When it comes to keeping tabs on the location of aircraft, radar has long ruled the roost. But radar range is limited, and long-haul planes become untraceable when passing over oceans and large deserts or polar regions. By equipping orbiting satellites with instruments that listen in on ADS-B signals, scientists think that it should possible to track aircraft over the course of their entire journey, and with the launch of Proba-V, they're ready to put the idea to the test.
ADS-B stands for Automatic Dependent Surveillance – Broadcast. It's a safety system designed to supplement, and perhaps one day replace, radar. Aircraft equipped with ADS-B monitor their own location using GPS, and constantly report that data to other aircraft and Air Traffic Control using a data link transmitter. According to Flightradar24, 70 percent of all passenger aircraft in Europe are equipped with ADS-B, and though the equivalent statistic is 30 percent in the US, by 2020 ADS-B will be mandated on all flights that require transponders today. Being more accurate than radar, it's hoped ADS-B will allow a higher volume of air traffic in already-busy airspaces. However, whether air-to-air or air-to-ground, its range is limited to about 200 to 250 nautical miles (370 to 460 km), which is insufficient for keeping a permanent eye on a long-haul passenger jet. From space, however, it could be a different story.
Though ADS-B requires GPS satellites to ascertain its position, they play no part in actual monitoring (GPS satellites are dumb, to all intents and purposes). But on Tuesday, the ESA's Vega rocket took off from French Guiana carrying the Proba-V satellite. It's main mission is to monitor changes in Earth's vegetation, but it is also equipped with a receiver to pick up ADS-B signals from its altitude of 820 km (509 miles). Over the next two years, scientists at the German Aerospace Center (DLR) will monitor proceedings to see if it might be possible for satellites to monitor aircraft over their entire journey.
The DLR has cause for optimism. In 2009, a high-altitude balloon drifting some 30 km (19 miles) above the North Sea equipped with the same ADS-B receiver aboard the satellite was able to pick up a flight en route to Amsterdam from Beijing at a distance of 1,100 km (680 miles). In 2012 a second balloon test at an altitude of 40 km (25 miles) investigated the potential for interference from multiple signals and radar.
Yet the work is in its early stages and the researchers say that the first tests will be to work out the limits of the receiver. From there they can start finding answers to their questions, such as the possible range of monitoring air traffic from space using ADS-B.
Later this year, DLR's AISat satellite will enter orbit to monitor maritime traffic using a 4-meter helical aerial. "With both systems – aircraft signals as well as signals from shipping – we want to contribute to closing the gaps in location data," says Jörg Behrens of the DLR's Institute of Space Systems. If he's right, the key to keeping tabs on global air traffic could boil down to looking at it from above rather than from below.