The world falls increasingly under the sway of satellite navigation, and GPS continues to improve for voyagers. Recently, I was talking to one of our contributing editors, retired Coast Guard Capt. Bill Brogdon, and he mentioned a recent non-boating trip to Europe. Just for fun, he brought along his hand-held GPS unit — Brogdon was once in charge of the Coast Guard’s Office of Navigation and loves all forms of navigation even when he isn’t out on the water. He turned it on while over there and paid close attention to the results. Almost immediately, he noticed something intriguing.
“The accuracy of the fix was just as good as in U.S. waters,” Brogdon said. That is, the fix Brogdon’s GPS was giving him was the same accuracy as if he were in North America, receiving differential GPS corrections via the Federal Aviation Administration’s wide-area augmentation system (WAAS). There was only one problem: WAAS signals can’t be received in Europe. So what was the source of Brogdon’s on-the-money fix?
The answer is good news for voyagers in European waters. The Europeans are implementing their own version of WAAS. Called the European Geostationary Navigation Overlay Service. This service will provide the same high-accuracy GPS corrections that WAAS does in North American waters. And the domain of improved GPS expands beyond that; a similar Japanese service, called MSAS will also provide satellite GPS corrections in the waters around Japan.
The best part of all this rests with a single word: interoperability. Voyagers with WAAS-equipped GPS receivers will be able to use all three systems as if they were a single system. The development of EGNOS means that, like WAAS in the North American cruising grounds, all of European waters from the Denmark Strait in the north to the Mediterranean in the south, as well as a sizable chunk of the eastern North Atlantic, will have approximately 2-meter accuracy.
What exactly are these systems? It’s not surprising that the average voyager might see the thicket of acronyms and wonder what they mean. Let’s look at differential GPS (DGPS) systems and what they do, and then examine how DGPS systems like WAAS and EGNOS work.
The first DGPS system developed to increase GPS accuracy was built (and is still operated) by the U.S. Coast Guard. It uses 60 marine radiobeacons (remember them?) nationwide to broadcast corrections that allow DGPS-capable GPS receivers to correct GPS signals and thus get more accurate position fixes. One of the driving factors in the design and deployment of the Coast Guard’s DGPS network was the Defense Department’s former policy of selective availability (SA). A purposeful error introduced into GPS signals, SA was designed to deny the full accuracy of GPS to those who would use it to attack the United States. The Coast Guard’s DGPS system removed the SA inaccuracy. And, in fact, DGPS does more than that — it also removes other sources of error inherent in the system. With the Clinton administration’s May 2000 decision to remove SA from the GPS signal, the DGPS network seemed to lose some of its raison d’être: Why did anyone need DGPS when GPS signals are broadcast in their full accuracy?
The answer to this was prompted by the needs not of boats and ships but of airplanes. In the 1990s, the FAA engaged in a process of certifying GPS as a sole means of navigation for aircraft. GPS, however, was not designed as a civilian navigation system with built-in redundancy. If one of the GPS satellites in view malfunctioned, there was no way for an airliner traveling at 400 knots to get an immediate warning that its position fix was in error. And the other issue involved pure accuracy. In order for GPS to be used for precision all-weather approaches and landings, the system needed the best possible accuracy, better than what was possible using GPS alone.
The FAA’s solution was a two-tiered system of correction signals: one tier uses satellites to broadcast correction signals over a wide area; the other tier uses local, ground-based transmitters. The wide-area element uses geostationary satellites to send a correction for every satellite in view. These corrections are then correlated inside a WAAS-capable receiver with the satellite signals to provide a highly accurate GPS fix, down to 2 meters. Since it uses geostationary spacecraft parked in orbits 23,000 miles above North America, users throughout the continent and voyagers more than 100 miles at sea can take advantage of these signals for correcting the GPS fixes. Unlike the Coast Guard system that requires a receiver than can tune to the medium-frequency signals of the Coast Guard’s radiobeacon network, WAAS signals are broadcast on the same frequency used by the GPS satellites themselves. This means that no additional tuning circuits or antenna are needed to use and apply WAAS signals, they can be picked up by a standard GPS receiver. Of course, to use the WAAS corrections, a GPS receiver must have WAAS-capable software.
So the satellite-based WAAS system gives users in North America and its offshore waters DGPS correction signals that, provided your receiver is equipped with the right software, are automatically applied to standard GPS signals. The result is impressive accuracy of 2 meters. That’s good for both pilots and for mariners.
Voyagers sailing in Europe are in luck, too. EGNOS provides the same capability as WAAS. During its test phase, the EGNOS system used transponders on Inmarsat satellites. Now the system is moving into its operational phase and uses transponders on two Inmarsat satellites and a European telecommunications satellite called Artemis.
The development of EGNOS resulted from the desire of European authorities to have their own system for correcting GPS signals, should the U.S. government reintroduce SA. EGNOS gives European authorities an independent method for eliminating GPS errors. But the Europeans aren’t stopping there. They have been moving ahead with their own satellite navigation system called Galileo. If Galileo does fly, it would be a great boon to navigators: multiple satellite navigation systems girdling the globe, GPS and Galileo providing accuracy and redundancy. That might be the golden age of navigation.