Precision Navigation In European Skies

Geostationary satellites provide differential GPS with a positioning accuracy of 1-2 meters

3 min read

A global system of air traffic control based entirely on global positioning systems, an inevitable idea that has been inching toward realization for more than a decade, came a centimeter or two closer on 6 June, with the first transmission of test signals from the European Geostationary Navigation Overlay Service (Egnos). The point of Egnos is to provide error correction to geopositioning signals, relying on dedicated equipment installed on three geostationary satellites and a network of ground reference stations [see map], so that locations can be determined at an accuracy of close to 1 meter.

For now, Egnos will error-correct signals from the U.S. Global Positioning System (GPS) and Russia’s Global Navigation Satellite System (Glonass). Eventually, though, the expectation is that it will error-correct signals from Europe's own constellation of global positioning satellites, dubbed Galileo. Galileo will rely on 30 low-earth-orbiting satellites, together with ground stations, and is expected to begin transmitting its first test signals in 2006.

Europe's insistence on proceeding with Galileo has raised hackles in U.S. policy circles, where the system tends to be seen as redundant at best and threatening at worst. In particular, because Galileo—unlike GPS and Glonass—will not be under military control, U.S. specialists have feared its signals could be hijacked by military adversaries or terrorists.

For now, however, those concerns seem to have been quieted by assurances from members of the European Commission (EC, Brussels) that system security will be maintained. And as it happened, the arrival of the first test signals from Egnos coincided closely with the commissioning on 10 July of the U.S. wide-area augmentation system (WAAS), which replaces U.S. commercial air navigation based on fixed flight corridors with much more flexible GPS-based navigation. Accordingly, two complementary systems of air traffic control based on global positioning are taking shape and in due course can be expected to embrace the world.

Egnos system elements

Egnos obtains corrected locations using 34 ranging and integrity monitoring stations (RIMSs). They measure the position of each Egnos geostationary satellite and compare accurate measurements of where the low-earth-orbiting satellites are with measurements taken from the signals themselves. Measurements in hand, the RIMSs relay the data to four master control centers over a dedicated communications network [for background, see "X Marks the Spot, Maybe," IEEE Spectrum, April 2000, pp. 26-36].

In turn, the master control centers determine the accuracy of the GPS signals received by the RIMSs, calculate position inaccuracies caused by ionospheric disturbances, incorporate the deviation data into a signal, and relay it to the uplink or navigation land earth stations. From there the signal is sent to the three Egnos satellites, which transmit it to Egnos-capable receivers.

The net result is that the positioning accuracy of Egnos-equipped GPS receivers is boosted to 1-2 meters from the 15-20 meters typical of civilian GPS receivers that rely solely on GPS or Glonass signals. (Military GPS receivers can access encrypted signals from the GPS with 1- to 2-meter accuracy.)

To be sure, much still remains to be done before Egnos officially begins operating in the spring of 2004. The first thing is to get the system's RIMSs on-line, plus the four master control centers, seven uplink stations, and a dedicated communications network for the whole project. So far, one master control center is operating and the three Egnos geostationary satellites are aloft: one Inmarsat-3 each over the eastern Atlantic Ocean and the Indian Ocean, and Europe's Artemis satellite over Africa.

On 6 June the European Space Agency (ESA, Paris) began running the first master control center for Egnos in Langen, Germany. Egnos is a joint project of the EC, ESA, and the European Organization for the Safety of Air Navigation (Eurocontrol).

Diverse applications

With the greater positioning accuracy of Egnos, trains and automobiles—as well as planes—can navigate based on a better awareness of their own position. For starters, trains could be spotted on a specific track rather than just running on any of three parallel rails. Or, if rail service is hampered by signaling incompatibilities across national borders, operators could use Egnos-based positions in place of conventional signaling.

Unmodulated positioning signals from Galileo, like those from GPS and Glonass, will be free. But error-corrected signals will cost: in general, the greater the accuracy, the higher the price. According to a 2001 study by PricewaterhouseCoopers LLP (New York City), Galileo should produce revenues of ¤17.8 billion (US $15.8 billion) from its 2008 start-up to 2020.

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