Bernese GPS Software Version 5.0 - Bernese GNSS Software

10.2 Defining the Geodetic Datum for a Tracking Network
The format of the ocean tidal loading file and the procedure how to add more stations is
described in Section 22.8.11. A file containing a subset of the IGS stations is available at the
anonymous FTP site http://www.aiub.unibe.ch/download/BSWUSER50/STA/FES2004.
BLQ (see Section 4.12). The file http://www.aiub.unibe.ch/download/BSWUSER50/TXT/
BLQ.README contains additional information and more details. Make sure to use a consistent
set of coefficients (e.g., the same ocean tidal model for all stations). If a station is not
found in the file a warning message is issued and no corrections are applied to this station.
10.1.4 Other Site Displacements
There are many other effects causing site displacements like atmospheric loading, nontidal
ocean loading, post-glacial rebound, or varying ground water level. The Bernese GPS
Software, Version 5.0 , does not apply models for these effects because they are rather
small, they change the station position only very slowly, or no conventional models are
available. Nevertheless, these effects are measurable with GNSS and may be investigated in
site position time series.
10.2 Defining the Geodetic Datum for a Tracking Network
GNSS is basically a differential technique due to the fact that the measurements are related
to absolute ranges between receivers and satellites through unknown clock corrections and
phase ambiguity parameters, only. The values of this large amount of parameters have to
be estimated, either explicitly or implicitly, irrespective of whether undifferenced or doubledifferenced
observations are processed. The only exception is the precise point positioning
where satellite clocks, known from other sources, are introduced as fixed, thereby defining
the geodetic datum of the solution.
The fact that for each observation epoch one clock bias per receiver and per satellite has to
be estimated (implicitly in the case of double-difference processing) as well as at least one
ambiguity parameter per link and satellite pass (or double-differences thereof) results in a
loose definition of the geodetic datum of a processed network. The smaller the geometric
extension of the network, the easier it is to compensate a translation of the entire network
by adapting clock and ambiguity parameters. As a consequence the geodetic datum of a
network solution has to be introduced as external information.
Using coordinates of one or several reference sites given in a well defined reference frame, the
estimated coordinates can be aligned to that frame. The way this alignment is performed
is called geodetic datum definition. In this section we discuss the different options provided
by the Bernese GPS Software.
In contrast to the absolute geometry, the internal geometry of the network is very well
determined by GNSS measurements because a shift of a single station in a network cannot
be compensated by simply adjusting clock and ambiguity parameters. The same is true for
the orientation of the network which is imposed by the reference frame in which the used
GNSS satellite orbits are represented (as long as no orbit parameters are estimated).
Constraining of sites on a rotated reference frame does, in fact, not change the orientation
of the estimated network, if orbits are introduced as fixed, but distorts the network.
Bernese GPS Software Version 5.0 Page 213