Ten Ninety Versus Nine Seventy-Eight, the Two-Frequency ADS-B System America Built That No One Else Copied

The United States is the only country that built its ADS-B network on two frequencies instead of one. High-flying aircraft use 1090 MHz (the international standard), while most general aviation uses 978 MHz, and the whole system only works because FAA ground stations translate between the two links in real time. The practical catch: you only get the full traffic picture if your aircraft is transmitting ADS-B Out, not just receiving In.

What Is ADS-B and How Does It Work?

ADS-B stands for Automatic Dependent Surveillance–Broadcast, and each word describes the technology precisely.

Automatic means it runs on its own, with no interrogation required. Dependent is the key word: it depends on your airplane knowing where it is. Rather than radar bouncing energy off your aluminum, your aircraft determines its own position from GPS and broadcasts it outward for anyone to hear.

Surveillance is the job it performs. Broadcast means it simply announces the data into the air, unencrypted, for any receiver in range.

The contrast with old radar is stark. Radar is a question and answer: a dish sweeps around, sends a pulse, and waits for the echo. ADS-B is a continuous announcement - position, altitude, heading, speed, and identity - updating about once per second instead of once every several seconds. On paper, it’s a straight upgrade.

Why Did the U.S. Build ADS-B on Two Frequencies?

The rest of the world settled on a single frequency: 1090 MHz, the same frequency a Mode S transponder already uses. Engineers bolted the position broadcast onto that existing signal, a technique called Extended Squitter (1090 ES). One frequency, one link, done.

The United States adopted 1090 ES only for aircraft that fly high. For everyone else, it built a second system on a completely different frequency - 978 MHz - called the Universal Access Transceiver (UAT).

The reasoning was sound. 1090 MHz was already crowded. Every transponder chirps on it, every radar interrogation happens on it, and airliner TCAS collision-avoidance systems interrogate each other on it. In a busy terminal area, 1090 MHz is a crowded room where everyone is shouting - a problem engineers call frequency congestion.

Dumping every Cessna, Cub, experimental, helicopter, and balloon onto that same band - each broadcasting position once a second - would have made the noise catastrophically worse, stepping on high-value airliner position reports.

So American designers gave the low-and-slow fleet its own clean frequency. Aircraft below 18,000 feet could use the 978 MHz UAT link, while airliners and anything in the flight levels use 1090 ES, because international flight requires speaking the world’s language. The rule became roughly: in Class A airspace above 18,000 feet, 1090 ES is mandatory; down low, where most general aviation lives, a pilot can choose either link.

What Free Weather Do You Get on 978 MHz?

Because the 978 MHz UAT link wasn’t fighting for room, it had bandwidth to spare - and the FAA used it to send pilots free data.

Ground stations broadcast a full package of information up to aircraft on 978 MHz: graphical weather radar, METARs, TAFs, TFRs, and NOTAMs. This service is called FIS-B (Flight Information Service–Broadcast), and it’s why a pilot with the right 978 MHz receiver gets free in-cockpit weather with no subscription.

1090 ES does not carry FIS-B. If you’re on the 1090 link, you get no FIS-B weather at all.

What Is the Translation Problem Between 978 and 1090?

The moment you have two frequencies, you have a translation problem.

Picture a Piper flying with a 978 MHz UAT system. A Bonanza three miles off its nose is broadcasting on 1090 ES. The Piper’s radio listens on 978; the Bonanza’s broadcasts on 1090. They physically cannot hear each other - two aircraft in the same sky, on the same surveillance system, completely deaf to one another. For a technology built to help you see traffic, that’s a genuinely bad outcome.

The fix is infrastructure most pilots never think about: ADS-B ground stations. The FAA built a national network whose job includes acting as interpreter. A ground station hears the Bonanza on 1090, hears the Piper on 978, and rebroadcasts each aircraft onto the other frequency so they can see each other. That relayed service is called TIS-B (Traffic Information Service–Broadcast).

Why Does an In-Only ADS-B Receiver Miss Traffic?

There are two halves to ADS-B. Out is your airplane broadcasting your own position. In is your airplane receiving other aircraft’s positions on a display.

Many pilots bought an In-only box - a portable receiver that sits on the glareshield, feeds a tablet, requires no installation, and provides no Out. They believe they’re seeing everything. They’re not.

Here’s the catch that every VFR pilot must understand: the ground station only builds you a TIS-B traffic picture if you are broadcasting ADS-B Out yourself. The ground station won’t waste bandwidth creating a custom traffic feed for aircraft it can’t see. It builds that tailored picture only for aircraft transmitting Out, because those are the ones it knows are there and knows where to aim the service.

So an In-only receiver shows only the aircraft broadcasting directly on your frequency. It misses the ones on the other link - the ones that needed translating. You get a partial, misleading picture that looks complete.

What Is the ADS-B “Hockey Puck”?

Pilots nickname the good version of TIS-B coverage the hockey puck. When you transmit ADS-B Out, the ground station serves you a cylinder of airspace centered on your aircraft, and inside it you see everybody - both frequencies, fully translated.

The puck extends roughly 15 nautical miles around you and 3,500 feet above and below. Outside it, coverage thins. Inside it, if you’re broadcasting Out, you see the whole room. Without Out, there is no puck. No box’s marketing states that clearly.

Who Built ADS-B and When Was It Required?

The mandate date was January 1, 2020. That’s when ADS-B Out became required to fly in most controlled airspace - the airspace that previously required a Mode C transponder. Roughly: inside the Class B and Class C shelves, above 10,000 feet, and within the Mode C veil around busy airports.

The FAA set the deadline years in advance, and the general aviation fleet spent the 2010s getting equipped - sometimes grumbling, because the boxes weren’t cheap and both the 978 and 1090 solutions carried installation costs.

On the manufacturing side, the major players are Garmin, Appareo (the Stratus line), uAvionix, and L3, among others. uAvionix did something clever for the budget end, building ADS-B into wingtip and tail position-light housings so an owner could get compliant without tearing apart the panel. The 978 MHz UAT path made many of those low-cost solutions possible, because it’s a simpler, cheaper radio than a full 1090 ES transponder.

Was the Two-Frequency Bet Worth It?

The engineering community genuinely disagrees, and there’s an honest case on both sides.

The case for it. Congestion on 1090 MHz is real, and offloading the general aviation fleet onto a clean frequency protected the system where it mattered most - busy terminal airspace and the flight levels. And FIS-B weather on 978 MHz has made flying measurably safer for thousands of small-aircraft pilots who never would have paid for a datalink weather subscription. Free weather radar in the cockpit of a trainer is a real safety gain.

The case against it. The complexity is the cost. Two links mean the system leans on ground stations to translate, so ADS-B In on the American system isn’t truly peer-to-peer the way the single-frequency world is. Elsewhere, two aircraft with ADS-B simply see each other directly, air-to-air, no ground station required. In America, cross-link traffic depends on being inside a ground station’s coverage and on your transmitting Out. Fly into a valley with no ground-station reception and the translation service can vanish exactly when terrain has already made traffic hard to spot.

There’s a subtler cost, too: every pilot now has to understand a distinction that shouldn’t exist - Out versus In, which frequency, whether you get the puck. That’s cognitive load the rest of the world’s pilots don’t carry. When a surveillance system requires a long explanation of the difference between the box that keeps you legal and the box that actually shows you traffic, the design has leaked complexity onto the user.

In fairness, the FAA made a defensible call for its conditions: a uniquely crowded 1090 environment, a uniquely large and diverse general aviation fleet, and a chance to give that fleet free weather. Two frequencies wasn’t crazy - it was a reasonable answer to an American-sized problem. No other country had that exact problem, which is why no other country copied it. When a design is genuinely universal, it spreads; when it’s a local optimization, it stays home.

What Should Every Pilot Do About It?

Know which link you have - 978 or 1090. Know that if you want the full traffic picture, the complete hockey puck with both frequencies translated, you must be transmitting ADS-B Out, not just receiving In.

That glareshield box feeding your tablet is excellent for weather and better than nothing for traffic, but it does not make you a full participant in the system. Treat the traffic it shows as a helpful hint, never as complete truth. Your eyes out the window remain the primary sensor.

Key Takeaways

• The U.S. is the only country running ADS-B on two frequencies: 978 MHz (UAT) for low-altitude GA and 1090 MHz (1090 ES), the international standard, for aircraft above 18,000 feet.
• FAA ground stations translate between the two links via TIS-B, rebroadcasting each aircraft onto the other frequency so they can see each other.
• You only receive the full “hockey puck” of traffic - about 15 NM wide and 3,500 feet above and below - if you are transmitting ADS-B Out. In-only receivers show an incomplete picture.
• FIS-B free weather (radar, METARs, TAFs, TFRs, NOTAMs) is broadcast only on 978 MHz; the 1090 link does not carry it.
• ADS-B Out became mandatory in most controlled airspace on January 1, 2020.