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Technical experts are once again predicting imminent doom caused by interference with Global Positioning System (GPS) sat-nav receivers. A nationwide UK network of detectors has reportedly discovered widespread employment of GPS jammer devices, and calls are being made for a harsh crackdown on users of such devices.

"Today's evidence from roadside monitoring shows that we have moved on from a potentially threatening situation to a real danger that we must address now," says Bob Cockshott, who is a Challenge Delivery Manager at the government's Technology Strategy Board ICT Knowledge Transfer Network.

Under SENTINEL, detector devices have been placed around the country to find out how serious the problem of interference with GPS actually is. The signals from the GPS satellites in the sky are very weak, so they are easily blotted out by other transmissions, either by accident or on purpose. In particular, GPS jammers can be bought on the internet for those wishing to defeat tracking systems which might be installed in vehicles: they are sometimes used by sophisticated car thieves, truckers wishing to evade monitoring by fleet managers, and by people who fear that they may be under covert surveillance or who wish to avoid inadvertently generating records of their location by other means (perhaps automatically by their new smartphone, for instance).

We learn from SENTINEL contractor Chronos Technology:

Jamming monitors have so far been placed at around 20 locations in the UK. At one particular location that has been monitored continuously for the past 6 months over 60 individual jamming incidents were recorded and the results at another have already led to the recovery of a device. The next step is to update the monitoring equipment to be able to differentiate between different jammers, giving researchers a better idea of how many individuals at a particular location are jamming GPS signals.

The frequency of a GPS jammer going past, then, can be as high as once every few days at some locations.

"The question for the authorities is what we are going to do once the owners of these jammers are identified and how can we prevent others using them," says Chronos founder Charles Curry.

In many cases the answer would be presumably nothing, as it was revealed in a previous satnav-alarm report by the Royal Academy of Engineering last year that a common cause of GPS interference is so-called "Blue Team" jamming, where "friendly forces" use GPS jammers to guard against the threat that they may be being tracked by the use of covert GPS bugs attached to their vehicles.

Apart from the new, partial SENTINEL information there is little new on offer at the conference today. The General Lighthouse Authorities (GLAs) - the sector-straddling quangocrats who run the UK's lighthouses and navigational-aids infrastructure - have predictably popped up again, rehashing some trials they did using GPS jammers against one of their ships in 2010. Those trials revealed some unexpected vulnerabilities in poorly-designed maritime equipment - the NLV Pole Star's GPS didn't just cease reporting its position, it also showed erroneous positions at times, and the ship's radar also proved to be dependent on GPS timing signals to work properly. However it isn't hard to design sat-nav receivers and radars without these vulnerabilities: military and aviation equipment is much more robust, for instance.

The GLAs and their various contractors typically suggest on these occasions that they should be given government cash to run a network of eLORAN transmitters as a backup for GPS navigation and timing receivers, but there are some serious weaknesses in their case - we've gone into them in depth before. Nonetheless Professor David Last, consultant-for-hire on GPS to various bodies including the GLAs, considers that a maritime disaster caused by GPS jamming is inevitable.

"The spread of the jamming technology used in these trials, with devices available online for only £50, makes a major incident at sea, whether accidental or intentional, a real danger. In the English Channel, the world's busiest seaway, I personally believe we will see such an incident in the next decade."

Similar predictions of doom were made by the GLAs four years ago, at which time they suggested - not very credibly - that pen-sized portable jammers could shut down major container ports.

Meanwhile, as this is written, the assembled nav-and-timing scare experts are also bigging up the fearful possibility that crooks or other miscreants might move on from mere GPS jamming to actual spoofing - in other words the satellite signals would not merely be blotted out but replaced by stronger ones designed to generate false position or time readings.

"So far no credible high profile attack has been recorded but we are seeing evidence of basic spoofing, likely carried out by rogue individuals or small groups," says prof Todd Humphreys from Texas uni, who "owns the world's most powerful civil GPS spoofer".

"Whilst the leap to more advanced, untraceable spoofing is large, so are the rewards," says Humphreys. "It's therefore guaranteed that criminals are looking at this. All it takes is one person to put one together and publish it online and we have a major problem."

Spoofing is actually ridiculously easy: you just buy a simulator of the sort used to test location/timing systems, which can record a load of GPS output data as desired (alternatively an artificial record can fairly easily be generated in a PC, a simple standardised format is used) and then rebroadcast appropriate radio signals, convincing any nearby GPS receiver that it is where and when the simulator record says. This might be used to convince a tracking device in an armoured security van that it was still en route during a robbery, or - as Humphreys points out, rehashing another oft-touted threat - to falsify a time stamp on a financial transaction, allowing nefarious and lucrative misdeeds in the markets.

All these cunning techno-crimes, as the earnest conference attendees tell us, could easily be carried out. But this is also true in the case of submarine-borne terrorists: it could happen, it could have been happening for years, but actually it hasn't and it may not really be on the cards at all.

It could well be that there are more important things to worry about and spend money on. ®

Lewis Page was a bridge watch-keeping officer in the Royal Navy from 1993 to 2001, and operated routinely with and without GPS. As a minewarfare precise-nav officer for some of this time, he became quite familiar with various other recondite location technologies of that era such as DGPS, Decca, LORAN, Hyperfix, Microfix etc.

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