A danger for seafarers!

GNSS signals transmitted to receivers are extremely weak, using a fraction of the power of a light bulb. They are therefore very easy to distort or jam. The L1 (1,575.42 MHz) frequency, which is the publicly available part of US GPS which will be duplicated in Galileo, is for instance subject to interference from two tv channels.

The encrypted L2 (1,227.60 MHz) and L5 (1,176.45MHz) frequencies can be distorted by a further tv channel each. Washington is particularly concerned with protecting the spectrum from unintentional interference and has at times been critical of what it sees as Europe’s inadequate plans to protect the signal integrity of Galileo.

The military applications of GNSS have produced counter-measures and the design of jammers are not only available on the internet but jamming devices are commercially available. It is also possible to “spoof” GPS receivers by sending counterfeit signals. The impact of interference, whether intentional or not, can be widespread.

For almost five days days in Mesa Arizona in December 2001, Boeing accidentally left running a jammer it was testing on the L1 frequency, radiating just 0.8mW. Within 180 nautical miles around the jammer, air traffic control operations were thrown into chaos. Because of a false reading one airliner 45 miles out from Phoenix airport made a 35 degree turn into other air traffic and some aircraft lost GPS signals entirely.

A few month later in Moss Landing Harbour, California a boat owner proudly installed a tv antenna with a pre-amplifier on his vessel at its mooring. As a result all GPS signals were jammed through Moss Harbour and a kilometre out to sea for 37 days, while engineers tried to locate the source of the interference. Vessels encountered extreme difficulties moving through the harbour in foggy conditions and had to rely on radar control.

Dont forget the basics
For Michael Swiek these problem highlight a danger for navigators:
– When you get into the more modern high technology types of navigation, a lot of times people overlook the cardinal rule of safe navigation which goes back hundreds and hundreds of years. This is never rely on just one system or one piece of equipment. When sextants were the primary means of navigation, they were great as long as you didn’t have cloud cover, which was why no mariner just went to sea with just a sextant. The same is true today.

Prudent navigation is always a suite of approaches. GNSS is a very powerful tool, but you have to understand its vulnerabilities like the vulnerability of any other system and accommodate those with backups.

That backup is coming from the enhancement to Loran, eLoran which is being designed to integrate with GNSS, for instance using satellite constellations to calibrate itself to account for Additional Secondary Factors (ASF). Because of its high-power low frequency signals, Loran has excellent penetration and is considered to be virtually unjammable.

It is already possible to obtain absolute accuracies of 8–20 meters using eLoran for harbour entrance and approach. eLoran transmission infrastructure installation began in the US last year and a variant of the system is already available in north-west Europe. The anticipation is that receivers integrating GNSS and eLoran will soon become widely available.

//Nigel Ash

Latest update 18-10-2006 8:49

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