GPS not affected by war(i guess)

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Posted by Fletch on March 28, 2003 at 00:54:55:

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Monday, March 24, 2003

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GPS unaffected by war
Chris Jenkins
MARCH 24, 2003

THE war in the Middle East will have no effect on the accuracy of global positioning systems (GPS) used in Australia, despite the US Department of Defence retaining the capability to block civilian signals.

First used by the US military, GPS uses satellites to provide accurate positional information on the ground. GPS is now popular with police and emergency service users in Australia and has become crucial to both the shipping and aviation industries.
The managing director of Magellan GPS Systems in Australia, Doug Lloyd, said improvements in satellite technology mean that the US military can now select "spot" areas to disable accurate GPS services.

He said that while this capability would now be switched on in the Middle East, resulting in GPS information that was either very inaccurate or totally unavailable.

A spokesman for the US Department of Defence recently told New Scientist magazine that technology to "blackout" regions was available, but that the US would not block GPS signals across the globe to fight the war with Iraq.


The US military (which owns the worldwide network of 27 GPS satellites) originally built deliberate inaccuracy known as "selective availability" (SA) into all publicly available GPS information in order to maintain a tactical advantage in the event of any conflict.

In May 2000, then US president Bill Clinton ordered that selective availability be turned off. Mr Clinton said removing selective capability was part of "an on-going effort to make GPS more responsive to civil and commercial users worldwide".

Mr Clinton said the decision to turn off selective availability was made in consultation with other government agencies, including those involved in intelligence and defence. "They realised that worldwide transportation safety, scientific, and commercial interests could best be served by discontinuation of SA," he said

"Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique."

Tthe US military is ensured accurate positioning through the use of what is known as "p-code" (precise-code) GPS, which is encrypted to ensure that information is not intercepted or jammed. P-code is only available to the US military and its allies. It features a set of codes which are unique each satellite and are reset every week. Normal civilian applications use "coarse/acquisition" or C/A-code".

Mr Lloyd said that accurate positioning requires the availability of four satellites. Australia is always covered by five satellites, with up to 11 being available at different times. Widely available handheld GPS units are normally accurate to within a metre, he said.


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