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Issue Date: June/July 2000

GPS - the lifting of Selective Availability and the importance of GPS users understanding their equipment

June 2000

A very concerning aspect of this navigational system is that people are willing to buy GPS equipment in a price range of between R1500 and R4000 - without giving a further thought as to how it actually works.

Many of these people do not have the vaguest idea of how the system functions, or even a fundamental understanding regarding the basic knowledge of the subject. Since the announcement of the lifting of Selective Availability, many have now started asking the question, "What is Selective Availability?" This proves that their initial knowledge was not as comprehensive as one would expect from a person who is prepared to purchase such an expensive and advanced instrument.
The following relevant background information concerning the Navstar System (Navigation Satellite with Timing and Ranging) will explain this concept. This system cost the US $10 billion and was fully operational in April 1995. The more common name used is GPS (Global Positioning System). The initial system was designed to be a three-dimensional navigation system. It would function 24 hours a day, 365 days of the year and be operational during all weather conditions, in the air, at sea, and on land. The accuracy requirement for plotting a position on the earth's surface was in the region of 35 m. The actual result however, turned out to be far more precise than expected. Firstly, Precise Positioning, reserved for the United States of America's armed forces and secondly, Standard Positioning, for others who use the system. Due to the accuracy of the Standard Positioning being better than expected, and not wanting this technology to fall into the wrong hands, Selective Availability was applied. The US has since announced that the restriction of Selective Availability on the GPS was suspended as from the 1 May 2000.
Selective Availability was applied throughout the world on the Standard Positioning frequency. This gave the user accuracies better than 300 m for 24 hours a day, better than 100 m 95% of the time and averaging between 40 and 60 m in South Africa. The signal information degradation was introduced by applying orbit errors at random.
For the average outdoor enthusiast this accuracy was adequate, as generally a person could visibly identify their destination from 40 to 100 m. This could be improved by taking a position average for a minute or two at a stationary point, that is, if your receiver had this function.
As mentioned earlier, on 1 May 2000, President Clinton ordered the US's armed forces to de-activate Selective Availability with immediate effect. On midnight of 1 May 2000 this restriction was lifted, giving the average GPS user an accuracy of between 10-15 m. (I have personally received an EPE (Estimated Position Error) of 5 m in Centurion a few days after the lifting of the restriction). Normally on most handheld GPS receivers, the EPE is displayed in the satellite configuration menu. It is the (mathematical) geometrical error of the signals received from the four best satellite signals.
What is also interesting, is that a few months ago, Vice President Gore announced that the existing system would be modernised. In the near future there will be an additional 18 satellites launched, increasing the systems capability and hopefully also the accuracy. In addition there will be two new civilian frequencies added to the system.
However, during the announcement, the President made it very clear that the US would not hesitate to re-activate Selective Availability if the need arose. This re-activation could and would be applied to regions if it became necessary and not to the entire world as before.
With this latest development, the GPS user will now, for the first time, really start realising the advantages of owning such an instrument. He will start to appreciate the usefulness of this small wonder as it will give him the accuracy of his position within an area 5 m across (which is about the same area as a double garage.) This, however, will only be accomplished if the user is properly trained in the use of the handheld GPS.
KJ Bolton Consultant specialises in Geographic Information Services. One of its focus areas is that of satellite navigation. Kevin Bolton has been involved with satellite navigation since 1982 and has presented numerous training courses in this field since 1993.
KJ Bolton Consultant
083 596 2366


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