Bernese GPS Software Version 5.0 - Bernese GNSS Software

10. Station Coordinates and Velocities
The geodetic datum has to be defined for estimated velocities, also. Datum definition of coordinates,
e.g., to ITRF, does not implicitly define the velocity datum to the same reference
frame. The velocity datum may be defined, as for positions, with minimum constraints or
by constraining velocity vectors of reference sites to a priori values.
10.2.1 Reference Frames Important for GNSS Analyses
The International Terrestrial Reference System (ITRS) is realized by the International Terrestrial
Reference Frame (ITRF). The ITRF consists of 3-dimensional Cartesian coordinates
and velocities for a set of globally distributed stations. The coordinates refers to a specific
time epoch. It is updated on a regular basis to include recent results from contributing
space techniques (GPS, SLR, LLR, VLBI, DORIS). The ITRF web site is advisable for
more information in this regard (http://itrf.ensg.ign.fr). Since ITRF97, the IGS uses
its own realization of the ITRF [Kouba et al., 1998]. This IGS realization is not necessarily
more accurate but more consistent.
All IGS products are based on the ITRF resp. IGS realization thereof (see Table 10.1
for an overview). Consequently, the ITRF can be accessed with an accuracy of below one
centimeter if IGS products are introduced in a GNSS analysis.
The broadcast ephemerides of GPS satellites refer to the WGS–84 reference frame. This
frame can only be realized via the broadcast orbits and clock corrections with a quality of
about 1 m in geocentric position. Nevertheless, the WGS-84 is close enough to the ITRF
so that corresponding coordinates may be adopted for reference stations when broadcast
ephemerides are used.
GLONASS broadcast information is given in the PZ-90 1 system which is rotated with
respect to the ITRF. This fact must be considered when GLONASS broadcast orbits are
used. However, GLONASS orbits provided by the IGS or CODE refer to the ITRF, thus
being consistent to the GPS orbits and needing no further transformation.
Satellite positions in precise orbit files always refer to a particular reference frame. The same
reference frame should be used for the ground stations during the analysis to ensure best
possible consistency. An analysis of the biases introduced into a regional solution if station
positions and satellite orbits are used inconsistently may be found in [Beutler et al., 1988].
A 1 m height bias at a fixed site will cause a scale effect of about 0.03 ppm. A bias in the
horizontal components causes a rotation of the network.
Bernese coordinate and velocity files are available at http://www.aiub.unibe.ch/
download/BSWUSER50/STA for the various ITRF and IGS realizations. A set of three files
are provided for ITRF2005, IGS05, and IGT05: coordinate and velocity files containing IGS
reference stations only (e.g., IGS 05 R.CRD and IGS 05 R.VEL) and a station selection file
listing these reference sites (e.g., IGS 05.FIX). The selection file can be used to easily select
the reference stations for datum definition in Bernese programs.
The change to the ITRF2005 reference frame within the IGS was associated with the switch
from relative to absolute antenna phase pattern modeling (see Section 16.1). This model
change has an impact on the estimated coordinates of the reference stations. The ITRF2005
coordinates are still derived using the relative model. IGT05 is the corresponding IGS
1
PZ-90 (ÈÖÑØÖÑÐ) is an Earth-centered and Earth-fixed reference frame.
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