Garmin’s GDL-39 ADS-B Receiver

ADS-B for under $1000

By Bill Cox

Besides all of the gee-whiz and looky-loo aspects, one of the truly outstanding benefits of shows such as Sun ‘n Fun in Lakeland, Florida, and AirVenture in Oshkosh, Wisconsin, is that you have the opportunity to get some hands-on experience with many of the recent technologies in avionics.

Last year, leave it to Garmin, the world’s pre-eminent avionics company, to come up with an innovative and inexpensive product. The company chose Oshkosh to premier 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, and (with a special cable) 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— usually on the right side of the glareshield. The unit consists of an oblong mounting base and four-inch antenna and can be powered by either direct connection to the aircraft or by its own internal Lithium ion battery.

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 glareshield and simply left the iPad on the right seat.

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 Jan 1, 2020.

The worldwide Air Traffic Control system made a giant leap forward with introduction of radar after World War II. However, as with the prototypes of any new technology, the first radars were still fairly primitive devices, often imprecise, hard to interpret, and easily attenuated by weather 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 system (Identification- Friend or Foe) 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 with speed and heading. Eventually, encoders added altitude to ATC’s bag of information on each aircraft.

The trouble with radar was (and still is) that 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 mile—hardly “good enough” for accurate control.

Satellite technology was the answer. GPS, the American satellite navigation system, has a constellation of 24 birds orbiting 10,800 miles above the Earth. Plus, there are another four backups that are usually operating as well. 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, 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. In fact, 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 has 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 installed a mode S Garmin 330 “squitter” transponder in the early 2000s on the premise that I’d have TIS information forevermore.

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. (Updating a Garmin 330 to the ES configuration to accept ADS-B information is possible for $1200.)

Unlike radar, the ADS-B signal is not attenuated by range, altitude, or weather conditions. By definition, an ADS-B position is determined looking down at the Earth from far out in 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 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, and the Alaska test had everyone raving about ADS-B.

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 through much of the state.

Keep in mind, buying the GDL-39 (for a remarkably inexpensive $800) will not 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. It can only receive that information from other aircraft already equipped. 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 vertical/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 to install and use 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 consider the weather picture in Southern California before I take the runway.

Good news, bad news. Most of the time, you won’t be able to do the same trick with ADS-B because the information is delivered via line-of-sight from the same ground stations that provide TIS. XM is heaven sent from above, just like GPS. I departed Compton on one flight with the 696 indicating all the local XM weather before takeoff; then, watched the iPad weather depiction begin to fill in the screen as I passed through 1500 feet.

Uncharacteristically, Los Angeles was ringed by thunderstorms dredged up from the Gulf of California, so there was plenty to paint. By the time I passed 3000 feet, both XM weather on my 696 and the GDL-39 readout on my iPad provided good depictions of the storms, with frightening shades of red and purple scattered across the screens.

In the end, there’s little question that ADS-B will provide a major improvement when it’s fully implemented in 2020.

Note: After this article was completed, Garmin announced a significant enhancement to the GDL-39 in the form of a 3D readout of flight information. This adds altitude, attitude, rate of climb, heading, and airspeed readouts at the bottom of the screen in Garmin’s typical pseudo panel readout. It’s important to remember that these indications are all GPS-based, so for example, speed will be groundspeed, altitude and ROC will be GPS-derived, and heading will be a computerized analysis of repeated track information.

GPS with a Russian Accent

GLONASS, the Russian equivalent of our GPS, became operational on a limited basis in 1995 and was recently expanded to its full 24 satellite universe in 2011. GLONASS actually offers a few advantages over our NavStar system, but it’s not compatible with our cockpit mounted GPSs.

GLONASS, an acronym for Globalnaya Navagatsionnaya Sputnikovaya Sistema (ask that at your next hangar flying session and see if ANYONE knows the definition), does essentially the same job as GPS and offers another 24 satellites with the same navigation features. GLONASS orbits at 11,900 miles, 1000 miles farther out than our system, and offers a lightning fast refresh rate of 10 times a second, twice as quick as our GPS. That’s position accuracy to about every 20 feet for a 200-knot aircraft.

Though GLONASS does not participate in the ADS-B system, Garmin recently produced a wireless receiver to add GLONASS satellite signals for interpretation by your iPad. Put the two systems together, and you have the ingredients for a very comprehensive navigation system that could conceivably provide position information on your iPad or Android device.

NOTE: Garmin’s GLO receiver was also announced at Oshkosh and has a list price of only $99. Buy the full package, including a cockpit mount, power cables, and Garmin Pilot six-month subscription, and you’ll pay $129. Remember, this will only play through an iPad, Android, or other laptop computer device, not a standard portable or panel-mount GPS unit.