Steve Nichols

How secure is satellite navigation?

The US Global Positioning System (GPS) has 24 satellites - 21 active and three spare - orbiting the earth in just under 12 hours.

A basic GPS receiver usually needs to "see" at least four to plot position and altitude to an accuracy of 15-100m.

The system can be more accurate than this, but "fudging" of the signals by the US Department of Defense, called selective availability, limits it. If you want better accuracy you usually need a further signal from a fixed ground station.

It seems that GPS signals can be jammed, either unintentionally or otherwise. Steve Gubbins, of GPS supplier, Garmin (Hall 3, Stand D19) says: "Someone in Russia is reported to have taken a jamming device to an airshow and the RAF has completed GPS anti-jamming trials.

"We have never had any problems with GPS on our flight systems - I don't think people do," he adds.

Wiped Out

Larry Speelman, of II Morrow, promoting the GPS-based Automatic Dependent Surveillance Broadcast (ADS-B) system at Farnborough, agrees: "You could probably stop a GPS system from working completely, but it would be difficult to make it report positions incorrectly. It is also unlikely that a whole region of airspace would be wiped out."

Satellite outages are possible and Eurocontrol has a web site - - for pilots to check GPS availability before flights. The US Coast Guard has something similar.

Solar activity can also wreak havoc with satellites - some have been knocked out by radiation and other emissions.

The major avionics companies are understandably reluctant to talk about their navigation systems being rendered useless, so it is hard to know how true these stories are.

But Mayflower Communications in the US does sell GPS anti-jamming systems, so there must be a market.

Source: Flight Daily News