Sect_4.3.4

4.3.4 GPS Surveying Software:

C0MMENTS ON PHASE DATA REDUCTION

This component is at the "heart" of the GPS software package. Phase data reduction software tends to fall into three broad classes:

Software developed by the instrument manufacturer, and offered as a "package" with the GPS receiver hardware to address standard land surveying applications. The so-called "commercial off-the-shelf" (COTS) software.

Software developed by third-parties, generally government departments or academic institutions intended for very high precision "scientific" (or geodetic) applications.

Software intended for "specialist" (or unusual) applications, such as to support airborne and marine operations, GIS data capture, multi-antenna attitude determination systems, GPS systems integrated with other sensors (including aerial cameras, inertial navigation systems, etc.).

There are several distinctions to be made between these classes of software:

The commercial and specialist software is invariably written to handle data from one instrument type. The scientific software is instrument independent, accepting data in the RINEX format.
The commercial software tends to be "user-friendly", requiring a minimum of analyst input and runs on PC computers. The scientific software tends to have been developed for research and precise positioning purposes, offering many options and requiring more analyst skill to use. In addition, because such software has many more features and supports more complex data modelling, the computer requirements are generally more severe.
The commercial software is optimised for GPS surveying accuracies (a few parts per million relative positioning accuracy), whereas scientific software generally addresses very high accuracy applications. The scientific software has more sophisticated modelling and processing strategies, such as the ability to adjust orbital parameters, estimate tropospheric scale factors, process more than one observation session simultaneously, etc.
The commercial software tends to use sub-optimal data processing algorithms, typically processing data on a single baseline mode (even if more than two receivers were operating simultaneously), whereas the scientific software has multi-baseline and multi-session capability.
The range of specialist software is growing rapidly. Some specialist software may be quite "polished" products, others may have rather crude interfaces.

Although these above remarks are generalisations, there is nevertheless a difficult compromise to be made between insisting that the operation of the manufacturers' commercial software be largely transparent, and: (a) having the data processed in as mathematically rigorous a manner as possible, or (b) the software having the necessary versatility so as to address many of the less commong applications as well. Such a tension is not evident with scientific software developed specifically for high accuracy applications, or software developed for specialist applications.

It is not the intention of these notes to address topics in "GPS geodesy", or to comment on many of the unusual applications that GPS is increasingly being called to address. The focus will instead be on studying the elements of GPS phase data reduction software which are commonly found in the COTS software provided by the instrument manufacturers as part of their GPS "package". Some of these elements are:

There is generally a sequence of processing steps from the least rigorous to the "optimal" solution, for example:
- triple-difference solution: moderate accuracy, but robust (insensitive to cycle slips) and hence ideal for preliminary station coordinate solutions,
- double-difference solution with free ambiguities: for long baselines this may be the "best" solution obtainable,
- double-difference solution with fixed ambiguities: for short baselines when the estimated ambiguities can be "resolved" to their nearest correct integer value, and the solution repeated using "phase-range" or "carrier-range" data.
The comments above are valid for conventional static GPS baseline solutions. Modern "rapid static", "stop & go" and "kinematic" survey techniques require fixed ambiguity solutions only.
Software for dual-frequency instrumentation allows for several options (depending on the baseline length), some of which lead to ambiguity-fixed solutions, others provide ambiguity-free solutions.
COTS software packages typically process only single baselines, even when more than two receivers had been operating simultaneously in the field. (Rigorous mathematical processing requires a multi-baseline approach in which the between-station correlations are taken into account.)
Processing options are generally fixed so that in an operational data processing environment phase data reduction is executed according to a "recipe" or "script".
Pseudo-range data is generally not used during phase data reduction, and certainly not for conventional static GPS surveys.
Data from more than one observation session cannot be rigorously processed in one step.
If the observation residuals indicate an unresolved cycle slip, then the data has to be rescanned, and the phase data reduction process repeated. This is not usually done in commercial software running in "automatic" mode.
The phase data reduction step (be it in the single-baseline or multi-baseline mode) results in a minimally constrained "mini-network".
The output of the phase reduction process (be it single-baseline or multi-baseline) is then input into a network adjustment program.
The output variance-covariance information is generally over optimistic and does not truly reflect the accuracy of the GPS adjustment (the accuracy will have to be artificially "deflated" in the network adjustment).
The output coordinate results are given in an approximate geocentric WGS84 coordinate system (these results can be transformed into geodetic or map projection coordinates), and refer to the groundmark over which the GPS antenna was setup.

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