Plane & Pilot
Tuesday, October 2, 2012

Garmin’s GDL-39 ADS-B Receiver

ADS-B for under $1,000

One of the truly outstanding benefits of shows such as Sun 'n Fun in Lakeland, Fla., and AirVenture in Oshkosh, Wis., is that pilots have the opportunity to examine and often get some hands-on experience with many of the latest technologies in avionics.

This year, Garmin chose Oshkosh to premiere their new GDL-39 ADS-B receiver. This is a system that currently may be mated to an Apple iPad or Android tablet, plus a Garmin 796. Not too far down the road, it will be capable of mating with the Garmin 696, G3X or aera GPS. The iPad, Android and 796 can all use Bluetooth connectivity, but the other Garmin products will need to be wired directly.

The GDL-39 is a compact unit that may be placed wherever it's convenient as long as it has a clear view of the sky. That will usually be on the right side of the glare shield. The unit consists of an oblong mounting base and four-inch antenna, and may be powered by direct connection to the aircraft or the system's own internal Li-Ion battery.

Immediately following EAA AirVenture, Garmin arranged for me to borrow a GDL-39 for a few weeks back in California, and I mated it to my iPad2 via Bluetooth. The iPad2 is a wonderfully talented device as a reference in the cockpit, but it may be too large to conveniently fit in the front office of a typical general aviation airplane. There's also a problem with display brightness in strong sun. For the purposes of this evaluation, I placed the GDL-39 antenna on the glare shield and simply left the iPad on the right seat of my Mooney.

Before looking at the specifics of the GDL-39, it's important to understand what ADS-B is and does. The letters stand for Automatic Datalink Surveillance-Broadcast. In conjunction with Communication, Navigation and Surveillance (CNS) technologies, ADS-B is the key to air-traffic management of the future. The FAA has mandated that most ATC functions will be controlled by ADS-B by January 1, 2020.

The worldwide Air Traffic Control system made a giant leap forward with introduction of radar after World War II. As with the prototypes of any new technology, the first radars were still fairly primitive devices by today's standards, often imprecise and hard to interpret, easily attenuated by weather and other atmospheric anomalies. Beginning in the early 1950s, ATC began maintaining separation with conventional radar augmented by pilot communications, reporting elevation and other information.

What had been the old military IFF (Identification—Friend Or Foe) system gradually gave way to transponders with discrete codes that allowed controllers to pinpoint precise information for specific aircraft. As the system evolved, ATC's surveillance radar would send out a signal to all aircraft within range, and the transponders would respond. ATC could then determine speed and heading of each aircraft. Eventually, encoders made it possible to add altitude to ATC's bag of information on each aircraft.


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