Bernese GPS Software Version 5.0 - Bernese GNSS Software

10.2 Defining the Geodetic Datum for a Tracking Network
Table 10.1: History of reference frames used for IGS products.
Used for IGS products between
Frame GPS week Date
ITRF92 0730 – 0781 02 Jan. 1994 – 31 Dec. 1994
ITRF93 0782 – 0859 01 Jan. 1995 – 29 June 1996
ITRF94 0860 – 0947 30 June 1996 – 07Mar.1998
ITRF96 0948 – 1020 08Mar.1998 – 31 July 1999
ITRF97 1021 – 1064 01Aug. 1999 – 03 June 2000
IGS97 1065 – 1142 04 June 2000 – 01 Dec. 2001
IGS00 1143 – 1252 02 Dec. 2001 – 10 Jan. 2004
IGS00b 1253 – 1399 11 Jan. 2004 – 04 Nov. 2006
IGS05 a 1400 – 05 Nov. 2006 –
a The switch to the IGS05 reference frame was associated with the change from
the relative to absolute antenna phase center modeling within the IGS. This
model change has an impact on the coordinates of the reference sites, see text.
realization, also to be used together with the relative antenna model. The coordinates in the
IGS05 are corrected for the difference between the relative and absolute antenna models. It
has to be used together with the absolute model. The velocity files (.VEL) and the reference
station selection files (.FIX) are identical for IGT05 and IGS05.
10.2.2 Datum Definition Types
10.2.2.1 Free Network Solution
A network solution generated without explicitly introducing any datum information is called
a free network solution in the Bernese GPS Software. The fixed orbits are then the only
external information defining the geodetic datum. The network geometry is derived from
the GNSS observations and not affected by (possibly bad) reference coordinates. However,
the resulting coordinates do not refer to a well defined reference frame rendering a free
network solution unusable to compute precise results.
The estimated network will show considerable translations for different days leading to significant
day-to-day coordinate variations. Therefore, the results of a free network solution
must be transformed into an appropriate reference frame using, e.g., Helmert transformations
(program HELMR1). The daily transformations, however, may accidentally remove
part of a geodynamical signal from the time series. Because the network translations are
an effect of correlations with other parameters and small modeling deficiencies, e.g., of
troposphere delays, an explicit datum definition is advisable in all cases.
There are two main applications for a free network solution. It may be used in program
GPSEST to create normal equations only, regardless of other results. The geodetic datum
must then be defined later in program ADDNEQ2. The second application is the precise
point positioning (see Section 10.5) where not only the satellite orbits but also the satellite
clocks are introduced. The resulting coordinates will then share the reference frame defined
by the orbits and clocks. The free network solution ensures that no additional constraints
are imposed on the estimated coordinates.
Bernese GPS Software Version 5.0 Page 215