Bernese GPS Software Version 5.0 - Bernese GNSS Software

6. Data Preprocessing
6.3.2 Preprocessing of Code Observations
If the option “Outlier detection” is enabled (see panel “CODSPP 4: Screening Options”) the
program CODSPP fully replaces program CODCHK which was necessary in old versions of
the Bernese GPS Software.
The program CODSPP processes the observation files epoch by epoch. The term observed
minus computed (O–C) is calculated for each observation of an epoch introducing the satellite
orbits, satellite clocks, and the a priori position of the station (or the position that
resulted from the previous iteration). Based on these O–C values the receiver clock correction
for this particular epoch is obtained by a majority voting. After correcting the O–C
values for all observations for the receiver clock correction the residuals are analyzed for
outliers. If the outlier detection is disabled or the biggest residual is smaller than a user
specified threshold (Option “Max. residual allowed”) all observations of this epoch are accepted
and will contribute to the coordinates and/or the GPS/GLONASS time offset.
If the residual of at least one observation of an epoch exceeds the threshold specified the
outlier detection procedure starts. From all residuals of the epoch a noise level (RMS without
the worst satellite) is then computed. Either this noise level or the RMS of unit weight from
the previous iteration (the smaller value is selected) is scaled by the user defined “Confidence
interval” to get a threshold for the biggest residual of the epoch. If the threshold is exceeded
the observation corresponding to the biggest residual is flagged and the majority voting to
compute the receiver clock correction is repeated without the flagged observations.
When the receiver clock corrections are written to the observation files also the flagged
observations may be marked in the code, phase, or both observation files (see option “Mark
outliers in obs. files”). Usually only the code observations should be marked because the outlier
detection is done for this observation type only. If you don’t use the code observation files
in further processing, marking of bad observations is not necessary.
Because the noise level of the raw code measurements may vary considerably between different
receiver types, the use of station observation sigma factors (station weighting) is highly
recommended to scale the outlier threshold. The threshold (option “Max. residual allowed”)
is multiplied with the station specific sigma factor to obtain the effective outlier rejection
threshold for the respective station. A sigma factor below 1.000 is set to 1.000. If you are
using smoothed code from program RNXSMT the use of the sigma factors for the rescaling
of the outlier rejection threshold is not required. See Section 22.8.14 for the definition and
Section 6.6.3 for the creation of a station observation sigma file.
If you use CODSPP to get receiver clock synchronization for a phase only based baseline
processing you should choose a high outlier rejection threshold (e.g., 30 m). You have to
keep in mind that an epoch where CODSPP could not compute a receiver clock value for
the station will be lost for all further processing.
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