Sect_4.4.2

4.4.2 Testing GPS Surveying Systems:

CALIBRATION TESTS

Some tests strive to determine the overall characteristics of GPS performance by testing the total system, others try to isolate as many of the "external" errors/biases as possible.
Some tests are conducted once, either in a laboratory (of the manufacturer, or an independent organisation) or "in the field", others are conducted on a continuous basis.
Some tests are conducted as a form of certification, others are conducted by individuals as they perceive the need to do so.
Some tests involve specific GPS instrument units, others are tests applied to an entire class or brand of instrumentation.

Three test strategies can be identified under this category:

"Integrity Monitoring"

Zero baseline tests

Laboratory testing

Integrity Monitoring

The following comments are made regarding this test procedure:

The impetus has come from the civil aviation community. As GPS is being promoted as the primary navigation aid for the Future Air Navigation System (FANS), concerns have been raised about the reliability, availability, integrity and accuracy of the system for such GPS users.
One strategy for monitoring "integrity" is to use a ground network of permanent GPS receivers, located on points of known coordinates, which track all visible satellites, monitor certain performance parameters, and issue a warning when the performance of the GPS system (Space and Control Segments) degrades to such a point that GPS navigation performance is adversely affected.
A record of the "integrity" of the GPS system from such an Integrity Monitoring (IM) Network can be referred to by surveyors when evidence is sought for possible periods of degraded GPS performance.
In Australia, IM for the civil aviation community (as well as for others) is the responsibility of the Federal Government's Australian Survey and Land Information Group (AUSLIG). AUSLIG has established for this (as well as for other geodetic purposes) the Australian Regional GPS Network, a permanent network of up to 15 GPS tracking stations (eight of which are located on the Australian continent) -- section 12.1.5. All tracking data is telemetered back to Canberra where real-time algorithms process the data and generate navigation warning messages, which can be transmitted to users via a dedicated channel on the OPTUS communications satellite.

It is expected that the GPS survey community will in the near future have easy access to GPS IM data. Although there may not always be a correlation between poor GPS navigation performance and poor GPS survey baseline results (as GPS integrity is generally very good), any periods of poor system performance as detected by the IM network must be considered with suspicion.

Zero Baseline Testing

A "zero baseline" test can be used to study the precision of the receiver measurements (and hence its correct operation), as well as the data processing software. The following comments are made regarding this test procedure:

The experimental setup, as the name implies, involves connecting two GPS receivers to the same antenna. The antenna "splitter" can be purchased from specialist electronics shops. Obviously this test cannot be applied to integrated antenna/receiver systems such as some of the Leica, Trimble and Ashtech GPS instruments.
When two receivers share the same antenna, biases such as those which are satellite (clock and ephemeris) and atmospheric path (troposphere and ionosphere) dependent, as well as errors such as multipath CANCEL during data processing. The quality of the resulting "zero baseline" is therefore a function of random observation error (or noise), and the propagation of any receiver biases that do not cancel in data differencing (section 6.3.2).
The impact of residual bias effects on the baseline solutions which are a function of baseline length (such as satellite ephemeris bias, the handling of observation time-tags and atmospheric delay) cannot be evaluated.
An important advantage of this test is that it is comparatively simple to administer -- no specialised software or "ground truth" data is required, and the location of the antenna is immaterial.
GPS surveying receiver manufacturers use this procedure to perform final product testing of all receiver units before they leave the factory.
No significant time-dependency to the quality of the zero baseline results should be evident, apart from a very small effect that is due to the daily variation in receiver-satellite geometry.

Laboratory Calibration Tests

GPS receiver manufacturers perform a variety of component tests; such as on the oscillator, onboard data memory (or memory cards), the receiver firmware, the antenna, batteries, etc. Furthermore, such tests may be carried out over a large range of temperatures (generally from -30° to +50° C), as well as under conditions of artificially induced signal jamming.

A commercial receiver testing service is on offer by the NAVSTAR GPS Joint Program Office. Unfortunately, not a lot is known of this service, but it is intended primarily for GPS receiver manufacturers and not for the general user community.

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