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New GPS sats to lack Selective Availability

Availability will still be selective though

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The US military announced yesterday that it will no longer procure Global Positioning System (GPS) satellites with the capability for worldwide civil sat nav degradation.

In a release dated yesterday, the Pentagon confirmed that "this capability, known as Selective Availability (SA), will no longer be present in the next generation of GPS satellites".

Until 2000, the accuracy of the GPS civil signal was reduced using SA, so that a normal sat nav receiver would be accurate to only 100 metres or so. There was and remains a separate military signal, encrypted so only those with appropriate keys - such as the US forces and their allies - can use it.

Despite SA, many civil users were able to achieve excellent accuracy using a technique known as Differential GPS, in which a ground station at a known location would calculate the SA error and transmit corrections to a mobile civil receiver in real time.

SA was switched off in 2000 on the orders of President Clinton, but it remained an option. In the years since, the US government has sought to assure civil GPS users that SA would never be used again, but its presence has remained a concern.

Now it has been confirmed that the next generation of satellites will not have SA built in. According to the Pentagon: "While this action will not materially improve the performance of the system, it does reflect the United States' strong commitment to users by reinforcing that this global utility can be counted on to support peaceful civil applications around the globe."

This announcement may reassure users somewhat, but the GPS civil signal still isn't a given. America will retain the option to degrade or deny it altogether over limited areas, and this was always much more likely to happen than the relatively blunt instrument of degrading performance worldwide.

The European Commission is just about to make its case for pushing ahead with the proposed new Galileo Euro sat nav constellation. One might speculate that the timing of this American announcement isn't a coincidence. ®

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