The Two Frequencies of ADS-B, Ten Ninety ES and Nine Seventy-Eight UAT, and Why America Split Its Traffic Picture Onto Two Radios That Can't Hear Each Other

SUMMARY: Why the US runs ADS-B on two frequencies - 1090 ES and 978 UAT - and how that split shapes what your traffic display can and can’t show you.

The United States is the only country that runs its cooperative surveillance system on two radio frequencies at once: 1090 MHz Extended Squitter (1090 ES) and the 978 MHz Universal Access Transceiver (978 UAT). Aircraft on one link cannot directly hear aircraft on the other, so the FAA relies on ground stations to translate and rebroadcast positions between them. That single design choice explains most of the confusing behavior pilots see on their traffic displays.

What does ADS-B actually mean?

ADS-B stands for Automatic Dependent Surveillance–Broadcast, and the name is essentially a spec sheet.

Automatic means it runs on its own - no controller has to interrogate the aircraft. Dependent is the key word: the system depends on the aircraft knowing its own position, which it gets from GPS. Surveillance means someone is watching, and Broadcast means the aircraft simply shouts its position into the open, unencrypted, to anyone listening.

This is the opposite philosophy from traditional radar. Radar is independent - a ground station sends a pulse, it echoes off the aircraft’s skin, and the station measures the return. Radar finds you whether you cooperate or not. ADS-B is you telling everyone where you are.

Why does the US use two ADS-B frequencies?

The rest of the world standardized on a single link: 1090 ES. The “ES” is Extended Squitter - squitter being an old radar-engineering term for a spontaneous, unsolicited broadcast. Because transponders already lived on 1090 MHz, engineers simply upgraded them to spit out a longer message containing GPS position. It reused a radio the aircraft already had.

The US added a second link - 978 UAT - for one main reason: radio congestion, not air traffic. The 1090 MHz band is crowded, shared by every transponder and every collision-avoidance system in the sky. The FAA worried that dumping every light aircraft in the country onto 1090 would choke the frequency with messages stepping on messages.

So the FAA made a bet. Keep the high-altitude, high-density world - airliners and jets - on 1090 ES, which is mandatory above 18,000 feet. Down low, in general aviation, offer a second option built from scratch with room to breathe: 978 UAT.

What do FIS-B and TIS-B give pilots - and on which frequency?

The 978 UAT link had extra bandwidth, and the FAA used it to send data up to aircraft for free. Two services came out of that.

FIS-B (Flight Information Service–Broadcast) is free weather. Ground stations continuously uplink a package that includes the regional radar mosaic, METARs, TAFs, winds aloft, TFRs, PIREPs, and NOTAMs - painted directly on your display at no cost. Cockpit weather once cost thousands of dollars a year in satellite subscriptions.

The catch: FIS-B exists only on the 978 UAT link. Equip with a 1090 ES box - the same link the airliners use - and you get no free weather at all.

TIS-B (Traffic Information Service–Broadcast) is the ground network taking everything it can see - including radar-only targets and aircraft on the other frequency - and rebroadcasting a traffic picture back up to you.

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

Picture a Cirrus with a 978 UAT box and a King Air with a 1090 ES box, three miles apart and closing. The Cirrus broadcasts on 978 and listens on 978. The King Air broadcasts on 1090 and listens on 1090.

Both are shouting. Both are listening. Neither can hear the other - different frequencies. Two fully equipped, perfectly working aircraft, invisible to each other. That isn’t a bug; it’s the unavoidable consequence of running two links.

The FAA’s patch is called ADS-R (Rebroadcast). When a ground station notices two aircraft near each other on opposite links, it translates: it takes the Cirrus’s position and rebroadcasts it on 1090 for the King Air, and the King Air’s position on 978 for the Cirrus. The two aircraft are effectively talking through a translator on the ground.

That means the entire cross-link traffic picture depends on being inside ground-station coverage. Down low, in a valley, or out in the mountains where no station can see you, the translator goes silent and the aircraft on the other link vanishes from your screen - not because anything failed, but because the geometry ran out.

Do I need ADS-B Out to receive a full traffic picture?

Yes - and this is the twist that bites people. To receive the full ground-based traffic service (TIS-B and ADS-R), you must be transmitting ADS-B Out yourself. The ground station builds a small bubble of traffic service around each transmitting aircraft and feeds its neighbors into that bubble.

If you bought a cheap portable receiver with no transmitter, you are not a client. The ground builds you no bubble. You’ll see the aircraft broadcasting directly on your frequency, miss the rest, and have no way of knowing what you’re not seeing. A traffic display that lies by omission looks exactly like a sky that’s empty.

Does the dual-link system actually work?

It does. The mandate went live on January 1, 2020, and the system now carries the busiest airspace on the planet. Hundreds of thousands of aircraft got equipped, and the ground network grew to more than 600 stations.

Controllers now get position updates roughly once per second, compared to the 4 to 12 seconds a rotating radar antenna takes to sweep. That faster, more precise picture enables spacing over the ocean and in the mountains that radar never could, and the free weather has almost certainly saved lives.

Still, the seam is real. Europe, Canada, and most of the world chose a single link - 1090 ES - so everybody hears everybody with no translator required. They absorbed the congestion hit and engineered around it, with Canada leaning hard into space-based receivers to fill coverage gaps. The US remains the outlier, carrying two links and the rebroadcast machinery permanently, because it bet on 978 and the free weather that came with it.

How to fly smart in a two-frequency world

Choose your link deliberately when equipping. 1090 ES is your only legal option above 18,000 feet and is the globally compatible standard, so it’s the choice if you fly high or internationally. 978 UAT gets you free FIS-B weather, which is why much of general aviation went that way for low-altitude flying.

Know your portable receiver’s limits cold. Confirm whether your aircraft is transmitting ADS-B Out, because that determines whether the ground builds you a full traffic bubble or leaves you half blind. A receiver with no transmitter will absolutely miss aircraft - treat it as a supplement to your eyes, never a replacement.

Remember that all of it is dependent. Every piece - both frequencies, the ground translator, the free weather - depends on GPS, on other aircraft being equipped and honest, and on you being inside coverage. Some aircraft legally fly with no electrical system and transmit nothing at all. The moment any link in that chain breaks, the picture goes incomplete. Look outside - the window is the one surveillance system that never needs a ground station.

Key Takeaways

• The US runs ADS-B on two frequencies: 1090 ES (global standard, mandatory above 18,000 ft) and 978 UAT (low-altitude general aviation).
• Aircraft on opposite links cannot hear each other directly; the FAA’s ADS-R service rebroadcasts positions between them, but only within ground-station coverage.
• Free weather (FIS-B) exists only on 978 UAT - a 1090 ES box gets none.
• You must transmit ADS-B Out to receive the full ground-based traffic picture (TIS-B); a receive-only portable will silently miss traffic.
• The mandate took effect January 1, 2020, backed by 600+ ground stations delivering position updates about once per second versus radar’s 4–12 seconds.