”, used up to 1996), and the new models (“B” and “D”). The orbit model is selected in
panel “ORBGEN 3.1: Options”, the planetary ephemeris file, geopotential model, and ocean
tides file are selected in panel “ORBGEN 1.1: General Files”.
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