UAT on 978 MHz explained: the second ADS-B system in the US and when you need to receive it

Anyone building an ADS-B receiver in Europe cares about exactly one frequency: 1090 MHz. In the US, it’s different. There is a second, completely independent ADS-B system there that transmits on 978 MHz and is called UAT, Universal Access Transceiver. Many small aircraft transmit exclusively on it. To a 1090 MHz receiver, they are therefore all but invisible.

In this article, you learn why the US uses two frequencies, who transmits on them, which additional services run over UAT and when a second receiver is worth it for you. The short version for readers in Germany, Austria and Switzerland right up front: You don’t need UAT here. It gets interesting when you spot in the US or run a feeder there.

UAT in one sentence

UAT is an ADS-B method on 978 MHz that the US aviation authority FAA designated for general aviation and that is approved as an alternative to 1090 MHz only below 18,000 feet.

The principle is the same as with the familiar ADS-B: The aircraft determines its position by satellite navigation and broadcasts it, together with altitude, speed and identification, at least once per second. How this works on 1090 MHz is covered in the basics article on flight tracking with ADS-B. The decisive difference lies in the radio path: UAT uses its own frequency, its own data format and its own devices. A receiver listening on 1090 MHz hears nothing of it.

Why the US has two ADS-B frequencies

The reason lies in history. When ADS-B took shape around the turn of the millennium, 1090 MHz was already heavily used: secondary radar replies, Mode S signals and collision avoidance systems share this frequency. How this fits together is explained in the article Transponder Mode A, C and S. In parallel, a data link of its own was developed with UAT, designed for ADS-B from the start.

UAT was tested in Alaska, among other places. There it already served in 2001 as the basis for 5 nautical mile en route separation, and from 2003 Embry-Riddle Aeronautical University equipped its training aircraft with it. In 2002, the FAA then made the fundamental decision: two data links. 1090ES for airlines and high-performance aircraft, UAT for general aviation.

Europe went a different way. In November 2012, the European Union Aviation Safety Agency EASA confirmed that the EU relies on 1090ES for the sake of compatibility. That left UAT essentially a US system. Satellite networks for ADS-B are also designed for 1090 MHz, as the article on space-based ADS-B shows.

What the US regulation requires exactly

The rules are in the US federal regulation 14 CFR 91.225. Since January 1, 2020, ADS-B Out has been mandatory in large parts of controlled airspace. The split is clear:

Airspace	Permitted ADS-B technology	Typical users
Class A (from 18,000 ft)	1090 MHz Extended Squitter only	Airliners, business jets, cargo
Class B and C	1090ES or UAT	everyone flying into large airports
Within 30 NM of certain large airports, up to 10,000 ft	1090ES or UAT	small aircraft in the surrounding area too
Class E from 10,000 ft MSL (48 contiguous states), except at and below 2,500 ft above ground	1090ES or UAT	higher-flying general aviation
Class E from 3,000 ft over the Gulf of Mexico, to 12 NM off the coast	1090ES or UAT	traffic over the Gulf, such as offshore flights

Aircraft without an engine-driven electrical system, such as gliders and balloons, are exempt from parts of the mandate. Outside the airspace named, flying without ADS-B altogether is also allowed. An aircraft you don’t see on either frequency is therefore not automatically flying illegally in the US.

Who transmits on UAT – and who doesn’t?

This is the core for spotters. Because UAT is not approved above 18,000 feet, everything that regularly travels at cruise altitude uses 1090ES. That applies to airline jets from Delta, United or American Airlines, freighters from FedEx and most business jets. Aircraft that travel internationally also need 1090ES, because countries with an ADS-B mandate outside the US rely on it.

UAT, by contrast, is common among pilots who fly mainly domestically and below 18,000 feet. The pilots’ association AOPA names as its main advantage the free weather reception in the cockpit, which only works via UAT. Typical candidates are therefore training and touring aircraft such as the Cessna 172, the Piper PA-28 or the Cirrus SR22. Which technology an individual aircraft uses, however, depends solely on the equipment the owner installed. You can’t reliably derive it from the type.

The technology: what UAT does differently

UAT is not a modified transponder but a data link of its own with its own logic. Three properties matter to you as a receiver builder.

Full position in one message. On 1090 MHz, the position is encoded in two alternating messages, an even one and an odd one. A decoder needs both, or a known reference position, to compute the location. UAT delivers the position in a single short transmission. Both methods update once per second.

One channel for the ground too. The 978 MHz frequency is shared by aircraft and ground stations. Aircraft send their positions downward, ground stations send data upward, the so-called uplink. Decoders like dump978 label both directions separately.

Different radio parameters. The sample rate for an RTL-SDR stick on UAT is a good 2 MHz, and the quarter-wave antenna at roughly 7.7 centimeters is slightly longer than for 1090 MHz. How to calculate the length yourself and build an antenna is shown in the guide Building a 1090 MHz antenna yourself. The principle holds for 978 MHz just the same.

FIS-B, TIS-B and ADS-R: the additional services

If you deal with UAT, you quickly stumble over three abbreviations. They describe services transmitted by the FAA’s ground stations.

• FIS-B (Flight Information Service – Broadcast): Weather and aeronautical information for the cockpit, including METARs, TAFs, radar images, SIGMETs, AIRMETs, NOTAMs and winds aloft. In the US, FIS-B runs exclusively over UAT. This service does not exist on 1090 MHz.
• TIS-B (Traffic Information Service – Broadcast): Ground stations send traffic information about aircraft that have no ADS-B themselves or transmit only on the respective other frequency. The data comes from radar or multilateration, for example.
• ADS-R (ADS-B Rebroadcast): The ground station receives the position of a UAT aircraft and rebroadcasts it on 1090 MHz – and vice versa. This way the two worlds see each other.

This bridge is intended for pilots with an ADS-B In receiver, not for hobby receivers on the ground. Ground stations send ADS-R and TIS-B specifically for nearby aircraft that request such data. Your 1090 MHz receiver therefore only picks up these rebroadcasts if such an aircraft happens to be in the area and you are within range of the ground station. That does not add up to a reliable picture of UAT traffic.

How UAT aircraft show up on flight tracker maps

On many maps, such as those based on tar1090, you can tell by color which source a position comes from, for example ADS-B, MLAT or TIS-B. Depending on the feed, however, UAT traffic appears there as ordinary ADS-B. What the colors mean in detail is explained in the article Symbols and colors on flight tracker maps.

For you, that means: If a small aircraft in the US shows up with the note TIS-B or ADS-R, your 1090 receiver did not hear the aircraft itself but the rebroadcast of a ground station. Such positions can be delayed and disappear suddenly when the ground station stops transmitting. If an aircraft is missing entirely, there can be many other reasons besides frequency. An overview is given in Why an aircraft doesn’t appear on the flight tracker.

Anonymous UAT flights

One peculiarity of UAT concerns the assignment to a registration. Normally every aircraft sends a fixed 24-bit address through which flight tracker services look up type and N-number. UAT devices, by contrast, usually have an anonymous mode. Under 14 CFR 91.227, a pilot may use a self-generated, temporary address and omit the flight identification if he has not filed a flight plan and has not requested air traffic control. The AOPA names transponder code 1200, the US standard code for visual flight, as an additional condition and clarifies that a VFR flight plan does not preclude anonymous mode.

On your map, such an aircraft then appears with an address that matches no record, and without a registration. There is no such mode for 1090ES. There, the FAA instead offers the PIA program, in which owners get an alternative address with a different callsign. For your logbook, this means: An anonymous UAT contact can only be assigned reliably via a photo of the registration.

When you need to receive UAT – and when not

Whether a 978 MHz receiver is worth it depends almost solely on your location and your interests.

A particularly rewarding case is the annual fly-in at Oshkosh. General aviation in the US gathers there in large numbers – exactly the class of aircraft in which UAT is common. What awaits you on site is shown by the articles on EAA AirVenture Oshkosh and on plane spotting in Oshkosh.

What a UAT setup looks like

Technically, a UAT receiver is close to the familiar 1090 setup. The most important difference: You need a receive branch of its own.

A practical bridge comes from the software itself. The tool uat2esnt, which is now part of readsb, converts UAT positions into the 1090 format. This way both sources end up on the same tar1090 map. How to set up readsb and tar1090 is covered under Installing readsb and tar1090.

Receiving UAT on the go

For pilots, there are dedicated receivers that cover both frequencies. The open-source project Stratux, for example, is built on a Raspberry Pi, receives 1090 MHz and 978 MHz and passes traffic and weather data via Wi-Fi to cockpit apps like ForeFlight. For a spotting day at the fence, that is more than necessary. But it shows that mobile dual-band reception is quite doable. How to receive at a spot with a smartphone, SDR stick and power bank in general is explained in Mobile ADS-B reception at the spot.

Common misunderstandings

“UAT is an older version of ADS-B.” No. UAT is a parallel method with a standard of its own. In the US, the DO-260B standard applies to 1090ES, DO-282B to UAT.

“My 1090 receiver does see small aircraft.” That’s true if they use 1090ES or happen to be rebroadcast via ADS-R or TIS-B. All other UAT aircraft stay invisible.

“MLAT finds the UAT aircraft.” Multilateration in the hobby field works with Mode S signals on 1090 MHz. An aircraft that transmits only on 978 MHz provides nothing for it. More on the method is under MLAT explained.

“UAT is available everywhere in America.” UAT is tailored to the equipment rules of the US. FlightAware explicitly points out that UAT tracking is only available in the US and that coverage can be thin depending on the region.

Conclusion

UAT on 978 MHz is the American answer to a question Europe answered differently: How do you bring ADS-B to general aviation? The US created a second data link for it, approved below 18,000 feet, which incidentally supplies pilots with free cockpit weather. Airliners stay on 1090 MHz, many small aircraft move to 978 MHz. If you are planning a spotting trip, you find more tips under Plane spotting trip to the US and Plane spotting in the US: the rules.