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User panel says US should scrap GPS off switch

Sat nav industry reckons degrading sat nav is bad

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American and international sat nav stakeholders have advised that the US government should never again degrade the civilian Global Positioning System (GPS) signal.

GPS is the satellite constellation used by the vast majority of the world's sat nav receivers. It is run by the US Air Force and paid for by the Pentagon.

The multi-agency National Space-Based Positioning Navigation and Timing (PNT) Advisory Board met for the first time at the end of March. The panel includes senior representatives from the aviation, space, and transport industries, retired US forces generals, and so on.

The minutes of the meeting (pdf) were released this month.

The sense of the meeting was that the Pentagon should abandon the option of reducing the accuracy of civilian GPS in future, mainly due to the huge number of civilian applications - many of which are economically or safety critical.

The 30 GPS satellites currently in orbit provide accuracy within 10 metres to civilian users in most circumstances, and receivers can often get a fix even with significant portions of the sky obscured. The US Air Force, which administers GPS, is only required to maintain 24 satellites in operation, and many panel members were concerned that the fleet could reduce to this number in future. It was said that users have become accustomed to the service offered by 30 satellites, and the consequences of a reduction to 24 would be problematic.

At present, GPS includes a feature called Selective Availability (SA), by which the accuracy of the civilian signal can be degraded. SA was turned off in May 2000 on the orders of President Clinton, but it could be turned back on again at any time. Many members of the panel said this feature should be dispensed with altogether.

President Clinton gave a firm undertaking not to degrade GPS in 2000, but in 2004, President Bush qualified this somewhat, saying that the US would "improve capabilities to deny hostile use of [satnav], without unduly disrupting civil and commercial access to [GPS] outside an area of military operations, or for homeland security purposes..."

PNT board members pointed out, however, that SA is fairly easy to counter. It isn't difficult to implement so-called Differential GPS methods, where the error in the GPS signal is worked out at a fixed ground station and corrections communicated to a mobile receiver in real time. It was suggested that SA simply be omitted from future GPS satellites.

Even ex-military board members didn't feel that SA was worth keeping. According to the record of the meeting, former US airforce general James McCarthy "stated that SA can be eliminated with the right set of arguments, which have not yet been made or articulated".

The general told the board "he has gained appreciation for the needs of non-defence users. The decision-makers in the [Pentagon] do not yet have a similar appreciation. There is no need for SA, although he would not have said this five years ago. He feels that the problem is that the message is not clear".

General McCarthy is nowadays an academic.

Decisions on satellites numbers and the inclusion (or not) of SA will have to be made soon. Proposals for the new "Block III" generation of GPS spacecraft are due by 27 August.

Such decisions could affect matters in Europe, too. American negotiators are in Brussels now seeking an agreement on integration of the proposed European "Galileo" satnav constellation with GPS, so as to allow a single receiver to draw on the combined resources of both systems.

Removing SA from GPS might, in fact, largely remove the case for Galileo, as many foreign users of GPS would then have more confidence that they could rely on the American system. ®

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