10.4.4 Quality Control Procedures for GPS Networks

REMEDIAL ACTION: SUMMARY AND SOME GUIDELINES

There are a number of remedial actions that can be taken with regard to quality issues:

• Remove the suspect station(s) from the session, and recompute the multi-session adjustment. This is possible only if there are sufficient redundancies (multiple occupancies) so that the network does not suffer by the removal of some of the baselines.

   

• Correct the problem. This is only feasible in the case of a wrong station occupation or an incorrect antenna height measurement. By using the station logs it may be possible to salvage the necessary information (eccentric station offsets or correct antenna height) without having to reobserve a session.

   

• Reobserve some or all of the stations in the session, particularly if the entire session is poor due to, for example, data insufficiency (and the ambiguities were therefore not resolved).

The effectiveness of multi-session quality control is enhanced if multiple occupations and care is especially given to the measurement of antenna height. Having different field parties (using different instrumentation) occupy the same station maximises the chance that any station error (such as groundmark misidentification) may be restricted to one setup, and hence is detectable in the multi-session solution. Having different observers note the height of antenna maximises the chance that the correct height is measured (at least once)!

Ensuring that quality control is being carried as the survey is progressing makes it feasible to reobserve sessions with minimum disruption (and cost)!

Data Quality Issues

DATA QUALITY PROBLEMS affect, in the first instance, the quality of a single session solution, and can hence be largely identified at this level. There is, in general, no satisfactory remedy but to discard the data and reobserve some or all the stations in the session. In summary, the factors affecting data quality, and their characteristics, are:

Data quality factors (insufficient epochs, satellites, etc.) affecting a baseline, but is an isolated incident due to poor field procedures.

Detect	: Preliminary processing and session solution quality indicators.
Remedy	: Reobserve and take greater care in future.

Data quality characteristic that is site-dependent, for example multipath.

Detect	: Preliminary processing and session solution quality indicators.
Remedy	: Improve site conditions, or change site location and reobserve. (Could have been prevented by better reconnaissance?)

Data quality factor which is due to an instrumental problem.

Detect	: Preliminary processing and session solution quality indicators.
Remedy	: Replace receiver and reobserve affected stations. (Regular calibration of equipment is advised.)

Data quality factor which affects all sites in a session, for example ionospheric problems or bad Navigation Message.

Detect	: Preliminary processing and session solution quality indicators.
Remedy	: Reobserve entire session.

Result Inconsistency Issues

RESULT INCONSISTENCY PROBLEMS affect the quality of a multi-session GPS network solution. They can be detected at this level only if sufficient redundancy is designed into the network, and only if the problem occurs at a station with multiple occupancies. In addition, the degree of redundancy will determine whether remedial action other than merely reobserving some or all stations affected can be taken. In summary, the factors affecting the internal consistency of a network solution, and their characteristics, are:

Incorrect station identification or antenna height error, being an isolated incident due to poor field procedures.

Detect	: Multi-session adjustment residuals, or loop closures. Detectable only if occurred at site with multiple occupancies.
Remedy	: Drop affected station occupation from multi-session solution, or if not possible, reobserve and take greater care in future!

Systematic effect at stations which is due to an instrumental problem, for example antenna incorrectly fitted so that electrical phase centre not where it is indicated on housing.

Detect	: Multi-session adjustment residuals, or loop closures involving the same receiver. Detectable only if different receivers are deployed at multiple occupations.
Remedy	: Replace receiver and reobserve affected stations. (Regular calibration of equipment is advised.)

If such problems occur at sites occupied only once (that is, no redundancies involving these sites) they are undetectable by any procedure.

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© Chris Rizos, SNAP-UNSW, 1999