Understanding ADS-B traffic

Garmin Pilot traffic display
The Garmin Pilot app, when paired with a GDL 39, can display ADS-B traffic.

ADS-B has suddenly become a household word among pilots, especially with the popularity of new devices like the Stratus and Garmin GDL 39 receivers. While ADS-B weather is fairly well understood, some new models (like the GDL 39) add ADS-B traffic to the mix. This subject is more complicated and there has been a great deal of confusion about when and how pilots can view this traffic information. In this article, we’ll try to explain in plain English what ADS-B traffic is, how to get it and what the limitations are.

Caveat: this is a very technical subject and we can’t possibly cover every piece of it. We will stick to the big picture and the pilot’s perspective. For complete details on ADS-B, check out Garmin’s ADS-B Academy.

Not like weather

The most important thing to understand is that ADS-B traffic is not like ADS-B weather. The weather product (technically FIS-B) is broadcast to anyone with a receiver–like an AM radio station. The only real limitation is that you must be in range of an ADS-B ground station. This is what some people call a “dumb transmission,” because you simply turn on the receiver and start receiving radar, METARs and TFRs. No additional equipment is required.

ADS-B traffic (called TIS-B), on the other hand, is very different–it is not broadcast to anyone and everyone. Instead, it is a “smart transmission,” meaning the ADS-B ground station sends a customized data package to a specific aircraft, and only in reply to an interrogation from specific types of panel-mount avionics. If you don’t have the right panel avionics, you probably won’t get reliable traffic.

Here’s why…

ADS-B 101

ADS-B ground station final map
ADS-B ground stations will eventually cover the entire US.

ADS-B is (for our purposes, at least) a way to transmit information. As such, there are two parts of ADS-B: In and Out.

  • ADS-B In is the receiver part of the system, and this is what Stratus and the GDL 39 are doing when they receive weather–they get ADS-B information in.
  • ADS-B Out is when a panel-mount transmitter sends a signal out to other aircraft and ground stations. This tells ATC and other aircraft what your position, speed and direction of flight are. It is sending data out. Note that ADS-B Out equipment is always installed in the aircraft and certified–never portable.

But there’s one more part to the ADS-B story. To avoid frequency overload, there are two frequencies that these ADS-B In/Out messages are transmitted on:

  • 1090ES is basically a modified Mode S transponder (using the transponder’s 1090MHz frequency) with Extended Squitter (ES). This is required above 18,000 feet, and is used by many airline and cargo jets. A Garmin GTX 330 transponder can be upgraded to a 1090ES ADS-B Out box, for example.
  • 978 UAT is newer, and is used below 18,000 feet in the US. It transmits on 978MHz, and is technically called a Universal Access Transceiver (UAT).

This means you can have multiple variations of ADS-B products: 978 In only, 978 In and Out, 1090ES Out only, etc. And while weather is only received on 978, traffic data is sent on both frequencies.

Enough technical jargon–let’s explain how to view traffic on your iPad.

ADS-B traffic on Garmin Pilot map page
Traffic can be viewed on the moving map page in Garmin Pilot

Two ways to get traffic

There are two basic ways to get ADS-B traffic with a portable ADS-B receiver–air to air and ground uplink. Air to air is straightforward: all airplanes equipped with ADS-B Out (so-called “participating aircraft”) will transmit their location, and the GDL 39 will pick up these transmissions directly. Because the GDL 39 is dual band (1090 and 978), it will receive all ADS-B Out transmissions from nearby aircraft. No ground stations ever come into play here.

But most airplanes (in fact, the vast majority of them today) are not equipped with ADS-B Out, so something has to be done to complete the traffic picture. This is where the ADS-B ground stations come into play. In addition to transmitting weather information (FIS-B), they can also send up traffic data (TIS-B). This traffic data includes all aircraft in radar contact–not just ADS-B Out aircraft.

Between the air-to-air traffic and the ground uplink traffic, you can get a very complete picture of traffic around you. Just like weather, you have to be in range of an ADS-B ground station to receive this data.

There’s a catch

Unfortunately, it’s not that simple–you will only receive this TIS-B information if you are equipped with ADS-B Out. The FAA wants to encourage pilots to equip their airplane with ADS-B Out, so they’re requiring this equipment in order to receive traffic information. Their hope is that this incentive gets more airplanes flying with ADS-B Out, sooner. Many pilots think this is a bad idea, but regardless, it’s the way the system works right now.

All is not lost, though. If you do not have ADS-B Out, but you are flying near another airplane that is transmitting ADS-B Out, you can be a parasite. That is, you can listen in on that airplane’s traffic message and display nearby airplanes on your iPad. That’s because each ADS-B Out airplane receives back an ADS-B In traffic package from the ground stations, and it is specifically tailored to their location. In particular, that ADS-B Out airplane will see all traffic within a 15 mile radius and +/-3500 feet:

ADS-B traffic

So if you’re flying in that “hockey puck” close to a participating airplane, you will have traffic uplinked from the ground, in addition to the air-to-air traffic. This is the best case scenario, as you have free traffic that rivals a $15,000 active traffic system. But as you can imagine, staying within 15 miles and 3500 feet of an ADS-B Out airplane can be a serious limitation. When you’re outside this hockey puck, you will only see air-to-air traffic, which is fairly limited.

Is TIS-B the same as Mode S Traffic?

No. Mode S traffic (sometimes called TIS-A) was popular in the early 2000s, with products like the Garmin GTX 330. This transponder received traffic information from terminal radar approach control, transmitted via Mode S. But the only traffic you receive with Mode S is the traffic in your local TRACON coverage area, and you only receive this information when you’re close to the TRACON. In addition, not all TRACONs support Mode S traffic.

TIS-B, on the other hand, does not depend on a TRACON. You see all traffic, even from en route radar facilities and TRACONs that do not support Mode S. In addition, the data is transmitted via ADS-B ground stations, not local TRACONs, so it is available over a much larger area of the country.

Traffic status page
Garmin Pilot indicates the quality of the traffic it is displaying.

ADS-B traffic with the GDL 39

How does this traffic system work in practice? Let’s look at the GDL 39 when paired with an iPad running Garmin’s Pilot app.

Once your iPad is connected to the GDL 39 (via Bluetooth), go to the map page in the app. Then tap Menu and turn on traffic. You’ll then see two indications of the traffic quality you’re receiving, located in the upper left part of the screen:

  • Air-To-Air–this is almost always displayed as a white airplane with blue shading. That means it’s working properly. Basically, if the GDL 39 is on and has a view of the sky, you have this.
  • TIS-B–this is a measure of the quality of ground uplink traffic you’re receiving.

The TIS-B indicator is more complicated, with four total display options:

  • Blue background with white tower–this is the best case. The app is telling you that either you or an aircraft close to you is equipped with the latest version of ADS-B Out hardware and software and you are receiving a high quality traffic package.
  • No blue background, but with a star–this means you or a nearby aircraft are equipped with ADS-B Out, and you’re receiving a fairly good quality traffic signal. But, your ADS-B Out may not be up to the latest revision of software. This usually isn’t a big deal, except for a few ground stations that may behave differently around the country. If you have an older ADS-B Out box, you may see this.
  • Tower with a yellow question mark–this means you’re getting “Degraded TIS-B.” Basically, you’re receiving ground uplink traffic and talking to ground station, but you are not close enough to a participating plane to get complete traffic coverage. Be skeptical in this case–you are only seeing part of the picture.
  • Red X–this is simple. You are not receiving any ground uplink traffic. This is either because you are not in range of any ground stations, or there are no ADS-B Out airplanes close at all. The only traffic you will see is air-to-air.
Garmin Pilot app settings page
The settings page in Garmin Pilot provides more information about ADS-B traffic.

On the Settings page, you can also view ADS-B traffic data:

  • Total Targets: the total number of aircraft the GDL 39 is tracking.
  • ADS-B Targets: air-to-air targets, received directly from ADS-B Out equipped aircraft.
  • ADS-R Targets: these are ADS-B Out equipped aircraft, but rebroadcast by the ground station. This exists because you could have a 978 In receiver that would not see the 1090 air-to-air transmission. So the ground stations sends the 1090 traffic back up to 978 In aircraft. Because the GDL 39 is dual band, these ADS-R targets are usually duplicates, and the number is 0. But since ground stations are higher power than the air-to-air transmissions, this could occasionally fill in the gaps you might miss.
  • TIS-B Targets: these are the rebroadcast targets sent up from the ADS-B ground stations, including non-ADS-B aircraft. These are the most valuable targets for GA pilots, generally speaking, since you’ll see almost everyone on ATC’s radar display.


There’s no doubt this subject is complicated. Here’s the one thing that is easy to remember: if you do not have ADS-B Out installed in your panel, you will not get reliable traffic on your iPad. That doesn’t mean it’s worthless, just incomplete. Most often, you’ll see lots of air-to-air ADS-B traffic, which is usually airline and cargo jets.

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.

As we get closer to 2020 (when all aircraft flying in controlled airspace will be required to have ADS-B Out), ADS-B traffic will get much, much better. Until then, your mileage may vary.


  1. I have a question from a renter’s point of view. When renting non-TIS (i.e. non G1000) aircraft, should I buy the ZAON pcas system and hook it to my iPad and use this until the ADS-B is implemented in 2020? There is a lot of talk about the patchy coverage of ADS-B at this time. I would appreciate any input as well as suggestions for where to direct this question if there are better forums for this.

    • Michael, the Zaon unit works well, and right now will give you better coverage than ADS-B traffic. It’s still not 100% in our experience (really nothing is short of an active TAS or TCAS system). But for the price and portability, the XRX is a good deal.

  2. WingX Pro7 appears to have been inadvertently omitted in this GDL39 ad (I mean ADS-traffic article). 😉 WingX Pro7 introduced ADS-B traffic months ago and works with the the following single and dual channel receivers (978 MHz and/or 1090 MHz): Skyradar ADS-B receivers, FreeFlight Systems XPLORER, the DUAL XGPS170, and Sagetech’s Clarity – none of which are sold by Sportys or mentioned in the article. Hopefully Sportys will sell the XGPS170 and Clarity when those become available (probably next month).

    Michael, FYI: WingX Pro7 works great with the Zaon XRX (WiFi adapter required).

    • Hilton, this article is written as an introduction to ADS-B traffic, not a review of the GDL 39. We’re just using that here as an illustration of the concepts at work. If you think we omitted WingX, I guess you could say we omitted 18 other apps too. I wouldn’t take it personally.

  3. John and Hilton,
    Thank you both for your comments. I agree no traffic system is perfect but I figure it can’t hurt to add one as long as vigilance (looking out the window) is continued. Since I am a WingX/ipad user now anyway (Zaon works with WingX) I will probably order the Zaon unit from Sporty’s to use for the foreseeable future unless there is a “portable ADS-B out” that you know of.

    Thank you John and Sporty’s for helping address this confusing topic.

  4. I think the big value with the Zaon is that at least there is traffic alerting. I haven’t been able to decipher the users manuals on other apps to determine if there is/will be traffic alerting when using ADS-B hardware. It’s nice to have traffic displayed on an iPad screen, and it may help you avoid traffic – IF you’re looking at it. But that means you have to spend more time looking in the cockpit when these devices are supposed to be decreasing workload so you can look out of the cockpit. The only portable system I’ve noticed that appears to provide traffic alerting from ADS-B is Skyvision Extreme.

  5. Mark, I totally agree. I try to keep my eyes looking outside as much as possible and the audible alerts of the Zaon mean I can flick my eyes to the iPad just when needed to look for traffic.

    Please Mark, John or anyone chime in with what you think of what I have come up with so far:
    1. When ADS-B is completely up and running (2020?) the traffic feature will be comprehensive and helpful.
    2. Until that time the ADS-B traffic feature on the iPad will provide spotty coverage at best.
    3. The existing portable traffic systems to bridge the gap until then that display on the iPad are the Zaon XRX ($1400) or the Skyvision Extreme ($4000?).

    Am I missing anything? Thanks, Mike

    • Michael,

      I think you’ve summarized it very well. No solution is perfect right now, but the Zaon does fill in some gaps.


    • Michael, I think there are more and more aircraft installing ADS-B capability every day. I’m active in the homebuilder community, and most of the folks I talk to are planning on traffic as soon as they feel it’s worthwhile, and that’s coming faster than you may think. Just look at the number of UATs that have been recently released making it more affordable than ever. SkyRadar is another solution that seems to be working well. A friend of mine has one on his dash in a C-182, and wouldn’t leave home without it. So to answer your questions, I think the ADS-B service will be completed by the next year so the value in using the service is increasing daily. The plethora of ADS-B UATs hitting the market will as well as an increasing awareness of the value of ADS-B will help greatly. And you also have many options for your iPad in addition to the ones you’ve mentioned. Check out Clarity, Dual and Navworx too. Now if we can get some of the iPad apps to incorporate alerting we’ll be golden.

      • Mark, you’re right that the ADS-B ground station network will be mostly complete by next year. But You’ll still have to have ADS-B Out (or be close to an airplane that has it) to get good ADS-B traffic. And I think there aren’t many of those. Lots of people are buying portable ADS-B boxes, like the SkyRadar and Stratus and GDL 39, but remember–those are ADS-B In only. Those portable devices will never be Out, so they don’t help with the traffic picture.

    • Currently use Tte Monroy ATD 300 with directional antenna to spot traffic; it is a good option before for traffic detection before 2020. Monroy is better than Zaon in one respect…it will show traffic for an aircraft that has a transponder but no Mode C. Zaon ignores the A/C without Mode C as they cannot calculate the altitude difference with your Mode C unit and the traffic.

  6. UAT is Universal Access Transceiver. As a receiver, it’s the box you must have to receive ADS-B traffic and weather. As a transmitter, it will broadcast your position and identification, and is one of the ways of meeting the ADS-B Out requirements. It has to be an authorized installation to do that, though. Most of what we’ve been talking about is the receiver part of a UAT. A bit of an oxymoron saying transceiver receiver.

  7. Why is Sporty’s pushing the Garmin &Stratus ADS-B rcvrs and never mentioning Skyradar, which many of us have. I hate to see an AOPA affiliated entity playing favorites.
    Bad deal boys.

    • Ron, we’re not pushing Garmin here, just using it as an illustration for the ADS-B traffic concepts discussed. Garmin Pilot has some nice traffic indicators, which makes it easy to see what data you’re getting.

      Also note that Sporty’s is not affiliated with AOPA in any way. We like and support AOPA, but it’s two totally separate organizations.

  8. I am a huge fan of WingXPro and the Skyradar dual freq ADSB reciever. Its flown about thirty hours five of whoch was fairly ugly IFR.


    It is comparable to the G1000 for situational awareness and beats the heck out of any xm product on price or performance.

  9. 1. ADS-B out plane broadcast GS, ALT, Position once per second to Ground Station
    2. Ground Station shoot back a packet of info tailored to the ADS-B equipment plane above for traffic +/-3500 FT and 15 mile radius.


    1. Does the ADS-B Out Plane rebroadcast the entire data package to any plane in range or just it’s position?
    2. Can the non ADS-B ‘out’ equipment plane pick up the ground station signal when it’s in the area of the ADS-B plane, but the data may not be reliable as it was meant to cover a disk of the size mentioned above a the location of the other plane?

  10. Ads-b traffic isn’t worth much without a uat transceiver installed. With a transceiver it provides by far the best traffic solution. The missing piece right now is a good ipad app for displaying that traffic. The display must be completely uncluttered with high contrast, like the Garmin software. Unfortunately the Garmin software is proprietary and all the other apps have displays so cluttered as to be nearly useless for effective traffic awareness. As soon as a decent app comes along that will give consumers a choice of hardware I’m ready to buy. But not before.

  11. It seems that sometimes the traffic information displays my own aircraft. Is this because I’m in someone’s hockey puck of information and the Stratus II receiver doesn’t sort out my ownship information??

    Thanks, Great information here!

    • The software should remove the duplication there, but sometimes it takes a second or two before it detects it and clears it.

  12. Currently receiving ADS-B In via Stratus/Foreflight and am pursuing an ADS-B Out solution.
    Question: If I install a GDL-88 (Garmin) or ADS600-B (NavWorx) will Stratus receive and display all the mode C/S traffic in the “hockey puck”?

  13. I use the Skyguard twx ADSB in my RV4. It is a portable unit that has both in and out. It is amazing and the support is superb. I am able to see all traffic seen by ATC regardless of whether the other airplanes are ADSB equipped or not. This affordable unit in conjunction with GPS are among the finest general aviation advances I’ve seen in my 48 years as a pilot.

  14. The new ICAO flight plan requires you to specify if you have a 1090 out, 1090 out/in, a 978 out, or a 978 out/in. It does not give you the option of having a 1090out/978 in or 978out/1090 in. If I have a 1090 out and a 978 in, but the system does not know that I have a 978 in (because there is no way in code it), will I still get all the traffic?

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