GPS freed

The US Department of Defense announced in September that it intends to stop procuring GPS satellites with the capability to intentionally degrade (desensitise) the accuracy of civil signals.

This capability, known as Selective Availability (SA), will no longer be present in the next generation of GPS satellites.

Although the US stopped the intentional degradation of GPS satellite signals by setting SA levels to zero in May 2000, on the orders of President Clinton, this action to permanently remove SA eliminates a source of uncertainty in GPS performance that has been of concern to civil GPS users worldwide for some time.

Until 2000, the accuracy of GPS civil signals was reduced using SA, so that a normal sat nav receiver would be accurate to only 100 metres or so, and long distance sailors have always feared the US could at any time desensitise the accuracy, without prior warning.

The move coincides with the US Air Force's solicitation to purchase the next generation of GPS satellites known as GPS III.

www.defenselink.mil

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