Bernese GPS Software Version 5.0 - Bernese GNSS Software

6. Data Preprocessing
is set to 2. All observations considered as good have a signal to noise ratio of 9. From this
convention you can derive the options in panel “RXOBV3 4: Input Options 2” for converting
smoothed RINEX files into Bernese observation file format using the program RXOBV3 (see
Figure 6.4). All other settings and checks in the program RXOBV3 are identical for original
and smoothed RINEX files. They are described in Section 4.2.3.
6.3 Receiver Clock Synchronization and
Preprocessing of Code Observations (CODSPP)
6.3.1 Receiver Clock Synchronization
In Section 2.3.5 we have seen that the receiver clock has to be synchronized with GPS time
even for a double-difference analysis. The receiver clock error δk has to be known with an
accuracy better than 1 µs. It would be possible to introduce δk as unknown parameters
during the final least-squares adjustment in program GPSEST, but this would increase
the number of parameters considerably. Fortunately, it is possible to compute δk a priori
with sufficient accuracy (< 1 µs) using zero-difference code measurements. This is the
main task of program CODSPP (”Menu>Processing>Code-based clock synchronization”), the second
important result from this program are the receiver coordinates. Looking at Eqn. (2.29) we
conclude that if we want to reach an accuracy of 1 µs in δk it is necessary to have the code
measurements available with an RMS error smaller than
c (δk)max = c (1 µs) ≈ 300 m
(c is the velocity of light). Obviously, even C/A-code measurements are sufficient for this
purpose. CODSPP will process P-code or smoothed code, of course, if available.
For the zero-difference analysis of the data in the parameter estimation program GPSEST
the results of the receiver clock synchronization are introduced as a priori values for the
station clock estimation. Because the dependency of the observation on the clock parameters
is almost linear, the clock a priori value is not very important. On the other hand the
receiver clock synchronization on the 1 µs level is necessary in the zero-difference case, too.
In addition, the numerical stability in program GPSEST is improved when the estimates for
the clocks are not too big.
Program CODSPP uses the standard least-squares adjustment to compute the unknown
parameters. The observables are the zero-difference code measurements. Usually, the L3
(ionosphere-free) linear combination is used. The most important parameters computed by
CODSPP are the receiver clock corrections δk. These parameters will be estimated in any
case. The resulting receiver clock corrections are written directly into the Bernese observation
files. This may be suppressed if the option “Save clock estimates” in panel “CODSPP 2:
Input Options” is disabled. This may be useful for test purposes. In the normal processing
this options has to be BOTH which is indicated in the program output file by the line:
CLOCK OFFSETS STORED IN CODE+PHASE OBSERVATION FILES
for each station. Otherwise a warning message is issued.
Page 108 AIUB