Vulnerable GPS puts sat-nav at risk

This was the stark warning made by technical experts at yesterday’s global symposium on GPS signal tampering in Teddington.

The one-day event, ‘GPS Jamming and Interference - A Clear and Present Danger’, was organised by the Digital Systems Knowledge Transfer Network (KTN), a Technology Strategy Board programme.

At the symposium, industry leaders and academics discussed the natural and sometimes criminal interference that can completely wipe out GPS reception.

‘The strength of a GPS signal is about as strong as viewing a 25W light bulb shining down from a satellite 10,000 miles away,’ said Bob Cockshott, a director of the Digital Systems KTN. ’It’s no surprise then that GPS signals are vulnerable to natural and, increasingly, criminal interruptions.’

Such criminal tampering is made easy, the experts warn, with jamming devices that are available illegally for under £100.

Prof David Last, a past president of the Royal Institute of Navigation and now a GPS consultant and expert witness to government and law-enforcement agencies, believes that the potential for serious disruption is a ‘clear and present danger’.

‘A portable jammer in a tall building such as the Gherkin could cover most of London and planes approaching its airports,’ he said.

The implications of GPS vulnerability are being investigated by the Technology Strategy Board grant-funded GAARDIAN.

Organisations such as the International Loran Association (ILA) are calling for a GPS backup technology that relies on ground-based systems to transmit time and location information. The technology, known as eLoran, uses terrestrial transmitters that operate at frequencies 10,000 times lower than GPS.  

GPS must operate at 1.5GHz, which is 10 times the frequencies operated on FM bands, because it must transmit through the Earth’s ionosphere. The eLoran frequency waves, on the other hand, do not need to operate at such high frequencies.

Robert Lilley, secretary of ILA, said it is much more difficult for a terrorist to jam an eLoran signal.

‘If he wanted to jam GPS he could do that with something that looks like a walkie-talkie,’ he said. ‘If he wanted to jam Loran he’d need something like a semi-truck, because it takes lots of power and very big antennas in order to generate a whole lot of Loran energy that would confuse receiver sets at some distance.’

While there are cases of intentional GPS jamming, Lilley points out that accidental incidents of interference are far more frequent.

Lilley, who is based in the US, said one example of this was in Moss Landing, California, where a homeowner’s television antenna preamplifier unexpectedly went into oscillation and generated so much noise it took out GPS over the entire harbour town.

Another incident in San Diego three years ago, Lilley said, occurred when a test generator was left on in a US Navy research centre – resulting in GPS signals disappearing over a large percentage of the city.

‘Aeroplanes didn’t fall out of the sky, but what happened was the cell-telephone towers around the city were unable to pick up the timing signals they needed and so they stopped operating,’ he said. ‘We have become so dependent on the timing and positioning signals from GPS that any little accident like that in San Diego can cause you not be able to make a 911 phone call from your cell phone.’

Some have argued that the yet-to-be-launched Galileo global navigational satellite system – which will be Europe’s answer to GPS when launched anytime after 2013 – will serve as a good enough backup.

However, Lilley said backing up a satellite with another satellite isn’t necessarily the solution when both will be susceptible to cosmic interference such as solar storms.

ILA president Sally Basker, who is the director of research and radionavigation at Trinity House in London, said Europe is well placed to deploy eLoran technology.  

There are currently nine locations across North West Europe that operate stations based on Loran C, which is less modernised form of eLoran, for maritime navigation.

Basker said the one Loran C station in the UK is currently undergoing testing as a prototype eLoran system that can now deliver positional accuracies of around about 10m.

For eLoran to be deployed across Europe, she said, all Loran C systems will need to be modernised and a data channel will need to be added to allow eLoran systems to kick in when GPS is interfered with. 

Basker said GPS is still by far the most accurate timing and positioning technology, but governments must realise that it is not perfect and needs a backup. 

‘The problem is we are increasingly finding GPS at the heart of our critical infrastructure, whether it’s power or telecoms or transportation or emergency services,’ she said. ‘When you lose it you’re pretty stuffed.’