ADS-B delayed down under
Australia's privatized air traffic control service has delayed deploying ADS-B (automatic dependent surveillance-broadcast) in lower-level airspace, but that decision doesn't affect U.S. plans to transition from radar to ADS-B.
"Australia was planning on implementing their ADS-B system differently," said Randy Kenagy, AOPA senior director of advanced technology. "That creates different issues for ADS-B down under."
ADS-B is an alternative to radar and transponders for air traffic control surveillance. ADS-B avionics in the aircraft broadcast the aircraft's location (as determined by GPS) to the ground and to other aircraft over a datalink.
In the U.S. implementation of ADS-B, that same datalink will be used to send free weather and traffic information back to aircraft. GA pilots will be able to see on a multifunction screen up-to-the-minute weather graphics - including weather radar - weather reports and forecasts, and the relative positions of nearby aircraft that are not already transmitting ADS-B on the datalink.
In Australia, there had been media reports that ADS-B could be "spoofed" - someone with a laptop computer and the right transmitter could supposedly add hundreds of fake ADS-B targets to a controller's screen.
That wasn't the "official" reason for delaying ADS-B, but many observers thought the press reports played a role in the decision. (For the record, Airservices Australia said that implementing ADS-B for lower-level airspace "raises a number of operational and policy issues that require resolution before a decision to proceed can be made.")
Spoofing isn't much of an issue in the United States, both for technical and regulatory reasons.
Here, the FAA plans to require ADS-B infrastructure to include a validation function. Simply put, that means there must be an independent way to ensure that ADS-B targets are legitimate.
Also, in the United States the FAA chose UAT - universal access transceiver - as the ADS-B datalink for general aviation. Australia chose to use as its ADS-B datalink the 1090-MHz Mode S extended squitter system. Mode S extended squitter does not have nearly the data capacity of UAT, nor is it as robust.
"We pushed hard for UAT because of the other benefits it could provide to GA pilots," said Kenagy. "The UAT datalink has more than enough capacity to send weather and traffic information back to the pilot. Also, it's technically easy to validate - and harder to spoof - a UAT transmission."
Validation is accomplished by looking at the GPS location report and seeing if that data makes sense compared with a position derived from calculating the time it takes for the signal to travel from the aircraft to the ground receiver.
Validation of the Mode S ADS-B system is possible as well, but it's unclear to some in the industry as to how Australia was planning to ensure that an ADS-B position was legitimate, although they undoubtedly had considered the problem.
"However, the fact remains that recent articles in the media have generated enough discussion for AOPA to ensure that the FAA has their 'validation' strategy well in hand," said Kenagy. "From what we've seen, it appears they do."
But is spoofing really that big of an issue? Consider how easy it is today to spoof an air traffic controller. Someone with an inexpensive aviation radio could pretend to be a controller and issue bogus instructions. It has happened once or twice, but it certainly isn't a widespread problem, and it didn't wreak havoc with the ATC system. Radar returns and ILS signals, theoretically, also could be spoofed. But in more than half a century of ATC experience, that hasn't been a problem either.
"It's easier said than done," said Kenagy, "and pilots and controllers are pretty good at recognizing a problem and working around it."
July 5, 2006
