Garmin Tips and Tricks: ADS-B In and Out Confusion

Another in a series of articles by Dr. Keith Thomassen on how to get the most out of your Garmin GPS Navigators.

There’s a lot of aviation media attention being paid to the upcoming 2020 mandate to upgrade avionics in your aircraft to provide your position via ADS-B “Out.” If you plan to fly anywhere that now requires a Mode C transponder (see Fig 1), you’ll need ADS-B Out. If you don’t plan to fly in transponder-required airspace, or file IFR flight plans (how can they track you without radar?), then ignore this.

Fig 1.  Airspace where the 2020 mandate on GPS position reporting is required.

Fig 1. Airspace where the 2020 mandate on GPS position reporting is required.

By now you’re probably learned the basics of ADS-B, but let’s review them.  The FAA wants to replace its system of radar sites with a set of relatively simple antenna sites to receive your GPS-derived position once per second. The network of sites is linked together into the ADS-B system of ground stations. Construction of the network is complete and maps showing coverage in the U.S. are readily found on the internet.

This system has the potential to locate you with much greater precision than does radar tracking (every 4-12 seconds), provided the ground stations receive a fairly complex set of data from you. Why must the data  be complex, and not simply report your position? Since you are moving, there are time delays from when your avionics receive satellite signals to when they process it to determine your position; there is some error in that determination from multiple sources; there is processing time in your ADS-B Out device; etc.

The required data that you need to send is listed in FAR 91.227 –– 19 items total. It includes your GPS position, velocity, and time (error) –– the so-called PVT solution. You also need to send appropriate info on integrity, accuracy, and latency of the solution, as well as a number of other items. If you really want to know about NICs and NACs, SIL, SDA, etc., read the FAR. The ground station needs data on these things to determine your position with the certainty or accuracy desired.

The required data can be transmitted from either a 1090 MHz Mode S transponder with Extended Squitter (good at all altitudes) or with a 978 MHz transmitter, allowed below 18,000 feet. The latter is often included in a combination transmitter/receiver, or UAT (Universal Access Transceiver). A key advantage of the UAT is that the receiver is broadband and can receive weather information, which 1090 MHz receivers cannot.

Several issues are associated with the requirements for data transmitted to ground stations. First, does your GPS send the required data to your transponder or UAT? Before the ADS-B mandate, your receiver computed those parameters from satellite signals but only used them internally for things like integrity warnings and accuracy measures (such as horizontal and vertical position levels –– HPL and VPL –– used for approaches).

Software updates to port the required data from many popular legacy GPS navigators have been completed and implemented (such as the GNS430W/530W), but as of this writing some have not (like the G1000 and an RS232 output for the GNS480). More recent GPS navigators generally have been designed with ADS-B requirements in mind. In any event, the GPS must be one that is an FAA-approved position source meeting TSO requirements.

You’re probably thinking, why do I need to know all of that? A simple answer is that without the right set of equipment, correctly and legally installed, you won’t meet the mandate. More to the point, there are already countless examples of installations done incorrectly, and you have to know enough to ask the right questions. A big learning process is still ongoing.

So, is a given combination of GPS source and ADS-B transmitter compatible? Are the signals from the GPS ported on channels (ARINC or RS-232) and in formats or protocols that are compatible with your choice of ADS-B Out? Compatibility among devices from different manufacturers is questionable and care must be taken in your selections.

But you can cut through all of this complexity because there must be an STC covering the combo, which ensures that the position source is approved and compatible with the ADS-B Out device. Here is a link to a list of some FAA-approved combinations:, but the list is not up to date and there may be critical requirements (such as software version requirements) that are not listed. Nonetheless, you should be able to determine whether there is an STC for your proposed combination from manufacturers and/or avionics shops.

Once installed and operating, you can request a compliance report from the FAA, generated from data on one of your flights. (Email to 9-AWA-AFS-300-ADSB-AvionicsCheck@

The flip side of ADS-B Out is ADS-B In, which is a receiver that gets signals from FAA ground stations and other aircraft broadcasting on your receiver frequency. There are stand-alone 1090 MHz and 978 MHz receivers as well as the UAT transceivers. On 1090 MHz you only get traffic, but as previously noted, on 978 MHz you can receive both traffic and FIS-B weather info.

If equipped with ADS-B Out you will trigger traffic reports from a nearby ground station on all reporting aircraft within a cylindrical boundary around you (15 nm radius, ± 3500 ft). Other aircraft near you, not equipped with ADS-B Out, can see any of that traffic in your volume with (say) a portable ADS-B In device. (I assume you won’t purchase a panel mounted “In” receiver without an “Out” transmitter.) However, as a parasite on the traffic volume of another aircraft, you don’t see all the traffic in “your volume” unless you are in very close formation with that aircraft. Nonetheless, with an ADS-B portable dual-receiver device you will see all ADS-B Out-equipped planes (either frequency) in your volume. You’ll miss other targets where your volume doesn’t overlap his/her volume.

The next complication is in choosing a panel-mounted display. Will your MFD or GPS display screen show the traffic and weather? Here again, mixing and matching among different manufacturers is problematic. Even Garmin’s GDL88 ADS-B In and Out solution (either with a Garmin GPS or built-in WAAS GPS-approved source) did not initially speak to its own G500/600 EFIS displays, or its own GMX200 MFD. Software updates are in the works to fix that, but the point here is that the compatibility issue again rears its head.

For that reason, many pilots are equipping with ADS-B Out and then using portable receivers that send info to a tablet via WiFi or Bluetooth. Those receivers are generally dual frequency and have a built-in WAAS GPS receiver, so they also provide more accurate and stable GPS position to the maps on your tablet. But there is an issue with altitude on the traffic information.

Altitudes used in aviation are pressure based (aircraft pressure using a standard model of the atmosphere, with a local baro correction). The GPS driving your tablet determines your GPS altitude. Differences between the two altitudes can be several hundred feet or more, and worse the higher you go. For that reason, the most reliable traffic alerts come from panel-mounted ADS-B In, which uses your encoder altitude. Garmin solves this problem in portables by including a pressure sensor in their GDL-39 unit to give you an apples-to-apples comparison between your pressure-derived altitude and that of the targets.

Of course, that won’t work if your aircraft is pressurized. Check out your combo of portable receiver and tablet program to see how they deal with this issue. Wing-X Pro, for example, allows you to input your altitude so that it can correct that difference.

Traffic is received in three different ways, as shown in an AOPA graphic in Fig 2: Aircraft-to-aircraft transmission on the same ADS-B frequency; ADS-R traffic (re-broadcast by the ground station) from aircraft on the opposite frequency; and TIS-B from the ground station (all area aircraft in radar or ADS-B contact with an FAA control facility). The majority of all traffic at the moment is from radar.


Fig 2. The three ways to receive traffic information through the ADS-B network; aircraft-direct, re-broadcast from the ground station, and transmission of radar positions from the ground station.

It’s beyond the scope of this article to explore the growing set of equipment options, but suffice it to say that the choices are expanding and will continue to do so. For example, the NavWorx ADS600-B, the FreeFlight FDL-978-XVR, or Garmin GDL88 are STC-approved UAT devices, some combined with an approved GPS position source and transceiver in a single unit. Bendix/King, Trig, Avidyne, and L3 are other manufacturers offering products.

Turning to the political/technical side, there is growing opposition to the cost of the mandate for general aviation aircraft since that cost will restrict some owners in their operations. That cost is a fair fraction of the value of many light aircraft flying today.

At issue is whether the severe technical requirements, placed on equipment for the desired accuracy needed to control commercial aviation, and associated certification costs, must also be placed on general aviation operations. Are there low-cost options the FAA can approve for most (but probably not all) of the affected airspace? If this opposition prevails it will likely dramatically impact your choices.

My goal in this article is to make you aware of all these issues in your decisions on this mandate. As I’ve tried to convey, the problem is complexity and intertwined systems, so your decisions are not easy ones.  You can’t avoid investing your time in homework since we’re not just talking about what color vehicle you want, but will it do the job?  Will the install do what you want, and will your choices lead to a system that works and is legal? Plan carefully, and good hunting!

Keith Thomassen is a pilot, instructor, and educator who specializes in teaching pilots how to get the most out of their GPS units. He has written several manuals on this and other subjects. For more see