Here’s a great article by Stephen Pope published in Flying Magazine Jul 01, 2013.

Making Sense of ADS-B Portables

A huge number of portable ADS-B receivers has hit the market in the last  couple of years, giving pilots instant access to traffic and weather information  on their iPad or Android tablets. Weather is broadcast continuously so there  isn’t much mystery there. But traffic data is somewhat limited. This can make  ADS-B confusing as pilots try to make sense of exactly how and where it  works.
Here’s a breakdown of how ADS-B operates from our friends at Sporty’s Pilot Shop that will hopefully  clear things up for those of us who are having a hard time keeping straight the  nuances of the technology.
There are three basic scenarios that will cover the vast majority of cases.  The first and most likely case is the situation where you’re flying with a  portable ADS-B receiver but don’t have an ADS-B Out transponder installed in  your panel. In this case, you’ll receive target information for any airplane  that is transmitting ADS-B Out via air-to-air, but you won’t pick up Mode C  target information. (Most airplanes do not have ADS-B Out, but this will change  after 2020 when the FAA’s mandate goes into effect.)
The second scenario comes into play if you are flying with a portable ADS-B  receiver and no ADS-B Out transponder but you are close to another aircraft that  is ADS-B Out-equipped and within range of an ADS-B ground station. In this case,  the ADS-B Out airplane can relay traffic information to your ADS-B receiver in a  30-mile bubble. In this case, you will see in-range Mode C and ADS-B  targets.
If you have an ADS-B Out transponder in your airplane, you’ll be  continuously transmitting to the ground stations and creating your own bubble of  traffic information. In this best-case scenario, you’ll see all radar traffic  within a 30-mile diameter and 3,500 feet of your altitude.
For more information, check out Sporty’s ADS-B 101 tutorial.  Also from  iPad Pilot News, is this very useful article entitled Understanding 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.

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