The Two ADS-B Links, Ten Ninety ES and Nine Seventy-Eight UAT, and Why the FAA Has to Rebroadcast Traffic So the Two Halves of the Sky Can See Each Other

SUMMARY: Why the US runs ADS-B on two frequencies, and how ADS-R and TIS-B rebroadcast traffic so every aircraft can see each other.

The United States is the only major country that operates ADS-B on two separate radio links at once: 1090 ES (1090 MHz) and 978 UAT (978 MHz). Because aircraft on one link cannot hear aircraft on the other, the FAA built a network of over 600 ground stations that translate and rebroadcast traffic between the two frequencies. That rebroadcast service is called ADS-R, and a companion service called TIS-B injects radar-only targets into the picture - but neither works reliably unless your aircraft is transmitting ADS-B Out.

What does ADS-B actually mean?

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

Automatic: it transmits on its own. No controller has to interrogate it and no button gets pushed - it just talks.

Dependent: this is the word that makes engineers nervous. The system depends on the aircraft knowing its own position and reporting it honestly. Old radar was independent - it bounced energy off your airframe and measured the echo, and it did not care what you thought your position was. ADS-B is the opposite: your aircraft reads its own GPS position and announces it. The ground doesn’t measure you; it trusts you.

Surveillance: it’s a position-reporting system that replaces and augments radar.

Broadcast: it doesn’t send a private message to one receiver - it shouts. Every aircraft with ADS-B Out broadcasts its identity, position, altitude, and velocity to anyone in range. Ground stations listen, other aircraft listen. It’s a party line in the sky.

Why does the US use two ADS-B frequencies?

When the FAA built the system for the mandate that took effect on January 1, 2020, it chose two links instead of one.

The first is 1090 ES, short for 1090 Extended Squitter, broadcasting on 1090 MHz. A “squitter” is engineering slang for a broadcast that goes out spontaneously, without anyone asking for it. The FAA chose 1090 MHz because it was already there - it’s the same frequency transponders have used to reply to radar for decades. So 1090 ES is essentially the old transponder taught to say more, and to say it without being interrogated. Every airliner uses it, and internationally it is the only standard. Fly in Europe, Asia, or Australia, and 1090 ES is the only game in town.

The second link, 978 UAT (978 MHz), is unique to the United States. UAT stands for Universal Access Transceiver, a purpose-built datalink designed for general aviation. Because it’s a wider, faster pipe, it has room to carry extra services back up to the aircraft: the free in-flight weather known as FIS-B, including the NEXRAD radar mosaic, winds, temperatures, and TFRs. 1090 ES cannot carry any of that - it’s too crowded and too narrow.

So the FAA gave pilots a choice. Fly below 18,000 feet and never leave the country, and you can equip with 978 UAT and get free weather as a bonus. Fly high or fly internationally, and you must have 1090 ES.

Why can’t two ADS-B aircraft always see each other?

That reasonable-sounding compromise created a real problem: if half the fleet broadcasts on 1090 MHz and the other half on 978 MHz, the two halves are deaf to each other.

Picture a Cessna 172 with a 978 UAT box. It listens on 978 MHz. A Bonanza is converging from the right with a 1090 ES transponder, broadcasting its position loud and clear on 1090 MHz. The Cessna cannot hear it - the Bonanza is transmitting on a band the Cessna’s receiver doesn’t even tune. And the Bonanza can’t hear the Cessna either. Two aircraft, both fully equipped, both broadcasting perfectly, completely invisible to one another.

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

The FAA had two ways to solve this. It could have required every aircraft to receive on both frequencies - expensive, and it doubles the receiver hardware on every airplane. Or it could fix the problem once, on the ground, and rebroadcast everything.

The FAA chose the ground. Its network of well over 600 ground stations listens on both frequencies all the time. When a station hears that Bonanza on 1090 ES, it repackages the position report and rebroadcasts it on 978 UAT so the Cessna can see it - and it does the reverse for the Bonanza, throwing the Cessna’s 978 report back up on 1090.

This service is ADS-R, Automatic Dependent Surveillance–Rebroadcast. It is, quite literally, a translation service in the sky, converting between the two links so both halves of the traffic picture can talk to each other. No other country needs it, because no other country split the band in two.

What is TIS-B and how is it different from ADS-R?

ADS-R only handles aircraft that have ADS-B. What about traffic that doesn’t - an aircraft with an old Mode C transponder and no ADS-B at all, legal in much of the airspace but silent on both new frequencies?

That’s the job of TIS-B, Traffic Information Service–Broadcast. The FAA already knows where that older aircraft is because its radar is painting it, the same way radar has painted aircraft since the 1950s. The ground station takes that radar track and injects it into the ADS-B broadcast so it appears on your display alongside everyone else. TIS-B is the bridge between the old radar world and the new broadcast world.

So when you glance at your traffic display and see three aircraft, you may be looking at three completely different plumbing systems:

• One target is direct, air-to-air - another aircraft’s raw broadcast reaching you on your own frequency.
• One came through ADS-R - translated from the other frequency by a ground station.
• One came through TIS-B - pulled off radar and injected because that aircraft has no ADS-B at all.

Three targets, three different paths, and the display makes them look identical.

Why do I need ADS-B Out to see traffic?

Here is the single most misunderstood point about the whole system: ADS-R and TIS-B are not always on.

A ground station does not spray the entire national traffic picture at everyone - that would flood the frequencies into uselessness. Instead, it only builds you a custom traffic package if it knows you’re there, and it only knows you’re there if you are transmitting ADS-B Out.

If you bought a cheap receive-only ADS-B box - one that shows traffic and weather but doesn’t transmit your position - you are a silent listener. The ground station has no idea you exist and will not build you an ADS-R or TIS-B package. You’ll see aircraft broadcasting directly on your own frequency and miss everyone on the other link and everyone that’s only on radar. To reliably receive the complete traffic picture, you have to be broadcasting yourself.

What is the ADS-B “hockey puck”?

When you do have ADS-B Out and you light up a ground station, it builds a bubble of traffic service around your position - roughly 15 nautical miles in radius and 3,500 feet above and below you. Pilots call it the hockey puck.

It’s a volume of airspace custom-built for your aircraft, following you across the country. Outside that puck, targets can flicker in and out. The service exists only because you announced yourself.

Is the two-link system worth it?

On the promise side, it works. It gave general aviation cheap access to free in-flight weather that would otherwise have cost a fortune, and it let the FAA replace aging radar with something more precise - position updates roughly once per second versus a radar sweep every several seconds, with far better accuracy than a radar echo.

On the problem side, the US built the only two-language surveillance system in the world and had to invent an entire ground-based translation network - ADS-R plus TIS-B - purely to paper over the frequency split. That’s real cost and real complexity, and it created a genuine safety trap in the receive-only box, where a pilot can look at a nearly empty traffic screen and believe the sky is empty when it isn’t.

This is not vaporware. The mandate has been in force since 2020, the ground stations are up, and well over 100,000 general aviation aircraft are equipped, plus the entire airline fleet. It is the surveillance backbone of the country today and will be for decades. The ground infrastructure was largely built and operated under contract for the FAA, while the avionics come from the established transponder and datalink manufacturers who had to build boxes for both frequencies - but the two-link architecture itself is pure FAA.

The practical lesson: if your box only receives, understand exactly what you’re not seeing. And if a controller ever calls traffic that never appears on your display, that aircraft may have been on the other frequency, outside your puck, or squawking nothing but Mode C. The system is brilliant, but it is not magic and it does not see everything. Fly like it doesn’t.

Key Takeaways

• The US runs ADS-B on two links: 1090 ES (1090 MHz) for high-altitude and international flight, and 978 UAT (978 MHz) for domestic GA below 18,000 feet, which also carries free FIS-B weather.
• Aircraft on one link cannot directly hear aircraft on the other, so the FAA built ADS-R to rebroadcast traffic between the two frequencies via 600+ ground stations.
• TIS-B adds a second layer, injecting radar tracks of non-ADS-B aircraft into your display.
• ADS-R and TIS-B only work when you transmit ADS-B Out - a receive-only box makes you invisible to the ground stations and blinds you to much of the traffic.
• Your traffic service arrives as a “hockey puck” roughly 15 NM in radius and 3,500 feet above and below your aircraft, built only because you announced your position.