Pilot Report: flying with the Garmin GDL 39 ADS-B receiver


Garmin generated a lot of buzz last month with the release of the GDL 39, a portable GPS and ADS-B traffic and weather receiver. This small antenna receives the signals from ADS-B ground stations in the US, and then wirelessly sends the data using Bluetooth to an iPad running the Garmin Pilot app. Along with the release of the GDL 39 Garmin also made a significant update to the Garmin Pilot app to support these new features, and they did an excellent job with the hardware/software integration.

For this product test we flew with both the Garmin GDL 39 and the Stratus from Sporty’s.

The Equipment

To test out the Garmin GDL 39 we recently flew a trip in a Piper Aztec from Cincinnati, OH, to Akron, OH. The weather was forecast to be VFR for the flight, though there was a good deal of precipitation just south of our route in Kentucky–a good test to check out the ADS-B weather radar display on the Garmin app. We also brought along a Stratus ADS-B receiver to compare the weather information with ForeFlight Mobile. It was easy to connect to both devices to the same iPad since the GDL 39 uses Bluetooth and Stratus uses Wifi.

The Aztec is equipped with a Garmin GTN750 panel mount GPS navigator that, among other things, displays traffic from an active-interrogation traffic advisory system (TAS). This displays ALL the nearby traffic (regardless of radar or ADS-B coverage), and allowed us to accurately assess what percentage of traffic was actually being displayed by the portable Garmin ADS-B system.

An important thing to remember with ADS-B traffic is that it’s not continuously broadcast to all aircraft like the weather products. Rather, you must be equipped with a certified/installed ADS-B Out system in order for the traffic picture to be sent back to you. But with the deadline for ADS-B out equipage still 6 years away, very few aircraft have this capability. So the only traffic you will most likely see when using the GDL 39 are those airplanes equipped with ADS-B Out (specifically those with 1090 ES transponders, which are typically airliners). This is no fault of the Garmin’s system, just a reality in this early phase of ADS-B adoption. ADS-B traffic is a complicated subject, and we’ll have more information on traffic in an upcoming post. Garmin also has some great information at their ADS-B Academy educational site.

ADS-B Traffic Performance

The dedicated traffic page on this flight was only showing aircraft equipped with 1090 ES transponders, which were typically airliners (FedEx flight 1780 in this example).

After pairing the GDL 39 with the Garmin app you’ll see a new option in the Home menu to bring up the dedicated traffic view. This sophisticated traffic display is very similar to what’s shown on Garmin’s panel mount traffic systems. You can configure several options here, including altitude filters to show unrestricted traffic, traffic above (useful during climbs), a normal view setting (useful during cruise), and a traffic below view (useful during descents).

You can also toggle the motion vector originating from the traffic to show either a relative vector (TargetTrend is what Garmin calls it) or absolute vector. The relative vector is particularly useful since it shows the aircraft’s motion relative to your position and groundspeed. For example, if you were overtaking an aircraft that was heading in the same direction as you but at a slower ground speed, the relative vector would actually be pointed opposite its direction of flight indicating that you are overtaking the aircraft. You can also tap any of the traffic targets to display N#, direction, ground speed, pressure altitude, climb/descent rate and closure information.

Traffic can also be displayed on the moving map, and this overlay is enabled with a setting located under the Menu button in the upper right corner of the screen. At the upper left of the map are status icons to indicate what type of traffic you are receiving, either Air-to-Air targets coming directly from aircraft equipped with 1090 ES out or TIS-B traffic originating from ADS-B ground stations. Since the Aztec does not have ADS-B Out capability and cannot ping the ADS-B ground stations for the TIS-B traffic picture, we only saw a few traffic targets show up with the system for the entire flight.

The Garmin GTN active traffic (left) shows all in-range targets over Columbus, while the ADS-B traffic picture Garmin app (right) displayed no traffic.

We were able to see all in-range traffic targets on the installed Garmin active-traffic system in the Aztec, and probably 5 – 10% of those showed up through the GDL 39 on the Garmin app. All but 1 of the traffic targets were airliners transitioning the terminal areas, and the 1 GA aircraft that did show up was about 70 miles away. At one point while passing through the Columbus Class C airspace we had 5 airplanes show up on the installed traffic system, while the ADS-B display on the Garmin app showed none of them. And we had 2 traffic callouts from ATC near Cleveland that showed on the installed active traffic system, but again neither showed up over ADS-B. This is no fault of the GDL 39, but just goes to show that a portable ADS-B traffic system provides a very incomplete traffic picture unless you have ADS-B out in your aircraft.

ADS-B Weather Performance

Most pilots will buy the Garmin GDL 39 for it’s subscription-free ADS-B weather capability, and the performance of this feature is excellent. The system began receiving data from ground stations just a few hundred feet after takeoff from the Cincinnati area, and began displaying regional radar not

The ADS-B weather performance was similar on both the ForeFlight app with Stratus (left) and the Garmin Pilot app with GDL 39 (right). Note that the radar opacity was lowered slightly on ForeFlight when the screens were saved.

long after. Similar to the ForeFlight/Stratus combo, the weather products can be displayed right on the moving map on top of sectional and en route charts.

One thing to take note of is that there are two options for radar on the map overlay settings–you need to choose the 2nd option called “Radar (FIS-B).” The “Radar” option shows internet-based weather radar when on the ground. And like ForeFlight/Stratus, the Garmin app combines the 250NM regional radar and national radar picture seamlessly with the same overlay toggle. See the screen at the right comparing the the same weather side by side (the radar opacity is turned down slightly on ForeFlight).

Garmin GDL 39 status page in the Garmin Pilot app. The map depicts location and signal strength of ADS-B ground stations.

The Garmin app displays all the ADS-B weather products, including NEXRAD radar, METARs, TAFs, Winds Aloft, Pireps, TFRs, Airmets/Sigmets and NOTAMs. The status page provides complete system information, including GDL 39 battery status (if installed), weather age, type and number of of traffic targets received, and even a map showing number, location and signal strength of ADS-B stations from which you’re receiving data. This is a handy display for tracking when you might fly out of ADS-B coverage.

One unique feature of the GDL 39 is that it continually caches the weather data to the antenna’s internal memory. This means you don’t have to keep the Garmin app active throughout the entire flight; once it wakes up from sleep mode or is switched to from another app, the GDL 39 will automatically send the latest weather data and not have to wait for the next data transmission from the ADS-B ground stations. Garmin Pilot can also loop the radar image, so you get an idea of a storm’s movement.


At $799 the GDL 39 is in the same price range as other portable ADS-B receivers on the market, and has the added benefit of integrating both 978 UAT and 1090 ES receivers for traffic. As mentioned above though, you will only see a small percentage of nearby traffic unless you are equipped with a compliant ADS-B transponder in your aircraft. The weather reception was nearly identical to Stratus, and the tight integration with the Garmin app is a big plus.

One other feature that may be of interest is the GDL 39 can display weather on Garmin portable GPSs in addition to the iPad, including the entire aera series, the 696/695 and 796/795. This is a nice way to get some added value out of an older GPS you may have sitting on the shelf. A different part number should be ordered for this application.

Garmin offers an optional battery for those who don’t have access to 12/24v power in their airplanes, and they’re even tossing in the battery for free if you order before August 31. Click here for more info on the Garmin GDL 39.


  1. […] For those without ADS-B Out, the traffic feature is most useful in the terminal area, where airlines are coming in to land. There, you’ll see a lot of air-to-air ADS-B traffic, regardless of what ground stations are around. This is handy if you’re flying into a major airport. But GA traffic is very limited with ADS-B right now, so you won’t see much at the country airport. For a real world pilot report on the GDL 39, read this article. […]

  2. You refer to transponders with ADS-B capability. Name some and their prices. Hope they are not too expensive.

    • Charles, the easiest option is a Garmin GTX 330 (if you happen to have one). This can be upgraded for about $1200 to ADS-B Out. Garmin also recently introduced the GDL-88, which will be available soon. It will cost around $3-4000.

    • Garmin sells the GTX 330es which runs around $5000 not including installation. I think that’s about an average price for other manufacturers as well.

  3. Hey John, enlightening! I just bought the GDL 39 and it’s great.

    Quick question do you need the difference between the traffic data provided by ground stations vs. aircraft? There isn’t anything online stating either way. Thanks

    • Nader, the only data provided by aircraft is the ones with ADS-B Out. The grounds stations show all Mode C aircraft, which is a lot more. So you could get by with only the ground station traffic, but not with only the air-to-air.

  4. I live in Idaho and was researching purchasing either the Stratus/Foreflight or Garmin GDL39/Garmin app for my iPad. This is the first webpage discussion that I have heard that ADS-B does not work in Wyoming, Idaho, Utah, Montana, most of the places I fly!! Can anyone expand on that?? Also I have a Garmin GTX330, does the $1200 upgrade include parts and labor or just parts?? Thanks

  5. Chad
    I am doing the exact same thing. I also have a GTX330. I have emailed back and forth with Garmin Aviation Support. They say a Garmin dealer must send off the GTX330 for the upgrade to ES. But that’s not all. A shielded cable must be added to feed the data from the Garmin 530W to the GTX330ES. To answer that question, I guess I need to contact a Garmin dealer. I am probably going to get the Garmin GDL39/app or Stratus/Foreflight. I also trying to find out the subscription cost for the app plus geo-referencing.

  6. I just talked to my Garmin rep in Idaho Falls. He is going to a conference in Vegas next week were he believes he will learn a lot. He has not done an upgrade on the 330 yet. I had read about the shielded cable which he said is not a major deal, but I don’t think he knew the unit had to be sent in to Garmin, I didn’t.
    I have a 530 and Mx20 and can use XM weather but I was wanting to get away from the monthly fee. I live 1/2 the time in Idaho and 1/2 in Arizona so I still think I will go with the stratus or Garmin GDL39. After much reading I still can’t decide which. Think currently Foreflight has the edge but Garmin might catch up. Which one are you leaning towards??

  7. I just purchased the GLD 39 and using my area 795 to display ADS B. ADS B weather worked perfect. I also have a Garmin 530W and 330 transponder. I will have my avionics shop send off my 330 to upgrade to 330ES then I will expect to see ADS B traffic. I will continue to use foreflight however for more for flight planning and back up in the air. I like the foreflight geo referenced approach plates.

  8. I have the GLD 39. The Nexrad radar is a little blocky compared with the Foreflight unit. This on an iPad 3. And the battery life stinks! Around 4 hours is it. I can stretch it out by turning the unit off during long cruises, but that sort of defeats the purpose.

  9. Richard thanks for sharing. You get 4 hours of battery life for your iPad 3, or, 4 hours on the GDL39? I think you mean GDL39, but just want to be sure.

    Also, anybody out there know how to get the Panel to work for Garmin Pilot while using Droid?

    I have the App Manual for Garmin Pilot and read through it today. It shows how to switch the Split-Screen to show the Panel, but, while using my Droid (Lenovo Thinkpad) with Garmin Pilot, one can not see any option for the Panel.


  10. Hello,

    I have a garmin 430w, a Garmin 330 and an MX20 MFD. With the 330 conversion, can the ADS-B information be displayed on the MX20 throught the 430W? I also use a 696 for XM weather i flight; if I do away with the XM weather, is the 696 a better option for a larger screen?



    • The GDL 39 will only show traffic on the 396, and only 4 traffic targets. No ADS-B weather. To me, that makes it a pretty bad deal (for this model GPS – works great on iPad).

  11. Any chance, Garmin will throw in a battery soon?
    My 60th BD is tomorrow (I’m off to the Bahamas in the Skylane)
    & I would like to order a GDL 39 to go along with my Nexus7 & Pilot app…:)
    That would be a great “BirfDay” present!

  12. I have a 496 and buy weather from XM, did I see weather may work on the 496 unit but I would need to order the GLD 39 with a different number?

  13. If you have ADS-B out using a Garmin 330ES that transmits on 1090, does the ground station transmit back to the airplane on both frequencies or just 1090?

  14. I just had my 330 upgraded and installed the GDL 39. So far I have had difficulty with it paring with my iPad and my Nexus 7. Once it pairs I can see traffic on the map, but often the traffic page says the GDL39 is not installed. Trying to find out the problem I have been reading the Garmin Manuals for their Pilot and the GDL 39. The manual says the 330 needs to be configured to send out a message that the aircraft can receive the 978 MHz as well as the 1090 ES. If that is not done only 1090 will be transmitted.

    • All ADS-B Out installations must be approved via a STC. Garmin’s STC for the GTX330 to output ADS-B was approved by the FAA May 1, 2013. The certified position source (GNS4xxW or GTN units) must be running the latest software (5.02 for a 430W) and have additional wiring installed (RS-232) between the position source and the 330. My 330 was modified in January and we still haven’t completed all the STC requirement.

  15. have both ipad and garmin 496. can you get a cable for bluetooth gdl 39 to feed 496 via cable and bluetooth to feed ipdad at sime time ?

    • Yes you can Dano. But the 496 will not show weather via ADS-B. Only traffic, and there it shows only 4 targets.

  16. I also have an Aera 510. The XM weather was great….but too expensive. Can I hook the GDL 39 to the Aera via a wire and bluetooth to a Android or iPad or both?

  17. Great information!
    Could we possibly hit on ‘remote’ mounting for the GDL-39? I’ve heard with a dedicated antenna you can mount the unit out of site / off the glare shield. Any suggestions?

    • John, you can remote mount the GDL 39, and Garmin offers this in some experimental aircraft. You basically run a permanent power wire to the unit (no battery) and screw off the ADS-B antenna and replace it with an extension cable.

  18. […] For those without ADS-B Out, the traffic feature is most useful in the terminal area, where airlines are coming in to land. There, you’ll see a lot of air-to-air ADS-B traffic, regardless of what ground stations are around. This is handy if you’re flying into a major airport. But GA traffic is very limited with ADS-B right now, so you won’t see much at the country airport. For a real world pilot report on the GDL 39, read this article. […]

  19. I think I understand the xpdr/gdl39 relationship but not the big picture. Does the ads-b xpdr somehow bluetooth to my ipad via the gdl39 for wx and tfc or does it have to go thru a panel-mounted nav unit.

    • The panel mount transponder is not connected to your iPad in any way. It simply transmits out your position (so it’s connected to your panel mount GPS). This lights up ground towers so you get a full traffic picture sent up to your airplane. But this traffic picture is received by the GDL 39.

      The weather part is easier. It’s always broadcast and your GDL 39 is always listening so weather is displayed anytime you’re receiving a ground station.

  20. i fly a baron 58 equipped with an MX_20, the Apollo CNX-80 and a garmin 330 transponder.Also hav TIS traffic installed.
    what is the best solution for providing ADS-B to this avionics package?

    • The software should figure this out on its own, and usually does. If that’s not happening, I’m not sure there is anything you can do to manually fix that.

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