Automatic Dependent Surveillance - Broadcast

In the Next Generation environment, starting in 2020, the FAA will require all aircraft (FAR 91.225) to continuously announce their WAAS GPS position and velocity, and baro-altitude, in order to operate in certain airspaces – those that now require Mode C transponders.  The airspaces include Class A, in and above B, and C to 10,000', airspace above 10,000' (but not below 2,500' AGL), and from the surface to 10,000' within 30 nm of certain airports listed in FAR 91.225.  This Next Gen system will eventually replace Radar ground stations for traffic separation. It requires a network of ADS-B ground stations with relatively simple transceivers and antennas (compared with radar sites) that are much less expensive.

There are two independent ADS-B technology paths to choose from; a 1090 MHz Mode-S transponder with Extended Squitter, or a 978 MHz Universal Access Transceiver (UAT). UPSAT (later purchased by Garmin) developed UAT's for the FAA-funded Capstone project in Alaska (1999 – 2006), and with the MX-20 MFD display, a GX-60 GPS and a Trans-Cal Blind Encoder, did a system demonstration of the concept. 

Position reports based on your transponder reply to surveillance radars have accuracy limitations – range and azimuth errors, latency, jitter, etc. Your position error must be less than 1 nm to permit 5 nm aircraft separation, but in terminal areas the separation can be reduced to 3 nm, if the position error is less than 0.2 nm.  The “ADS-B Out” position reporting (to the network of ground stations) should be considerably more accurate, allowing reduced traffic separation and therefore improved use and traffic flow in the system.

Transponder Based

Transponders are interrogated on 1030 MHz and reply on 1090 MHz with a message that includes your transponder code and barometric altitude. Mode S transponders add a 56-bit squitter message that uniquely identifies you (it's tied to your tail number), and the 1030 MHz channel has data link capability to receive Traffic Information System (TIS) reports from terminal radars.  The squitter message is sent automatically (without being interrogated) at least every second, depending on the data being sent.

By extending the Mode S squitter message to 112 bits (extended squitter, or ES) you can transmit the 19 required parameters as described in FAR 91.227 to meet TSO-C166B. In addition to your WAAS GPS position/velocity/time (PVT) solution, the message must include your Navigation Accuracy Category for Position (NACp) and Velocity (NACv), your Navigation Integrity Category (NIC), Source Integrity Level (SIL), and System Design Assurance (SDA).  Also, you need to indicate the uncompensated and total latency (time) of you PVT solution.  This data comes from the WAAS GPS, generally requiring software upgrades to existing units.

A 1090ES transponder sends position information twice per second to an ADS-B ground station and directly to any aircraft that can receive it. The extended squitter can send 47 individual parameters, compared to 3 for Mode C and 7 for Mode S.  These extra parameters can also transmit your flight plan route, a future capability being planned.

To receive the ADS-B traffic reports, either directly from another aircraft or from ground stations, requires a 1090 MHz ADS-B “ In” receiver in addition to a 1090ES transmitter. The ground stations then send ADS-B Rebroadcast traffic that you don't receive directly -- Mode C traffic, and any traffic reporting on the 978 MHz frequency described below.

ADS-B reports are available anywhere you can receive the Ground Stations now being constructed around the country.  The latest map of locations (30-Nov-2012) still has holes west of the Mississippi, but completion of the network is scheduled for the end of 2013.  Note the coverage in the gulf for oil rig operations.

ground stations

UAT (Universal Access Transceiver) Based ADS-B

The 978 MHz transceivers for ADS-B are a variant of the one used in Alaska , with both “In” and “Out” channels (separated by timing of the code messages).  You can also purchase transmit only or receive only units. These transceivers must meet the requirements of TSO-154C. The system consists of the transceiver and antenna, a moving map display, and WAAS GPS and altitude encoder signals to the transmitter.

There are both advantages and disadvantages to the 978 MHz choice. The 978 MHz receiver has a much larger bandwidth (1.2 MHz) than the 1090 MHz channel, so weather products and NOTAM information, in addition to traffic, can be sent to you from a ground station for display on a moving map.  That’s not possible on the 1090 MHz channel, which has a much narrower bandwidth and therefore sends traffic only..

But even if you don't have ADS-B Out, you get weather on the 978 MHz channel and traffic reported directly from an aircraft with 978 MHz Out. You may also get ground reports being sent to nearby participating aircraft (one with ADS-B Out), including traffic broadcast to the ground station on 1090 MHz, and rebroadcast reports received by the ground station from Mode C aircraft. To be a parasite on that nearby participating aircraft you need to be within 15 nm of his postion.

But 1090 MHz is the ICAO standard for traffic reporting, so you can't use it for a position report above 18,000', or outside of the US . You could report position on a 1090 MHz unit, and purchase a 978 MHz “receiver only” for traffic and weather. The weather information is primarily for the area around the ground station (the resolution is lower for regions beyond that). One downside of ADS-B reports is that at most airports you won't see a ground station until you’re in the air.  So generally you won't have traffic and local area weather before takeoff and in the initial climb, or in an area of sparse stations. However, the data is free. 

What to Do?

First, if you plan to fly above 18,000' you must have a 1090ES transponder after 2020. If you have TCAS you don't need an ADS-B In receiver for traffic, but you may also want a 978 MHz “receiver only” that will give you free weather and traffic.  Alternatively, if you choose to pay for satellite weather and have TCAS, a simple extended squitter transponder is all you will need to meet the 2020 requirement.

Since you can receive free ADS-B weather without having “ADS-B” Out, one option is a portable unit like the Garmin GDL 39, which has both 1090 and 978 MHz for direct traffic reports. You also have the parasitic reports if you don't have ADS-B Out. Weather and traffic are displayed on the Garmin Pilot App (on an iPad or Android tablet) or the Aera 795/796.

There’s a built in pressure transducer in the GDL 39 to correctly compare traffic altitude with your baro-altitude (instead of your GPS altitude).  To date, other portable ADS-B receivers (Dual, SkyRadar) compare traffic altitude with your GPS altitude when displayed on a tablet, a significant error.  (If displayed on a screen that also has baro-altitude available then these other devices then give a correct comparison).

Like the GDL-39 the Garmin GDL-88 has dual frequency receivers for traffic, and a 978 MHz transmitter for ADS-B Out. This works fine if you don't plan to fly above 18,000 ft. Traffic and weather are displayed on the GTN 750 to date, but it is not clear what other display options may be in the future.

Having made these basic choices you can now wade into the swamp of equipment choices and compatibilities, the latter being a “big deal”.


Whether you fly certified or experimental aircraft, you will need panel-mounted, certified (TSO-C166B or TSO-154C) ADS-B Out equipment. The Table here shows some current manufacturers and their certification status (late 2012).

adsb equipment

Clearly the equipment issue involves the WAAS GPS receiver as well, since the required position info sent to ground stations originates in the GPS.  Originally they were not configured to send this info, only to use it internally for map displays. At this date only the FreeFlight units meet requirements as a driver of the ADS-B “Out” unit. Garmin expects most of its GPS units will be certified by 2013.

Compatibility is another major issue.  What combination of GPS, ADS-B, and Display units will talk to each other? As an example, which ports are used in the GPS units to send the required info to the ADS-B units?  Some use ARINC 429 ports and others used RS 232 ports.  The GNS 480 can send the required info from one of its two 429 ports (in configuration 5, or ARINC 743A, to a GTX 330ES).  But the Trig TT31 doesn't support this format and must receive the info on an RS 232 (MAPCOM) input port. However, the MAPCOM format does not include the NIC's and NAC's required in 2020.  So, while the Trig works now for ADS-B out, by 2020 there will need to be changes.

With a software update to the GNS 480 it could be used to support ADS-B Out.  But if you have an Aspen or G500/600 PFD, driving the HSI requires both 429 output ports (configuration 8 or 9 for GPS and configuration 4 for ILS/VOR). So you can’t split one of these outputs to feed an ADS-B Out transmitter. This example clearly illustrates the complexity in choosing an ADS-B system.   Incompatibility extends to display units as well, so check your combination of equipment (or proposed purchases) thoroughly to find out what works with what.

The Garmin 430W/530W units send the required data on one of its RS 232 ports and the GTX 330ES can accept that (both units still need software updates to do this legally in 2020).

For operations below 18,000' the Garmin GDL 88 offers a dual receiver (1090 and 978 MHz) and 978 MHz transmitter.  It can be ordered with an optional GPS, which eliminates any compatibility issue.  However, while it will display on the new GTN touch screens, its new format is not compatible with the MX-20 or GMX-200 MFD's. 

All of these equipment issues are in a state of flux, and in the 7 years until required implementation, there will be many changes and new options.  But compatibility will still be an issue that must be thoroughly understood when planning your panel upgrades.