Bernese GPS Software Version 5.0 - Bernese GNSS Software

10.5 Precise Point Positioning
10.5 Precise Point Positioning
The precise point positioning (PPP) is a special case of zero-difference processing. As opposed
to other applications, the satellite clock corrections are not estimated (or differenced
out) but assumed to be known. They are introduced in the processing together with orbit
information and Earth orientation parameters. Parameters left to estimate are station clock
corrections, coordinates, and troposphere parameters.
This modus operandi has several consequences:
• PPP is not a differential technique. The geodetic datum must not be defined by means
of any constraints on reference stations. It is given by the introduced orbit and clock
information and can not be changed.
• Consistency of introduced orbit, EOP, and satellite clock information is of utmost
importance to reach high accuracy. It is mandatory to use the information from the
same source. Mixing of orbits and clocks from different Analysis Centers degrades the
PPP results.
• The models used to estimate the orbits and clock corrections should be the same as
the models implemented in the actual analysis software. This issue poses no problem
when IGS or CODE products are used together with the Bernese GPS Software.
The quality of the introduced information should be as good as possible for PPP as all
errors directly propagate in the results. One kind of inconsistencies may arise when using
orbits from other sources as CODE, e.g., from the IGS. CODE final orbits can be fitted by
program ORBGEN on the sub-cm level whereas this can not be guaranteed for other orbit
products due to slightly different orbit models employed. In such cases the orbit fit may
be improved by setting up not only one but several orbital arcs per day, e.g., two or four
(“ORBGEN 5: Orbital Arc Definition”).
Furthermore, the satellite clock corrections should be used as provided and not be interpolated.
Thus, data processing is limited to the sampling rate of the clock corrections.
I.e., using satellite clock information from IGS clock RINEX files means a data sampling
of 5 min, only. CODE provides high-rate satellite clocks with a sampling of 30 s allowing
higher sampling rates. Use these clocks together with CODE orbits. Section 14.3.1 describes
the necessary steps to extract satellite clock corrections from clock RINEX files.
PPP is a very fast and efficient means to generate good station coordinates. Nevertheless,
it is not possible to reach a coordinate quality as obtained from a network analysis. This
is mainly due to two facts, the impossibility to resolve phase ambiguities and the neglect
of correlations between stations and clock corrections. The latter needs some further explanations:
in a network solution all parameters are estimated to fit all observation data
best possible under consideration of all correlations. This is not the case for a PPP, where
the influence of the stations on the satellite clocks can not be accounted for anymore. As
a consequence, in particular for local and regional networks, small modeling deficiencies
(e.g., concerning troposphere modeling) may be absorbed in the satellite clocks estimated
implicitly in a double-difference solution while this mechanism does not work for PPP. A
network solution is therefore the first choice to obtain highest quality positioning results.
Bernese GPS Software Version 5.0 Page 231