Reliability Analysis of GPS Geodetic Control Networks
Reliability theory was introduced in 1968 by Dutch geodesist Professor Willem Baarda (Baarda, 1968). Lachapelle and Ryan (2000) state that: "Reliability refers to the controllability of observations, that is, the ability to detect blunders and to estimate the effects that undetected blunders may have on a solution."Ê This can be explored further as reliability theory is comprised of two main components: Internal and External Reliability. Internal reliability relates to the amount of gross error in an observation, not detectable at a certain probability level; External Reliability relates to the effect of non-detectable blunders on the estimated quantities (for example coordinates).
The application of Baarda's work to a GPS situation has initially been hindered by Baardaâs mathematical generalisations, which make the assumption that the dataset to be checked would have uncorrelated measurements, which is untrue of GPS. Steps have been taken by academics such as Wang & Chen (1994) and Schaffrin (1997), among others, to provide an unabridged and more comprehensive version of Baarda's work, which can be applied to the correlated observables of GPS. With the increasing use of GPS as the major observation tool for large geodetic control networks, the application of these unabridged versions has become necessary to ensure the integrity of results. This is advantageous to the surveying community, as it provides a means to understanding the effects of input data on output data, as well as the accuracy and precision of results in terms of the resolving power of the network.
The main purpose of applying reliability theory to GPS is to gain an understanding of, and to derive statistics about the quality of a GPS network. Pelzer (1979) says that "The quality of a geodetic network is described by its accuracy and reliability", and currently in NSW, and possibly other places, little is known about the reliability component of most large, and particularly geodetic, control networks.
To complete the
necessary testing required for this thesis, a Least Squares program incorporating
Reliability Theory from Baarda, Wang and Chen and Schaffrin's papers was developed. This was undertaken by the
NSW Department of Lands Survey Branch, and is called ORCA. More information
pertaining to ORCA can be gained from the Department of Lands,
The statistical tests present in ORCA are a practical and appropriate method for monitoring the presence and effects of gross errors in GPS networks. The use of such tests also allow for network optimisation at a design stage, which is advantageous when dealing with geodetic networks that require large amounts of time, economic and personnel resources to observe.
There were two significant outcomes from this thesis:
2. Using the results from conducting reliability theory analyses on different datasets, a series of network design guidelines have been recognised. Various recommendations relating to network design optimisation can be made based on the research, testing and conclusions arising from this thesis. The main points recognised were that:
á A design stage analysis should be undertaken on a network
á Determination of the approximate error detection levels from Marginally Detectable Error values.
á Determination of the effect of errors on coordinates using the external reliability matrix.
á Use of the Êcorrelation matrix to determine where undesirably high correlations exist.
á Perimeter stations should be connected by at least three baselines.
á Introducing eccentric marks will improve the redundancy of a network.
Reliability Theory proved to be an effective method for network monitoring and a useful design tool when applied to GPS geodetic control networks. It should be considered when designing GPS control networks, especially those with specific requirements and error budgets.
W. (1968) A Testing Procedure for use in
Lachapelle, G. and S. Ryan (2000) Statistical
Reliability Measures for GPS, IMA Workshop
on Mathematical challenges in GPS,
Pelzer, P. H. (1979) Some Criteria for the Accuracy and the Reliability of Networks, XVII General Assembly of the International Union of Geodesy and Geophysics, Canberra, Australia, 2 -15 December: 19 pages.
Schaffrin, B. (1997) Reliability Measures for Correlated Observations, Journal of Surveying Engineering, August: 126-137.
Wang, J. and Y. Chen (1994) On the Reliability Measures of Observations, Acta Geodaetica et Cartographica Sinica, English Edition: 42-51.
Wang, J. and Y. Chen (1999) Outlier Detection and Reliability Measures for Singular Adjustment Models, Geomatics Reasearch Australasia, No. 71: 52-72.
For more information, please contact:
Ms. C. E. McAlister
For more information on ORCA, please contact:
Department of Lands