Sect_4.3.5

4.3.5 GPS Surveying Software:

COMMENTS ON NETWORK ANALYSIS SOFTWARE

The processing of GPS phase data may be carried out for the smallest unit possible: a single baseline, or in a large simultaneous adjustment of all data collected in a survey, or in any combination in between. In any case, there are generally several common features of all phase data reduction:

They are typically minimally constrained solutions based on the coordinates of one station in the network (or baseline) being held fixed.

The results are in the form of 3-D coordinates (either in the Cartesian system X,Y,Z, or as geodetic coordinates ,,h referred to some ellipsoid).

The coordinate results are referred to the WGS84 reference system (at the level of accuracy that this is defined by the fixed datum station).

If the phase data reductions are performed piecemeal, that is in the single-baseline processing mode or (the preferable) single-session processing mode, the individual sets of results must be combined together in a subsequent network adjustment. Hence the secondary network computation software must be able to:

Combine individual baseline (or multi-baseline) solutions into a network. As there are invariably redundancies in the GPS survey (for example, multiple GPS station occupancies, repeated baseline observations, etc.), the best way of obtaining an optimal coordinate solution is to input the GPS results (and associated variance-covariance information) into a network analysis program. The combined solution could then be left as minimally constrained -- the coordinates of only one station are held fixed.
Include external station control coordinates within the GPS network solution to provide both a means of relating the GPS-only solution (in the quasi-WGS84 system) to the local geodetic datum, and in order to constrain the GPS-only solution to fit the local control. The former requires the determination of the transformation parameters between the GPS and local geodetic system.
Convert the GPS ellipsoidal heights into the more useful orthometric heights through the input of geoid height information for some or all of the surveyed stations.
Report writing and presentation software able to support survey report generation with coordinate lists, network diagrams, table formatting and spreadsheet capabilities.

The above tasks can be performed by network adjustment software which may be written explicitly for this purpose, or by conventional geodetic network adjustment software which has been modified to handle the GPS output (from the phase reduction software), as another type of geodetic "observable". This is in addition to terrestrial survey data such as horizontal directions, distances, zenith distances, etc. Some of the characteristics of network adjustment software used in GPS survey computations are:

The software may be developed by the GPS instrument manufacturers, and offered as an additional module within the overall GPS software "package". However, in the past, the preferred software packages have been written by geodesists experienced in conventional geodetic adjustments and are offered as a separate package.
The major modifications to conventional geodetic adjustment software necessary to accommodate GPS baseline solutions are: (a) to permit a 3-D adjustment capability (most conventional network adjustment programs can only handle 2-D data), (b) to partition coordinate datasets into several different datums, related to each other through the appropriate selection of transformation model, and (c) permit full VCV matrices to be input.
The most popular software packages have "toolkits" that can read the output files of most commercial phase data reduction software, thus obviating the need to manually type in the "data" (GPS coordinates and variance-covariance information).
It is often necessary to "rescale" the VCV information provided by the phase reduction software in order obtain more realistic estimates of GPS accuracy (as it is invariably over optimistic).
The network adjustment software is a useful means of monitoring the overall quality of the GPS survey as it progresses. The results of a baseline, or session, adjustment can be incrementally fed into the network program, and the solution checked. If an outlier is detected (for example, a bad baseline) then this could require a modification to the survey plan (perhaps a reobservation of the suspect baseline, or the survey of additional stations).
The output of such a program is a set of station coordinates (on the WGS84 datum or transformed into a local geodetic datum) and the VCV information, generally presented in the form of absolute or relative error ellipses (or ellipsoids).
Some adjustment programs have a simulation mode. GPS survey plans can be tested (or "optimised" in the parlance of geodetic adjustment) by specifying which stations will be occupied so as to fulfil some predefined accuracy and reliability criteria.
Some network adjustment programs may not have a geoid computation capability built-in, hence a separate package may be required. Geoid computation programs are relatively uncommon, though there are geoid height datasets. For example, there is a program available for interpolating from a precomputed gridded Canadian geoid, as well as one for Australia. In addition, programs have been developed that can compute a geoid height value using a geopotential model as input (though these are nowadays being increasingly incorporated within the network adjustment software). A small number of programs are able to compute geoid height values from surface gravity data.
In Australia, the commonly used network adjustment packages are NEWGAN^TM (developed by J.S. Allman, a former staff member of the School of Geomatic Engineering, UNSW), COMPUNET^TM (developed by F. Smith) and GEOLAB^TM (a Canadian product). These packages can process conventional geodetic observations, either alone or together with GPS results. There are many other "inhouse" programs that can handle GPS-only solutions (with and without transformation parameter determination capability).
Trimble, Ashtech and Leica all offer network adjustment software as modules within their commercial software packages.
There is a trend to append result presentation software to manufacturers GPS packages to aid client report generation, by being able to plot station networks, prepare coordinate lists, etc.

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