The wreckage of AirAsia Flight 8501, which disappeared with 162 people onboard while flying from Surabaya in Indonesia, was discovered in the Java Sea this morning. Air traffic controllers lost radar contact with the Airbus A320-200 on Sunday after the pilot requested permission to divert around a storm. And the lengthy search that followed highlights that airliners still cannot be tracked precisely over the sea.
The aviation industry has lived with the fact that aircraft are devilishly difficult to track over large bodies of water since the dawn of transoceanic flights in 1919. In recent decades new satellite tracking technologies have emerged, but most are considered too expensive within the airline industry.
Aircraft are tracked over the ocean as pilots report their positions via long-wave high frequency (HF) radios. Unlike radios that operate in the very high frequency (VHF) and ultra-high frequency (UHF) bands, HF radios are not limited by line-of-sight because their radio waves bounce off the Earth’s ionosphere. But these frequencies have to be shared, so it isn’t practical to continually stream updates over HF.
One reason that aircraft are tracked this way is due to the curvature of the Earth. “Many [radars] are located on mountaintops so that they see as far as possible,” says Rick Castaldo, a former U.S. Department of Transportation surveillance engineer who has consulted extensively with industry around the world. “But the performance for targets is usually limited to 250 miles. After that, the radar return is so small you don’t reliably detect and process targets.”
AirAsia 8501 was within radar range when it disappeared, although it had somehow fallen off radar tracking. The accuracy of radar depends on the radar cross-section, which is to say the size, of whatever is being tracked.
Air navigation service providers like the U.S. Federal Aviation Administration are gradually moving over to new GPS-based technology in the form of automatic dependent surveillance broadcast (ADS-B). An aircraft automatically generates positional data based on the input from its GPS navigational system and broadcasts its information to ultra-high frequency receivers around a country. The GPS signals are used to calculate the plane’s own position, velocity, and altitude.
That system currently also requires a ground station within 250 miles to pick up its signals—and therefore cannot be used over the sea. It does, however, have an air-to-air capability designed for use in environments like the ocean that allows aircraft to “see” each other—the FAA is now trying out this technique.
Another option is an automated satellite-based position reporting system. An aircraft provides controllers with all pertinent aircraft data such as identification, position, altitude, speed, and heading. These reports are usually sent every 15 minutes to 30 minutes, and such tracking can be expensive, involving contracts that require the airline to pay by the message.
Some emerging technologies could make it easier—and cheaper—to track airliners over the ocean. Two companies, Virginia-based Aireon and Alaska-based ADS-B Technologies, are working on new satellite-based surveillance solutions.
Aireon is expected to launch an ADS-B receiver onboard a communications satellite in 2015. As more satellites go up, the Aereon system could be operational as early as 2017. Aireon’s technology does not require any changes to airliners operating around the world. Nor should the service cost airlines money—instead, air navigation service providers like the FAA would pay to use Aireon.
ADS-B Technologies is also aiming to use Globalstar’s existing constellation of satellites and existing radio frequencies. However, Globalstar’s satellite constellation has not yet been fully deployed and does not have worldwide coverage.