Plane & Pilot
Tuesday, October 2, 2012

Garmin’s GDL-39 ADS-B Receiver

ADS-B for under $1,000

Connecting the GDL 39 with the FAA's ADS-B weather link, your compatible device, such as the GPSMAP 696, can display subscription-free weather reports and more.
Not. The feds have already turned off many TIS transmitters around the country and are gradually making it more difficult to operate without ADS-B. For better or worse, I fly in the world's busiest airspace, the Los Angeles Basin, so TIS in my neck of the sky will probably remain available for a while. It's nevertheless exasperating that the FAA feels the best way to gain acceptance for ADS-B is to eliminate a current traffic alert system. (Updating a Garmin 330 to the ES configuration to accept ADS-B information is possible for $1,200.)

Unlike radar, the ADS-B signal isn't attenuated by range, altitude or weather conditions. By definition, an ADS-B position is determined looking down at the Earth from far out on space, so terrain also has little effect on an ADS-B signal.

For those very reasons, the FAA launched a critical test of ADS-B between 1999 and 2006 in Alaska known as the Capstone project. I spoke at the Alaska Airmen's Association Convention in Anchorage in May 2000, and I had the opportunity to talk to a number of bush pilots who were testing ADS-B for the FAA. Alaska was an ideal location to evaluate the system's ability to locate and direct aircraft in bad weather and mountainous terrain.

The 49th state has plenty of tall mountains that make standard line-of-sight radar control ineffective for low-flying aircraft. GPS is a look-down technology, however, so it allowed ATC to track an aircraft flying in a mountain canyon pretty much anywhere in the state.

Keep in mind, buying the GDL-39 (roughly $800) won't allow you to transmit ADS-B information to other aircraft and to Center. The GDL-39 is strictly an ADS-B "In" receiver. It can't advise ATC or other aircraft of your position. You'll need an ADS-B "Out" transmitter for that (optional at extra cost).

I flew my tests over two weeks around Southern California, which means I was nearly always within 15 miles of an airliner, either horizontally or on slant range, as they negotiated the busy airspace around LAX, Ontario, Orange County and 14 other airports in the immediate Los Angeles area. Most airliners are already equipped with ADS-B Out transmitters, so I could hitchhike on their signals and read all the traffic they were seeing.

Of course, another major incentive of installing and using ADS-B is free weather information, compared to the subscription fees associated with XM weather (starting at about $30/month). I have XM weather available on my Garmin 696, and it can be illuminating to sit on the ramp and evaluate the weather in Southern California before I take the runway.


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