The Two ADS-B Frequencies, Nine Seventy-Eight UAT and Ten Ninety ES, and the ADS-R Rebroadcast That Had to Stitch America's Split Datalink Back Together

The United States is the only country that built its ADS-B traffic surveillance on two incompatible radio frequencies: 978 MHz (UAT) for general aviation flying low, and 1090 MHz (Extended Squitter) for airliners and high-flyers. Because a receiver on one frequency physically cannot hear a transmitter on the other, the FAA had to invent a ground-based translation service called ADS-R to rebroadcast each fleet’s position to the other. The critical catch: that translated traffic only reaches you if your aircraft transmits ADS-B Out - a receive-only box leaves you with a traffic picture that looks complete but isn’t.

What Does ADS-B Actually Mean?

ADS-B stands for Automatic Dependent Surveillance–Broadcast, and each word describes exactly how it works.

Automatic - it runs continuously with no pilot input and no controller interrogation. The box just talks.

Dependent - this is the key word. The system depends on the aircraft knowing its own position. Your transmitter pulls a position straight from a GPS receiver and reports it. That’s the opposite of old radar, which was independent: it bounced a signal off your aircraft and didn’t care what you thought your position was. ADS-B trusts the airplane to report itself honestly.

Surveillance - it’s how you get watched, by both controllers and other aircraft.

Broadcast - this is the part that matters most. Your position isn’t a phone call to a specific controller or airplane. It’s shouted in every direction, and anyone with the right receiver tuned to the right frequency can hear it.

Why Does the US Use Two ADS-B Frequencies?

Everywhere else in the world, ADS-B runs on a single channel: 1090 MHz, known as 1090 ES (Extended Squitter), meaning the transponder squirts out extra data alongside its normal reply. One frequency, one standard, for the entire rest of the planet.

The US chose two frequencies to protect 1090 MHz from saturating. That band is already crowded: it’s where Mode S transponders live, where ATC radar interrogates, and where TCAS (the Traffic Collision Avoidance System) interrogates other aircraft and coordinates escape maneuvers. Every airliner, business jet, and ground radar is already shouting on 1090.

FAA engineers realized that pushing hundreds of thousands of general aviation aircraft - Cessnas, Pipers, Cubs, homebuilts - onto 1090 as well would jam the frequency solid. So they built a second road: 978 MHz, the Universal Access Transceiver (UAT), a clean channel reserved for GA flying down low.

What Do You Get on 978 UAT That 1090 Can’t Offer?

The 978 UAT channel has the bandwidth to carry more than just position data, and that extra room is the whole reason it exists.

On 978, you get free weather and information services beamed up from the ground. That service is FIS-B (Flight Information Services–Broadcast), and it’s how your tablet receives NEXRAD radar imagery, METARs, TAFs, TFRs, PIREPs, and winds aloft with no subscription and no monthly bill.

If you’ve ever wondered how free weather appears in ForeFlight or Garmin Pilot from a portable receiver, the answer is 978 UAT. That free weather pipe does not exist on 1090 - there simply isn’t room for it.

The Problem: The Two Frequencies Can’t Hear Each Other

A broadcast on 978 cannot be heard by a 1090 receiver, and a broadcast on 1090 cannot be heard by a 978 receiver. They are different radios on different frequencies - like two groups in one room speaking languages neither understands.

Picture yourself in a Cessna 172 with a 978 UAT box. Above you, a business jet climbs out with a 1090 ES transponder. You’re both broadcasting your positions perfectly and honestly - and you cannot hear each other at all. His signal sails past your antenna in a language your receiver doesn’t speak, and yours sails past his.

For a collision avoidance system, that’s a catastrophe. The entire point was to see each other, yet half the fleet was invisible to the other half.

How Does ADS-R Fix the Split?

ADS-R stands for Automatic Dependent Surveillance–Rebroadcast, and the “R” is the fix. It works from the ground.

The nationwide network of ADS-B ground stations has ears on both frequencies. Each station hears the business jet on 1090 and hears your Cessna on 978, then acts as a translator: it repackages the jet’s 1090 position and rebroadcasts it to you on 978, the frequency you can actually hear - and rebroadcasts your 978 position up on 1090 so the jet can see you.

There’s a companion service called TIS-B (Traffic Information Services–Broadcast). It takes aircraft that ground radar sees but that aren’t broadcasting ADS-B at all - older airplanes with just a Mode C transponder - and injects those targets into your picture too.

So the traffic on your screen arrives by three different paths: direct airplane-to-airplane on your own frequency, ADS-R translated across from the other frequency, and TIS-B pulled from radar. Three streams, stitched into one seamless-looking picture.

Why a Receive-Only ADS-B Box Leaves Holes in Your Traffic Picture

Here’s the part every pilot flying ADS-B In needs to understand: ADS-R and TIS-B are client-based services. The ground station only sends you translated traffic once your aircraft has announced itself by transmitting ADS-B Out.

When you transmit Out, the ground builds a service volume - a bubble roughly 15 nautical miles around you and 3,500 feet above and below. It figures out which nearby traffic you can’t already hear on your own frequency and beams just that traffic to you.

If you fly with a receive-only box (ADS-B In with no Out), you’re an eavesdropper. You never announced yourself, so the ground station doesn’t know you exist and never builds a bubble for you. You’ll still hear direct broadcasts on your own frequency, but the entire translated stream - all traffic on the other frequency and all radar-only targets - never comes to you.

The danger is that nothing on your screen warns you about the gap. Your display looks calm and complete while an airplane on the other datalink slides right through your area, totally invisible. A receive-only box gives you a partial picture that looks like a full one - the single most dangerous misunderstanding in ADS-B.

What Are ADS-B “Ghost” Targets?

Because ADS-R is targeted to your specific aircraft, you can occasionally see a ghost target near the edge of two ground stations’ coverage - the same airplane painted twice, slightly offset, because both stations decided to serve you and the geometry didn’t line up perfectly. It’s uncommon now as the system has matured, but that phantom twin is the seam of the rebroadcast architecture showing through.

Is the Two-Frequency System Worth It?

Despite the awkwardness, the benefits are real and large. Since the ADS-B mandate took full effect in 2020, coverage has been excellent. Ground stations reach down into valleys and canyons where line-of-sight radar never could, and controllers get position updates about once per second instead of once every several seconds from a sweeping radar dish.

Search and rescue, terrain coverage, and mountain separation all improved, and the free 978 weather has genuinely changed how safely GA pilots make weather decisions. The two-frequency gamble also succeeded at its original goal: it protected 1090 from saturating.

The honest accounting is that each layer fixed the one beneath it and added a new wrinkle on top. A spectrum-crowding problem was solved by creating a translation problem, which was solved by a ground relay that only works if you play by its rules. That’s what real engineering looks like when you retrofit an entire national airspace instead of designing on a clean sheet - the new system had to be built on top of the transponders and radars already in place.

Who Builds and Runs the System?

The FAA owns the architecture and standards. The US ground station network was largely built and is operated under contract. The avionics in your panel come from familiar names - Garmin, uAvionix, Appareo, and others - spanning certified installs to portable receivers you can velcro to the glareshield.

The Practical Takeaway

Know what’s in your panel. If you have ADS-B Out, you’re a full participant: the ground builds your bubble, and you get the translated traffic and the complete picture, subject to normal range and line-of-sight limits.

If you fly behind a receive-only box, understand exactly what you have - great weather and some traffic (the direct stuff on your own frequency), but not the full stitched picture. You’re missing the other datalink and the radar-only airplanes, with no warning about the gap. Keep your eyes outside. The traffic display is an aid; it was never a promise.

Key Takeaways

• The US runs ADS-B on two incompatible frequencies: 978 MHz UAT for low-flying GA and 1090 MHz ES for airliners and high-altitude traffic - every other country uses only 1090.
• A 978 receiver cannot hear a 1090 transmitter and vice versa, so the FAA created ADS-R to have ground stations translate and rebroadcast each fleet’s position to the other frequency.
• 978 UAT carries free FIS-B weather (NEXRAD, METARs, TAFs, TFRs) that 1090 has no bandwidth to provide.
• ADS-R and TIS-B are client-based: you only receive translated and radar-only traffic if you transmit ADS-B Out, which triggers a service volume of about 15 NM and 3,500 feet above and below.
• A receive-only ADS-B In box shows a picture that looks complete but silently omits the other datalink and radar-only aircraft - always keep your eyes outside.