8.1.2 GPS Baseline Processing

SETTING UP THE SOLUTION

With reg ards to initialising a triple- or double-differenced data solution, the following usually can be influenced by the analyst:

• Defining the apriori groundmark coordinates, including that for the "datum" station to be held fixed (for example, from a pseudo-range point position solution, or a triple-difference phase solution, or a previous solution when "chaining" baselines). This involves correctly setting up the station file (usually from the recorded field data), with information on antenna height, etc.
• Identifying the ephemeris file to be used (may be a Navigation Message file, or the Precise Ephemerides).
• Any satellites to be excluded from solution (for example, because of known health problems).
• Identifying the baseline to be processed, by selecting the data files to be used (generally from a database of GPS files).
• Inputting the standard deviation of the differenced observations.
• If option is available for taking correlations into account, this may be exercised.
• Minimum elevation cutoff angle for data culling to low satellites.
• Data selection for solution (all data or some sample rate, for example every 5th data epoch).
• Tropospheric refraction model for bias may be activated, based on input met data or "standard atmosphere" values.
• Dual-frequency processing options to be exercised.
• Whether to attempt ambiguity resolution or not (in the case of double-differenced solution), and the test/validation parameters associated with the algorithm.

In general standard data processing in an operational environment is largely automatic, offering the analyst little choice. Such "blind" or "batch" processing is usually controlled by default processing options. These can be changed from time to time by experienced operators to activate appropriate processing options when the survey conditions demand it (for example in the case of very short baselines, or dual-frequency observations, or short session lengths, sessions with bad satellite geometry, etc.).

There is, however, a limit to the options offered by commercial software packages. This can lead to some frustration in a small number of cases, when the processing strikes problems and "trouble-shooting" through the exercise of radically different options is not possible. The best option in such cases may be to reobserve the baseline! (Research or scientific software used in academic or government organisations offer many more options.)

As the best solution results are normally provided by a double-difference solution, the "hard-wired" processing options for double-differenced data solutions are the ones of greatest interest. The following are likely to be internally defined by the software:

• Differencing strategy (between-satellites) to be used.
• Ambiguity parameter model, usually single- or double-differenced model.
• Criteria for judging success of ambiguity resolution on a parameter-by-parameter basis.
• Solution convergence criteria.
• Internal modelling of the satellite orbit.
• Ordering of the satellites (influences the between-satellite differencing process).

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