Bernese GPS Software Version 5.0 - Bernese GNSS Software

6.2 Preprocessing on the RINEX Level
the arc with a maximum RMS value specified by the user. Outliers are only temporarily
removed in this step. The difference between the two (clean) parts is estimated and the arcs
are connected using the estimated cycle slip. All points that were considered outliers during
the cycle slip detection are included again. The RMS is recomputed to check whether there
are more cycle slips in this observation arc.
After the detection of all cycle slips, the observations are screened for outliers. Outliers are
removed (using a 4σ-threshold) until the RMS of the observation arc is below the specified
maximum. The specified maximum RMS is typically 0.4–0.6 wide-lane cycles (34–52 cm).
If an outlier is detected, all four observation types (code and phase on two frequencies) are
rejected.
6.2.2 Data Screening Based on Geometry-Free Linear Combination
Only those observation arcs in which cycle slips have been detected are screened using the
geometry-free combination of the phase observations (see Eqn. (2.46)). At this stage the size
of the wide-lane cycle slip (n1 − n2) is known. The geometry-free linear combination (L4)
allows us to compute the size of the n1 and n2 cycle slips because it gives us: L4 = L1 −L2.
To determine the size of the cycle slip on the L4 linear combination two linear polynomials
are fitted through m points (m is defined by the user in option “Number of L4 observations for
fit” in panel “RNXSMT 2.3: Options”), before and after the cycle slip. The difference between
the two polynomials at the time of the cycle slip is computed. If the fractional part of
the difference is smaller than a user specified limit (option “RMS of L4 for fit and cycle slip
correction” in panel “RNXSMT 2.3: Options”) the n4 cycle slip is accepted and the cycle slips n1
and n2 are computed. Typically a value of m = 10 is used and a difference smaller than
10 mm. This procedure is executed to be able to connect the code observations during the
code smoothing step. Because cycle slips occur rarely, no attempt is made to connect the
phase observations. For the phase observations a new ambiguity is set up at the epoch of
the detected cycle slip.
6.2.3 Data Screening Based on Ionosphere-Free Linear Combination
Sometimes the described procedure is not fully successful to clean the data due to systematic
errors in the Melbourne-Wübbena combination. These systematic errors are most likely
caused by the filtering and smoothing procedures employed in the receivers. Therefore, an
additional data screening step is necessary.
The difference between ionosphere-free linear combinations of the phase and code observations,
Eqns. (2.41) and (2.42), i.e., L3 − P3, is used in this step. As in the case of the
Melbourne-Wübbena combination this linear combination should consist of noise only, too.
The disadvantage is an amplified noise (about 3 times the noise of the P1 observations),
which is about 4 times larger than the noise of the Melbourne-Wübbena combination. Nevertheless,
the check is useful for removing errors caused by systematic effects. It consists of an
outlier rejection scheme very similar to the one used for screening the Melbourne-Wübbena
combination. The starting value for the maximum RMS is larger (typically 1.6–1.8 meters
for option “RMS of an arc in ionosphere free LC (L3-P3)” in panel “RNXSMT 2.3: Options”) to
account for the higher noise of these observations.
Bernese GPS Software Version 5.0 Page 105