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Could pen-sized GPS jammers paralyse UK shipping?

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Analysis Contractors and sector-straddling quangocrats sought yesterday to convince politicians of the need to spend defence or security funds on radio-based backups for GPS satnav, according to reports.

The BBC, covering recent Commons defence committee testimony, gives serious play to the contention by VT Communications and Trinity House that Loran radio-station signals - while not, perhaps, making much commercial sense - are worth funding for the security benefits they bring.

The Beeb quotes Doug Umbers, of VT Communications, as saying that a pen-sized device could prevent ships in a port from receiving satnav signals, whereas a huge field of jammers would be required to deny Loran service.

Sally Basker of the General Lighthouse Authorities (GLAs) told the Beeb that "GPS and Galileo have the potential to do great things ... satellite navigation has got into our critical infrastructure but we know that it has some weaknesses".

VT Communications has a contract from the GLAs to run trial Loran transmissions at its government-services radio station in Cumbria. The GLAs are an alliance of the lighthouse authorities covering the UK and Ireland: Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights. The alliance is funded by a special tax on shipping, the "light dues" charged to ships calling at UK and Irish ports. Apart from its role as an arm of government, however, Trinity House also operates as a commercial offshore contractor, deep sea pilot bureau and mariners' charity.

Since the advent of GPS as a serious contender for maritime navigation in the late 1980s, most other electronic navaids have withered on the vine. The UK formerly maintained chains of Decca radio stations, using similar technology to Loran but shorter-ranged, erratic, expensive to run and typically difficult to use. (Your correspondent, as a navigating officer in the early 1990s, was occasionally forced to use Decca).

The only thing that really kept Decca and the rest going was the fact that for a long time the USA deliberately degraded the accuracy that civilian GPS users could get - using so-called "Selective Availability" (SA) technology. However, in the early 1990s SA became largely irrelevant as a workaround called Differential GPS (DGPS) became common. With DGPS, a ground station at a known location works out the error in the civil GPS signal and continuously transmits corrections to GPS users in the region. This makes an ordinary GPS user, without the codes needed to read the military signal, able to get accuracy as good as 1m - as opposed to say 150m under SA.

As paid-for commercial DGPS became widely available, the always fairly rubbish Decca became totally outmoded and it was finally shut down in 2000. (The only people who really liked it by then were old-school fishermen.) Funnily enough, SA was switched off on the orders of President Clinton the same year, making ordinary free GPS accurate to 20 metres or better. In 2007, the US announced that it wouldn't even build SA kit into its future GPS satellites. For those worried that civil GPS service might be denied altogether - an option the US reserves, though extremely unlikely to be exercised off the British coast - it now appears certain that the interoperable European Galileo system will be operational during the next decade.

These developments could well be said to have made the GLAs' free to air British-Irish network of 14 DGPS stations - which they only got round to setting up rather late, in 2002 - as much things of the past as SA and Decca. Certainly the shipowners who are compelled to pay for them probably wouldn't choose to do so voluntarily. Nor, most likely, would they choose to pay for Loran.

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