Satellite Navigation– Truths & Myths

in common with computer viruses. We know about viruses; we live in a world in which a socially-inadequate teenager with acne and no girl-friend, can cause havoc in the Pentagon and panic in the banking system! There are designs for GPS jammers on the Web.

Jamming does not take Al Qaeda, it takes a spotty kid in his bedroom. Satellite navigation is like the computer business before the fi rst virus. And we have no McAfee and no Norton!

It could get much worse when people really try to stop satellite navigation. Many European countries now plan to charge road users. Motorists hate the idea, just as they hate radar speed traps. A jammer in a motorist’s car could disable GPS road user charging across a city – and at the same time disable GPS for the rest of the population. Who would track it down? How long would that take? That is the potential problem. The response of each nation tells us a lot about whether they have woken up to the new satellite navigation world. Initially, the UK and other countries, responded to the threat of losing GPS by stating that they required multiple navigation systems for aviation and shipping – the traditional navigation applications. These are indeed safetycritical areas, and governments have responsibilities for them. But there is no sense here of governments recognising the role satellite navigation is now playing in their economies. What about telecommunications timing, mobile phone users and their emergency calls? The UK and other countries are saying that these millions of new users (perhaps 96% of the market) are not critical and do not really need satellite navigation. If there is a GPS problem, they can just go back to doing what they did before. But they no longer can! Of course, we could lose our car navigators. But whole swathes of industry and commerce have committed to the cost savings and efficiency benefits offered by GPS. Our telecommunications rely on it totally. It is simply too late to go back to what we did before.People do not maintain their old systems alongside the new and pay for both! A recent European report showed that fewer than 40 of 137 GPS applications would remain operational if GPS were lost. So as we commit to the widespread dependence
of our industry and commerce and navigation on satellite systems, we need to retain some terrestrial backup.

A leading candidate, suggested in the Volpe Report, is Loran-C, especially the new Enhanced Loran (eLoran). Loran is a terrestrial system with transmitters of hundreds of kilowatts operating at the low frequency of 100kHz. It is in place across all the US and much of Europe and Asia. The strengths and weaknesses of Loran (Fig 6) are very different from those of GPS. GPS is vulnerable to interference because the distant satellites deliver so little power to our receivers. Loran, with its megawatt stations and tall transmitting antennas, is at least 10,000 times harder to jam. Then, microwave signals from satellites and lowfrequency signals from Loran prevent single-point failures. On land, where buildings and mountains block GPS signals, Loran travels along the earth’s surface, deep into city centres, even into buildings. Like GPS, eLoran is a complete navigation and timing system.

Integrate them together in a single receiver (Fig 7), and eLoran will take over seamlessly when GPS fails.

eLoran in that mode, like GPS, works with an accuracy of metres; it delivered harbour entrance accuracy of better than 10m in recent UK trials. Loran has recently been modernised right across the US to act as a backup to satellite navigation and Europe has to decide whether to do the same. Both the US and Europe will shortly set out their future polices on eLoran.

Many Europeans, though, believe that there is a different solution to the problem of GPS vulnerability: Galileo. The European Commission says:“Galileo will be far less vulnerable than GPS, so eliminating most concerns expressed in the Volpe report … because Galileo will use a variety of different and separate frequencies.

Why is Europe is producing a GPS look-alike? Like GPS, Galileo will be a Global Navigation Satellite System. It is broadly similar to GPS, using the same principles and radio frequency bands, so our receivers
will pick up both. But they are very different in certain important respects.

GPS started life as a US Air Force“weapons aiming system and force enhancer”, and though it now has both military and civil roles, at times of crisis the US military requirement always prevails. Galileo was a wholly civil concept; but gradually questions of its possible military role have emerged, and are now a hot topic. GPS belongs to a single nation, and the rest of us use it on terms that suit US interests. Galileo belongs to the many nations of the European Community who have sought the active participation of other nations.

There are important technical differences between the two systems: frequencies, codes, time standards, modulations, and geodetic frameworks. GPS is free of charge. With Galileo, the user pays for additional services. One is run by the military, the other is to be a public-private partnership. And, of course, GPS is a mature system, the fi rst satellite launched in 1978, a stunning success. Galileo is still essentially a proposal; only the first test satellite has yet been launched. The first true Galileo satellite will be a full 29 years behind GPS!

Tensions between US military control of GPS and its growing civil use led to the imposition of Selective Availability (SA), the intentional reduction of the accuracy available to civil users. Despite SA, civil GPS use fl ourished, and Differential GPS (DGPS) was developed to give high accuracy (Fig 8). In DGPS, a receiver at a differential reference station measures where GPS says it is. It knows where it truly is, so it can compute and broadcast corrections for the GPS errors to users in the region, who apply them and so get metrelevel or better accuracy. The US Coast Guard were pioneers of DGPS. So, by the mid-90s we had the bizarre situation that the US government had spent some $20 billion creating a highly-accurate navigation system, millions more to deny that accuracy to most of the users, and was spending yet more on DGPS. This not only defeated SA, but actually gave civil users higher accuracy than the military GPS that SA was there to defend!