GPS equipment accuracy testing

In general, measurements are only “legal” if they are “traceable” to primary standards of measurement. A GPS measurement is “legally traceable” if: (i) it is carried out using the various test/calibration procedures as required by the Guidelines, and; (ii) the survey has followed the “recommended practices for field and office procedures” as described in the Guidelines.
A GPS system testing/calibration program is considered a prerequisite for demonstrating “competence” and for assuring that GPS-derived coordinates are of a uniformly high quality. We recommend three tests: (a) a zerobaseline test, (b) calibration of the GPS equipment on an existing first order EDM/Total station baseline, and (c) connections to several existing first order geodetic control stations. It is essential that all GPS equipment be tested not only initially but periodically depending upon volume of usage of particular GPS equipment in case of surveys of first and second order accuracy. Details of these tests have been described in this paper.
A pair of dual frequency or single frequency geodetic GPS receivers or used mostly in relative positioning mode and also in differential mode (DGPS) for all applications requiring first or second order control surveys. Whether the GPS equipment (GPS receivers, antenna, cables, processing software etc.) yield the required results, should be found by carrying out testing, on receipt of the equipment and before any significant project is taken up. The following tests are recommended for the testing of GPS equipment: –

Zero base line test

The two GPS receivers should be connected to single antenna using an antenna splitter appropriate for the particular brand of receiver and antenna. The slope distance on processing should be less than 3 mm. This test is very simple and should be performed at regular intervals and also before any control survey project is undertaken.

Known base line test

The pair of GPS receivers should be tested by measurement against a known base line of first order accuracy. In India Survey of India has a 13 km base line of first order accuracy near Dehra Dun. Most of the GPS equipment purchased by SOI is tested against this base line. Such test sites can be prepared by number of zero order observations between two inter visible points/stations anywhere by Geodimeters/ Precise EDM instruments. GPS equipment should be checked against such base line periodically. The resulting difference between slope distances should yield accuracy of better than 1 in 50000.

Network test

India has a geodetic control network of first order accuracy in which control points are available nearly every 50 km apart. GPS receivers to be tested should be used for observations at such points and processed coordinates can be checked against these. Indian control points known as GTS stations are in Indian system whereas GPS gives coordinates in WGS 84. Therefore a network of at least a braced quadrilateral can be observed, data processed, and adjustment done by least squares. Processing shall yield accuracy also which should be of the first order. This test is most comprehensive and considered the best but it takes considerable time.

Conclusion

It is essential that at least two tests preferably all the three tests should be performed on receipt of the instruments and periodically say every year and before any large control survey project is taken up.

References

Guidelines for GPS Cadastral urveys in Malaysia