Plane & Pilot
Tuesday, October 2, 2012

Garmin’s GDL-39 ADS-B Receiver

ADS-B for under $1,000

The GDL 39 provides portable ADS-B "In" reception for access to traffic and weather on compatible Garmin portables and other mobile devices, such as the Apple iPad, shown here.
The trouble with radar was/is it's only modestly accurate. Even on its best day, radar simply isn't quick enough to stay ahead of the aircraft. A typical radar dish revolves once every five seconds. In five seconds, an aircraft travelling at 440 knots will have moved .7 miles, hardly good enough for accurate control.

Satellite technology was the obvious answer. GPS, the American Navstar satellite navigation system, has a constellation of 24 birds (plus four backups that are normally also operating), orbiting 10,800 miles above the Earth. At any given moment, roughly half those satellites may be within "sight" of your aircraft. GPS has a refresh rate of about five times a second. In other words, it updates its position every fifth of a second. The same 440-knot jet using ADS-B would therefore be sending out position information every 150 feet.

Just as you can use the multiple triangulation signals from so many satellites to establish your position in three dimensions to within a few cubic feet above the Earth, the satellites can do the same thing in conjunction with ATC, coordinate your changing location as you move across the planet, and report that information to the ground. ADS-B operates on two frequencies, 978 and 1090 mHz, and the Garmin GDL-39 receives both of them.

Of course, the new universal GPS-based, all-encompassing air traffic control system introduces levels of complexity undreamed of by most mere mortals. The level of electronic sophistication required to coordinate signals from 24 satellites and thousands of aircraft is a job for extremely accurate computers, part of the justification for delaying full ADS-B implementation until 2020.

The true joy of full ADS-B is that each aircraft broadcasts its position to every other aircraft with an ADS-B transceiver, plus it sends its position to ATC, as well. When the system is fully implemented in 2020, it will represent the best of all possible worlds.

In their ultimate wisdom, the FAA decided one way to encourage everyone to upgrade is to reduce the level of safety for current users. To that end, the FAA is selectively turning off the current TIS (Traffic Information Service) available to owners of mode-S transponders so they can no longer receive traffic alerts. This is supposed to encourage everyone to get on board with ADS-B as quickly as possible.

Like so many other aircraft owners, I was seduced into buying a mode-S Garmin 330 "squitter" transponder in the early 2000s on the premise that I'd have TIS information forevermore.


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