ADS-R, the Rebroadcast That Vouches for You, and Why the Airplane on the Other Frequency Is Invisible Until a Ground Station Ties the Picture Together

ADS-R (Automatic Dependent Surveillance–Rebroadcast) is the ground-station service that lets aircraft on the two incompatible ADS-B frequencies “see” each other. Because the United States runs ADS-B on two separate radio links that cannot hear each other, an airplane transmitting on one frequency is invisible to an airplane receiving on the other - unless a ground station translates and rebroadcasts the traffic. Critically, that rebroadcast is only built for aircraft transmitting ADS-B Out, which is why a receive-only display can show you a dangerously incomplete picture.

Why did that Cessna never appear on my ADS-B traffic display?

If you have ADS-B In and still watched an airplane pass a thousand feet beneath you without ever painting on your screen, the answer is almost always the dual-link design. That Cessna was likely broadcasting on the opposite frequency from the one your receiver listens to, and no ground station built a rebroadcast to bridge the gap for you.

This isn’t a malfunction. It’s the built-in consequence of a decision the FAA made around 2007 that no other country copied: running ADS-B on two frequencies at once.

What are the two ADS-B frequencies (1090ES vs. UAT 978)?

The United States operates ADS-B over two completely separate broadcast standards.

1090 MHz - called 1090 Extended Squitter (1090ES) - is the global standard. It shares the frequency your Mode S transponder already uses, and it’s what airlines and the turbine fleet fly. If you go international or fly above 18,000 feet, this is the link that matters. It carries no free weather.

978 MHz - the Universal Access Transceiver (UAT) - is a US-only link the FAA built specifically for light general aviation: piston singles, trainers, and airplanes that stay below 18,000 feet. The FAA didn’t want tens of thousands of new transmitters crowding an already-busy 1090 MHz, so it created a roomier, higher-bandwidth pipe down low.

That extra bandwidth is why UAT gets the free goodies. Free weather via FIS-B - the NEXRAD mosaic, METARs, and winds - rides down on 978, and so does free traffic. 1090ES carries none of that.

The trade-off is clean: 1090ES gets you global compatibility and legal operation above the flight levels but no free weather. UAT on 978 gets you the free weather and traffic services but keeps you US-only and capped at 18,000 feet.

Why can’t the two frequencies hear each other?

The moment you split the fleet across two links, you create a problem that only infrastructure can solve. An airplane broadcasting on 978 is completely deaf to an airplane broadcasting on 1090, and vice versa. They are shouting in two different languages on two different radios.

Picture it physically. You’re in a 978 UAT airplane. A Bonanza a mile off your nose is broadcasting flawless ADS-B Out on 1090ES - its position, altitude, and track streaming into the sky at the speed of light. Your receiver hears none of it. As far as your panel is concerned, that Bonanza does not exist.

If the FAA had stopped there, the entire promise of ADS-B In - seeing traffic in the cockpit - would have been half-broken from day one.

How does ADS-R fix the two-frequency problem?

The FAA built a bridge: a network of more than 700 ADS-B ground stations scattered across the country. They are the quiet workhorses of the system, and almost nobody thinks about them.

Each ground station listens on both frequencies at once. It hears the 1090ES airplanes and the 978 UAT airplanes, then performs the translation:

• Traffic heard on 1090 is rebroadcast down on 978, so UAT airplanes can finally see it.
• Traffic heard on 978 is rebroadcast up on 1090, so 1090ES airplanes can see it.

That translation service is ADS-R - the “R” is for Rebroadcast. It’s the ground station standing in the middle, hearing both languages and repeating each one to the other side.

Why does ADS-R only work if I have ADS-B Out?

Here is the hinge of the entire system, and the most important safety point to understand: ADS-R is a service the ground station provides only to aircraft broadcasting ADS-B Out.

ADS-R is deliberately not a firehose. If the ground station rebroadcast every airplane in the state to every airplane in the state, it would flood the frequencies right back into the congestion the FAA was trying to avoid. So the service is targeted - the ground station only builds a rebroadcast for you if it knows you’re there and knows what you need.

And it only knows you’re there because your ADS-B Out transmission told it. When your Out signal reaches a ground station, the system learns three things: your position, your altitude, and which frequency you’re listening on. Now it can find the traffic on the other link near you and rebroadcast just that traffic, tailored to a bubble around your airplane.

What is the ADS-B “hockey puck”?

That tailored bubble has a nickname pilots use: the hockey puck. It’s a cylinder centered on your aircraft, roughly 15 nautical miles in radius and about 3,500 feet above and below you.

The ground station takes the cross-link traffic inside your hockey puck and sends it your way. Traffic outside the puck it ignores - you don’t need it, and the frequency doesn’t need the noise.

So return to that invisible Bonanza. If you’re transmitting ADS-B Out, the ground station knows you’re there, sees the Bonanza inside your hockey puck, and rebroadcasts him down to you on 978. He appears. The system worked exactly as designed.

Is a receive-only ADS-B In display dangerous?

Yes - and this is the single most important takeaway of the topic. If you bought a portable receive-only box that feeds your tablet - ADS-B In but no Out - the ground station does not know you exist. You’re a silent listener, so it never builds an ADS-R rebroadcast for you.

Every airplane on the opposite frequency stays invisible. You’ll happily watch the traffic that happens to be on your own link and quietly miss everyone on the other one. Your screen looks alive. It looks like it’s working. And it is lying to you by omission.

That false confidence is arguably more dangerous than having no traffic display at all, because the pilot with no gadget keeps his eyes outside and assumes he can’t see everybody.

To be fair: if a nearby airplane that does have Out is close to you, the ground station is already rebroadcasting for that aircraft, and you may catch that rebroadcast as a bystander. So you aren’t guaranteed to be blind - but you’re relying on someone else in your neighborhood to have switched the lights on. In quiet airspace with no one nearby to trigger a rebroadcast, you can be genuinely alone with a traffic screen showing you nothing but the occasional target on your own link.

What is TIS-B, and what is a “ghost target”?

Riding alongside ADS-R is a companion service with the same catch: TIS-B (Traffic Information Service–Broadcast). TIS-B takes traffic the FAA sees on its regular radar - airplanes with a transponder but no ADS-B at all - and injects those targets into the ADS-B stream so you can see them.

It works on the same principle: it’s client-based. The ground station only builds a TIS-B feed for aircraft it knows are there, which again means aircraft transmitting ADS-B Out. No Out, no tailored TIS-B.

TIS-B also has a famous quirk - the ghost target. Ever seen a traffic symbol appear right on top of your own airplane, matching your altitude and shadowing every turn? That’s usually you. It happens when radar sees you and the ground station doesn’t perfectly reconcile that radar track with your own ADS-B report, so it hands you back a picture of yourself as an intruder. Modern systems filter this out far better now by matching your reported position, but a ghost is the surveillance system briefly failing to realize the airplane it’s tracking and the airplane it’s talking to are the same one.

Did the 2020 ADS-B Out mandate fix the blind spot?

Largely, yes. The ADS-B Out mandate took effect January 1, 2020. To fly most busy airspace - Class A, B, and C, and above certain shelves - you now need ADS-B Out. That rule quietly pushed huge numbers of airplanes into transmitting, and every airplane that added Out to comply also, as a bonus, switched on its own ADS-R and TIS-B services.

But aircraft that don’t have to comply are still out there: the airplane on a rural grass strip in Class G with no electrical system, no transponder, and no ADS-B - perfectly legal. The system cannot show you what nobody is broadcasting and radar isn’t painting.

That’s the defining limitation. ADS-B is a cooperative surveillance system. Cooperative is the key word. It only works when the other airplane is playing along, or when a ground station or radar is vouching for them. It is not radar - it does not bounce energy off metal to find things that don’t want to be found.

Who builds and runs the ADS-B network?

The US ground infrastructure was built and is operated under contract for the FAA. It’s the same backbone behind the Aireon space-based system over the oceans, but over land and down low - where most GA flies - it’s the 700-plus ground stations doing the 1090 and 978 translation hour after hour.

On the airplane side, it’s the avionics houses - Garmin, Avidyne, and uAvionix - building the boxes that speak both links and sort out the hockey puck for you.

Key Takeaways

• The US runs ADS-B on two incompatible links - 1090ES (global, no free weather) and UAT 978 (US-only, free weather and traffic, capped at 18,000 ft) - that cannot hear each other directly.
• ADS-R uses 700+ ground stations to translate traffic between the two frequencies and rebroadcast it inside your ~15 NM / ±3,500 ft “hockey puck.”
• ADS-R and TIS-B are only built for aircraft transmitting ADS-B Out - a receive-only In box can leave you blind to cross-link and radar-only traffic.
• The January 1, 2020 ADS-B Out mandate fixed much of the blind-spot problem by pushing most aircraft into transmitting.
• ADS-B is cooperative surveillance, not radar - it can’t show non-transmitting aircraft, and your traffic display supplements the visual scan but never replaces it.