Bernese GPS Software Version 5.0 - Bernese GNSS Software

6. Data Preprocessing
The ambiguities from both zero-difference phase observation files are merged when forming
a single-difference observation file when the options “SETTING OF NEW AMBIGUITIES” in
panel “SNGDIF 3: Options” are disabled (empty input field for “After a gap in the observations
larger than”). This may be of interest if your zero-difference observation files are already
cleaned by the program MAUPRP in the zero-difference mode or by RNXSMT. Usually the
ambiguities are redefined when the single-difference phase observation files are screened with
program MAUPRP (see Section 6.5).
6.4.1 Algorithm for Baseline Selection
Let us assume that m receivers are used simultaneously. Let us further assume that the
same satellites are tracked by all receivers (these assumptions are true in local campaigns).
We have thus m zero-difference measurements to each satellite at each epoch (and each
carrier). If we use single-difference observations, only m − 1 independent single-differences
may be formed.
If the assumption that the same set of satellites is tracked by all receivers is not correct
(global campaigns), it would be better to optimize the forming of single-differences for each
epoch. However, the data handling would be tremendous in such case. We use a compromise
in the Bernese GPS Software. We create only one set of m − 1 baselines for the entire session
(and store the observations into the single-difference files — each single-difference file
corresponding to one baseline and one session), but we optimize the selection of these independent
baselines. The algorithm used is known as maximum (or minimum) path method.
First, all possible baselines are ordered according to a user-defined criterion (either baseline
length or the number of available single-difference observables — see Section 6.4.2). Then,
all the receivers active in the current session are given the initial flag 0. We take the “best”
baseline into the optimal set, the two corresponding stations receive the flags 1. The variable
“maximum flag” is set to 1. Now, we proceed to the second baseline. If the corresponding
stations have flag 0, we change them to 2, and 2 is the value of the “maximum flag”, too. If
one station has flag 0 and the other 1, both flags will be set to 1 and the “maximum flag”
remains 1. From now on we proceed as follows: we select the next baseline according to our
criterion and make the distinction between the following four cases:
(1) Both stations of the new baseline have flags 0:
in this case, the two station flags are set to “maximum flag +1”, and we have to
increment the “maximum flag” accordingly.
(2) One station has flag 0 and the flag of the other station is not equal to 0:
in this case, the station with flag 0 receives the (non-zero) flag of the other station.
The “maximum flag” is not changed.
(3) The two flags are not equal and no flag is 0:
let us assume that the first station has a lower flag than the second one. We have
to change the flags of all stations which have the same flag as the first station. The
station flags are set to the flag of the second station.
(4) The two flags are equal and different from 0:
this means that the baseline is dependent and can not be added to the set.
This procedure is repeated until m − 1 independent baselines have been formed.
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