Garmin’s GDL 39 portable ADS-B receiver offers a simple solution for pilots who want to receive free weather and traffic information without a lot of complication. The GDL 39 sells for $799 and receives both types of ADS-B signal that are a unique feature of the U.S. ADS-B landscape. Not all portable ADS-B receivers are dual-band; some receive only the frequency that provides free weather information and certain traffic targets.
The GDL 39 is small enough to sit on a glareshield without obscuring the outside view, or it can be mounted on a plate (included) or placed in an empty seat when using remote antennas. A remote GPS antenna is included, and the GDL 39 can be hooked up to a permanently installed external ADS-B antenna (DME and transponder antennas are generally compatible). Without the remote antennas, the GDL 39 does need a clear view of the sky.
The U.S. ADS-B network is filling in rapidly, with much of the coastal and central states already covered and full coverage in the entire U.S. (including Hawaii and Alaska) due by year-end. The chief advantage of portable ADS-B receivers is the ability to receive free weather information over the 978 MHz universal access transceiver (UAT) datalink frequency. This frequency was designed to offload light aircraft ADS-B signals from the 1090 MHz extended squitter (ES) datalink frequency designated for larger aircraft that typically fly above 18,000 feet. Only 978 UAT provides ADS-B in weather information. Both 978 UAT and 1090 ES datalinks provide traffic information.
While portable ADS-B receivers do not meet any ADS-B mandates (ADS-B out equipment is required in most U.S. aircraft by Jan. 1, 2020), they are a good way to start learning about the ADS-B network and available IN services. Portable receivers do not have transmitters, which are part of installed ADS-B out equipment that meets the mandate.
In addition to ADS-B features, portable receivers usually contain a highly accurate GPS receiver. The GDL 39’s Waas GPS connects via Bluetooth to the iPad and delivers a rock-solid GPS signal to iPads running the Garmin Pilot app and Garmin’s aera 796 portable GPS. The GDL 39 can connect to multiple Bluetooth devices, so everyone in the cockpit can view traffic, weather and navigation data on his own iPad/iPhone and Android device.
I suggest spending time learning how to use the GDL 39 somewhere on the ground where you can receive signals from an ADS-B ground station before flying. While there’s nothing unusual about the GPS feature, the traffic displayed in Garmin Pilot from the GDL 39 will likely be a new experience. In a busy metropolitan area like Los Angeles, the amount of traffic that shows up on the iPad is amazing (and not unexpected).
Because it is dual-band, the GDL-39 receives traffic from other aircraft broadcasting ADS-B out signals, from both 978 UAT and 1090 ES datalinks. (If you’re using a single-band 978 UAT receiver, you won’t see traffic broadcasting on 1090 ES air-to-air.) For now, few aircraft are equipped with 978 UAT ADS-B out systems, so you won’t see many of those in terms of air-to-air signals. However, you will likely see a lot of larger aircraft broadcasting the 1090 ES ADS-B out signal because that is the same as a mode-S transponder, with which most of these aircraft are equipped.
The other main source of traffic information is the FAA’s feed, traffic information service-broadcast (TIS-B), which comes from FAA radar and is then rebroadcast through ground stations to aircraft receivers. There is a caveat for TIS-B: the rebroadcast isn’t sent until an aircraft with ADS-B out “wakes up” the ADS-B ground station to let it know to start sending information. For this reason, Garmin recommends that operators not wait for the 2020 mandate to install ADS-B out equipment. In a busy metropolitan area, there is a good likelihood that an ADS-B out-equipped aircraft will wake up ground stations. TIS-B traffic supplied by the FAA includes targets within plus/minus 15 nm and 3,500 feet of your aircraft.
With multiple sources of traffic data, one might wonder what happens when, say, an air-to-air datalink target is the same as one displayed on controllers’ radar scopes and then redelivered via TIS-B. Garmin handles this by automatically displaying the target with the “highest-integrity traffic data, preventing ghost or duplicate targets.” Each displayed ADS-B target shows TargetTrend relative motion tracking. Instead of displayed targets oriented to the way they are tracking across the ground, TargetTrend shows traffic movement in relation to your flight path, a much more intuitive way to view traffic. The GDL 39 also provides visual and aural traffic alerts.
Note that Garmin warns users: “The GDL 39 is an aid for visually acquiring nearby aircraft. It should never be assumed that the GDL 39 is providing complete information about the traffic in the area.”
The subscription-free weather information–Flight Information Service-Broadcast (FIS-B)–delivered via the GDL 39 worked well. Services include Nexrad (Conus and regional), TFRs, TAFs, Metars, Pireps, Notams, Sigmets/Airmets and winds/temperatures aloft. Weather information was easy to pull up using Garmin Pilot on the iPad.