Figure 1. ADS-B surveillance coverage, courtesy FAA (Next Gen Implementation Plan, March 2012).

Garmin Tips and Tricks: Automatic Dependent Surveillance–Broadcast

Number 15 in a continuing series on how to get the most out of your Garmin navigation system.

In the Next Generation of its Air Transportation System the FAA intends to replace your radar-determined location with your GPS/WAAS position, which you will transmit to a nearby ground station (about 700 of them by 2014). This position report is given far more accurately than radar-determined position, and given more often — about once per second. As a result there can be reduced traffic separation, improved traffic flow, and fuel savings.

Of course, this comes at your expense and the equipment is not cheap, but by January 2020 you will need to report your ADS-B “Out” position in all airspace now requiring Mode C transponders. This includes not only Class A, B, and C airspace, but also altitudes above 10,000 ft (unless you are less than 2500 ft AGL). While 2020 is still far away, it’s not too early to understand the requirements in order to plan future equipment additions or upgrades. The complexity of the situation, and the myriad of choices, demand a detailed understanding of the issues.


Before this recent ADS-B rulemaking (Part 91.227), your GPS (navigation unit) and your transponder (position reply) performed separate functions, but now they will be linked because a special transponder can be used for the position report. There are separate requirements laid on the driving GPS/WAAS for reporting your position via ADS-B “Out” than those to certify your GPS for primary navigation. In Part 91.227 there are 19 required items the GPS/WAAS must send via the transponder to a ground station.

All requirements are spelled out in the recently finalized TSO documents governing ADS-B “Out” transmitters, and described in AC 20-165 for those interested. These describe what information must be sent (and therefore what they must receive from your GPS).  Manufacturers are scrambling to make both their GPS/WAAS units and ADS-B “Out” units compliant with the 2020 rules. None of the Garmin units are currently compliant, but most will be by year-end, and Avidyne units that can replace a GNS 430W/530W will also be compliant by next spring.

Two ADS-B “Out” Systems

There are two allowed systems for satisfying the 2020 reporting requirements, but for those who fly in the Flight Levels, your position report must be done with a 1090 MHz transponder having an extended squitter (ES). Mode S has 56 more bits than a Mode C, and adding another 56 bits makes it an ES, or 1090 ES for short. These extra bits are for the position data.

The other option, for those who always operate below FL 180, is a 978 MHz transmitter operating over a broadband channel (compared to the narrow bandwidth of a 1090 MHz transmitter). This additional channel offers new possibilities for ADS-B “In”, that is, the reception of weather as well as traffic information from the ground stations for display in the cockpit. But even with 978 MHz “Out” for 2020 compliance, you’ll still need a transponder.

ADS-B “In”

You can receive traffic information from the ground stations on either frequency if you are a legal ADS-B participant. This means you are transmitting your position with equipment that meets the TSOs for the respective frequencies. They know if you’re legal because what you send must include the information (§91.227) on accuracy, integrity, latency of your GPS position and velocity, and a lot of other stuff.

The traffic you receive will include reports directly from other ADS-B aircraft and rebroadcast traffic from the ground station (ADS-R)–traffic they receive from Mode C aircraft or from aircraft on whichever ADS-B frequency you don’t have. Position reports from those targets which are ADS-B equipped will be considerably more accurate than those based on their Mode C report.

You will receive free weather information only on the broadband 978 MHz channel whether you send ADS-B “Out” or not. The transmitter/receiver combination on 978 MHz is called a Universal Access Transceiver (UAT), and with that you get both traffic and weather. These ADS-B “In” systems are where your benefits accrue for the investment you make.

Figure 1.  ADS-B surveillance coverage, courtesy FAA (Next Gen Implementation Plan, March 2012).

Figure 1. ADS-B surveillance coverage, courtesy FAA (Next Gen Implementation Plan, March 2012).

The ground stations are scheduled for completion by 2014. The map in Figure 1 shows station locations in the U.S. as of February 2012. There is clearly a hole over several Western states that will be filled, but coverage may be sparse in some areas. Even in areas of good coverage, you probably won’t receive signals on the ground, or for hundreds of feet (or more) above the airport.


The display of ADS-B traffic and weather is a significant issue because each manufacturer has its own self-consistent mix of GPS, ADS-B, and displays designed to work together. Mixing and matching of different manufacturers may work, but you must determine what does. The GDL 88 from Garmin, for example, is a dual frequency (978 MHz, 1090 MHz) unit for ADS-B “In”. It has no transmitter, and is designed to display on the GTN series and not on their GMX 200 (its GDL 90 does, but is not 2020 certified). Displays include large screen GPSs, MFDs, and EFIS screens, so be sure to include this element in your decision matrix.

One option is a portable ADS-B “In”, which has the advantage of being very inexpensive and will display via Bluetooth on an iPad. This gives you an additional free weather source, and when you equip with ADS-B “Out” you get ADS-B traffic. A number of portable, dual-frequency, ADS-B “In” systems currently are available for under $1000. They show weather around your area at a somewhat lower resolution than satellite weather (and even lower resolution weather elsewhere).

Dual, Sagetech, Stratus, and SkyRadar make these, but the Garmin GDL 39 is unique in that has an internal pressure sensor (as well as an internal GPS to geo-reference your iPad maps).  The pressure sensor lets you compare the reported baro-altitude traffic with the baro-altitude of your aircraft, critical for the traffic report. Other units without a pressure sensor compare baro-altitude traffic to your GPS altitude (major errors here).

What to Do?

For Twin Commander owners, who fly in the flight levels, the questions are a little different than for single-engine low flyers. Clearly you’ll need a 1090 ES transponder for operation above FL 180. Your choices now are the Trig TT31, FreeFlight FDL-1090 TX, or Garmin GTX 330 modified for ES (Garmin is not yet certified, the other two are).

If you have radar and TCAD you could decide to do nothing for seven years, since you have traffic and weather covered and much will change between now and then. If you also have satellite weather there is little case to be made to get a 978 MHz system, since the trade is between lower-resolution weather–and then only when you receive a ground station–versus the extra monthly subscription fee (and the value of your satellite receiver if you sell it).

Figure 2.  Some certified ADS-B “Out” units, UATs, and Certified GPS/WAAS for ADS-B

Figure 2. Some certified ADS-B “Out” units, UATs, and Certified GPS/WAAS for ADS-B

You might want a GDL 39 (I recommend it for its internal pressure sensor) for your iPad to try out the ADS-B system as it develops. Then you can decide to keep or toss the satellite weather based on experience. If you do want a 978 MHz UAT there are choices now, and certainly many others will appear. Figure 2 is a short and incomplete list of certified and pending units.

In summary, beyond the choice of which 1090 ES you want, I think the only question is whether you want to use the ADS-B ground system for traffic and weather. If the answer is no, you can ignore all this fuss.