8.1.7 GPS Baseline Processing

SOLUTION OUTPUT

The first remark that must be made is that there is, as yet, no standard form for the baseline solution output. Different software packages produce differently formatted output files. Not only are the coordinate results likely to be variously labelled, but in addition, different terminology is often used (this is partly cultural as well, as most software is developed in the U.S.). For example, it may not be clear if the "baseline components" are antenna-to-antenna or groundmark-to-groundmark, "ambiguities" may be labelled "biases", and the source of the apriori coordinates may not be given. Furthermore, the types of statistical tests applied and the exact (mathematical) definition of some of the "quality" indicators may not be given. (They may be found in the software manual, but these generally suffer the problem of all manuals, that is they may be user-unfriendly and sometimes the source of misinformation, or at least out-of-date advice.)

There is a SINEX (Solution INdependent EXchange) format presently being used within the geodetic community, and may find favour with survey users as well (section 9.3.7).

With regards to the output of a baseline solution, the following information is usually provided in some form or another:

• Type of solution: whether triple-, ambiguity-free or ambiguity-fixed.
• Input and output coordinates (solve-for and fixed), in various systems, for example Cartesian, geodetic, baseline components.
• Echo of receiver (serial numbers, etc.) and station information (site I.D., antenna height, etc.).
• Standard deviation of estimated coordinate components.
• The correlation matrix or variance-covariance (VCV) matrix for all coordinate parameters (and perhaps for the ambiguities as well).
• Optional estimate of quality of satellite geometry, for example the RDOP value (derived from the VCV matrix).
• Tracking data acquired, logging times at individual sites, tracking channels used, satellites tracked, signal quality flags, etc.
• Number of observations used in solution, as well as those rejected, the sampling rate used, and the data edit criteria (usually some factor times "RMS tracking").
• Summary of ephemeris information, health warning flags in the Navigation Message, etc.
• Any data pre-processing performed (for example, tropospheric delay model).
• Indicator of fit of observations to final solution (that is, the residuals),usually in the form of an "RMS tracking" value (though the label may vary for different processing software). The magnitude is an indicator of the quality and reliability of solution, and whether ambiguity resolution is possible (or resolution was successful).
• Results of statistical tests of the residuals may also be displayed.
• If an ambiguity-fixed solution was attempted, then a summary of the number of ambiguity parameters resolved.
• Possibly a summary recommendation on the quality of the solution.

Solution output may be scanned to assess the quality of the results, but in order to be able to compare one baseline solution to another, some "rule-of-thumb" remarks may be kept in mind:

• In conventional static GPS surveying successful ambiguity resolution is basically a function of baseline length. For baselines over 15km in length it can be a problem. If ambiguity resolution was not possible for baselines < 15km, check the continuity of tr acking, length of observation session, data "noise", and take steps such as reduce the resolution decision making criteria, and perhaps rerun the solution.

   

• Having sessions of the same length (each day and throughout the day) is not sufficient to ensure similar quality results. Nor are sessions observed at the same time each day a guarantee of consistent quality results, even though the receiver-satellite geometry are the same. Residual biases due to orbit error, atmospheric effects, etc. are dynamic in nature and will influence the results as much as (and often more than) the receiver-satellite geometry during a session.

   

• The "RMS of tracking" quantity tends to increase with increasing baseline length. (This quantity may have a different label in some software.)

   

• The accuracy of the baseline components, expressed in metres, will degrade with increasing baseline length. Expressed as "parts per million" it should be a constant.

   

• A "total" quality indicator, generally on the basis of a number of internal "tests" such as whether the ambiguities were resolved, the "RMS tracking", etc. may be provided, but is invariably software dependent -- therefore READ THE MANUAL.

   

• The coordinate standard deviations are lower for an ambiguity-fixed solution than for an ambiguity-free solution, which are in turn lower than for a triple-difference solution. (Though that does not mean that the ambiguity-fixed solution is correct!) For example, the following results were obtained for a baseline of 6.8km length:

  	x	y	z
  Triple soln.	.415E-01	.920E-01	.329E-01
  A-free soln.	.398E-02	.108E-01	.327E-02
  A-fix soln.	.352E-02	.175E-02	.182E-02

   

• The solution stochastic information can be presented in one of two forms, for example, in the case of the above 6.8km baseline (ambiguity-fixed solution):
  • as standard deviations and correlations:

  x = .352E-02	y = .175E-02	z = .182E-02
  xy = -.52198	xz = .65125	yz = -.72810

   

  • or as the full variance-covariance matrix:

  .12461E-04
  -.32245E-05	.30625E-05
  .41840E-05	-.23190E-05	.33124E-05

   

• Solution statistics based on the VCV information are often optimistic. Beware of software packages that may artificially inflate the uncertainties of the parameters in order for them to appear more "realistic".

   

• There is no measure for "reliability", the output VCV information will not reflect the influence of systematic biases.

   

• It is good practice to first carry out preliminary baseline reductions with a minimum of options. This would permit the output to be assessed for bad data, residual cycle slips, etc., and the final adjustment may be carried out using the best dataset.

   

• If there is any doubt about the quality of the ambiguity-fixed solution, it is preferable to accept the ambiguity-free solution in its place. If the ambiguity-free solution indicates high a "RMS tracking" value (for example because the baseline > 50km), the triple-difference solution may be the preferred solution. However, check that the recommended standards & practices for a certain class of GPS survey will accept such a solution (see section 10.3.1).

   

• Improvement in the modelling of biases (for example, through the use of dual-frequency observations), or increase in the sophistication of the solution ("rigorous" adjustment, inclusion of additional parameters, etc.) leads to better and more reliable results for the same length baseline and observation session compared with the basic single baseline processing strategy.

   

• Additional statistical testing may be carried out (for example, using "chi squared" and "variance factor" tests). In addition, external tests may be based on loop closure statistics, or the results of a network adjustment. This is dealt with further in Chapter 9.

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