Bernese GPS Software Version 5.0 - Bernese GNSS Software

10.2.2.4 Fixing Reference Coordinates
10.3 Coordinate and Velocity Estimation in Practice
If the coordinates of reference stations in the observed network are not estimated but kept
fixed they define the geodetic datum for the GNSS network solution. All other estimated
coordinates then refer to that reference frame.
There are certain risks involved in fixing station positions. The reference coordinates may be
incorrect or less accurate than the computed GNSS solution would allow, a reference station
may have tracking problems and show a bad performance. In either case the estimated
network will be distorted decreasing the quality of all parameters. On the other hand if you
have very accurate coordinates of reference stations and your current network solution is
less accurate (e.g., very short observation time) the network solution may be improved by
fixing the good quality reference site coordinates.
In any case, fixing site coordinates is not recommended because the corresponding coordinate
parameters are removed from the normal equation system. Thus the datum definition
is frozen and can not be changed anymore. Use tight constraints instead (e.g., 0.01 mm).
10.2.3 Reference Site Verification
There are several reasons why the coordinates of reference stations may become invalid
for datum definition, e.g., co-seismic deformations, human interaction, or other influences
damaging the antenna installation. Therefore, it is advisable to verify the coordinates of
reference stations before using them for datum definition. This is especially important if
the coordinates are to be fixed or tightly constrained.
In a minimum constraint solution, the resulting network geometry is independent from the
reference sites. Thus, it is well suited to check the coordinates of reference sites by comparing
the estimated coordinates with the a priori values based on a Helmert transformation. The
program HELMR1 provides the possibility of an automatized reference site verification and
selection. A detailed program description is given in Section 10.6.2.
An abrupt change in the reference coordinates may also be detected by comparing coordinates
from the current session with those from previous ones using program COMPAR (see
Section 10.6.4). Be aware that a datum definition problem may affect the repeatability of
all sites in the network.
10.3 Coordinate and Velocity Estimation in Practice
Before detailing on specific aspects let us first address some general issues concerning coordinate
and velocity estimation with the Bernese GPS Software. There are several programs
in Version 5.0 which may be used to estimate station coordinates and velocities. Table 10.2
lists these programs together with the respective estimation approach.
Station coordinates estimated by the two preprocessing programs CODSPP and MAUPRP
have a limited accuracy because they are based on code measurements, only, or on result
from an epoch-difference solution, respectively. If the PPP procedure has been executed
Bernese GPS Software Version 5.0 Page 217