The Dual-Link Gamble - Why American ADS-B Runs on Two Frequencies, and the Ghost Targets That TIS-B Was Built to Chase Down

The United States runs its ADS-B surveillance network on two separate radio frequencies at once - 1090 MHz for airliners and turbines, and 978 MHz for general aviation - while most of the world uses only one. That dual-link design solved a frequency-congestion problem, but it also created the need for ground-based relay services and produced the “ghost targets” pilots sometimes see: a single airplane painted twice on the display because the system received it through more than one path.

What ADS-B Actually Means

The name explains the system. ADS-B stands for Automatic Dependent Surveillance Broadcast, and each word carries weight.

Automatic: the equipment transmits on its own, without being interrogated, firing off a position report roughly once per second whether or not anyone is listening.

Dependent: this is the critical distinction. Old radar bounced energy off your aluminum and measured the echo - it did not care what you thought your position was. ADS-B does the opposite. Your airplane reads its own GPS position and announces it. The system is only as honest as the aircraft doing the talking.

Surveillance: someone is watching - air traffic control, and increasingly the airplane next to you.

Broadcast: it is an open transmission. Anyone with the right receiver can hear it.

Why the US Built ADS-B on Two Frequencies

Most of the world settled on a single link: 1090 MHz, the same frequency legacy transponders already used, upgraded to what engineers call 1090 Extended Squitter (1090 ES). A “squitter” is simply an unprompted broadcast. If you fly an airliner or a turbine up high, 1090 ES is your link, and it is the global standard - it works in London, Tokyo, and everywhere else.

The FAA saw a problem for general aviation. The 1090 MHz frequency was already crowded with every transponder, radar interrogation, and airliner in the country. Packing tens of thousands of additional light aircraft onto it, all squittering once a second, would cause frequency congestion - reports stepping on each other in a channel filled with noise.

So the FAA opened a second road for light aircraft: a separate frequency, 978 MHz, running a different data link called the Universal Access Transceiver (UAT). Because 978 had bandwidth to spare, the FAA also poured extra services into it - free weather and free traffic broadcast straight to the cockpit from the ground.

The dual link was a deliberate bet: that splitting traffic across two frequencies tuned to two kinds of flying would serve everyone better than forcing one standard onto all of it - and better than making every Cub and Cessna owner buy an expensive high-power transmitter to punch through the noise at altitude.

How TIS-B and FIS-B Connect the Two Links

The two-frequency design created an obvious hazard: an airplane down low broadcasting only on 978 MHz and an airliner up high broadcasting only on 1090 MHz are in the same sky, possibly converging, but speaking on frequencies that cannot hear each other. With only one receiver, you would be blind to half the traffic around you.

The fix is a nationwide network of more than 700 ground stations that listen to both frequencies and rebroadcast, so everyone hears everyone. This relay carries two named services:

TIS-B (Traffic Information Service Broadcast) handles traffic. A ground station hears the airliner on 1090, knows you are listening on 978, and rebroadcasts the airliner’s position down to you on your frequency. The ground station acts as a translator between the two languages so the full picture assembles on your screen.

FIS-B (Flight Information Service Broadcast) handles weather - radar mosaic, METARs, TAFs, TFRs, and winds aloft, pumped up from the ground on the 978 link at no charge. Free in-flight weather alone sold many pilots on the UAT box.

Why Receive-Only Boxes Give You an Incomplete Picture

TIS-B has a catch every portable-receiver user needs to understand. The ground station does not relay traffic to everyone all the time - that would flood the channel again. It builds a custom traffic picture around you only if it knows you are there, and it only knows you are there because your own ADS-B Out is transmitting. Your outbound broadcast is what triggers the relay.

So if you fly with a receive-only box - the kind stacked on exhibit tables that picks up traffic but does not transmit your position - the ground station does not know you exist and never builds you a traffic picture. You will still see aircraft transmitting directly nearby, but you get a partial, sometimes badly incomplete view.

The danger is that it looks complete. The screen appears confident while it is lying by omission.

What Causes Ghost Targets on ADS-B Displays

Ghost targets are the direct consequence of stitching two links together. Say you have a properly installed system that transmits, so the ground station builds you a traffic picture. Now imagine a nearby airplane that is dual-equipped, or that is tracked by both radar and ADS-B. The system can receive that aircraft through more than one path - once directly, once relayed. If the software does not perfectly reconcile those reports, the same airplane appears twice: two targets, one airplane.

Early in the rollout, pilots reported exactly this - a target shadowing a real airplane, offset slightly, maneuvering whenever the real one maneuvered. It is unsettling to hunt the sky for an airplane that exists only inside the data.

Engineers have largely solved it. Correlation algorithms match the reports by position, velocity, and identity code, then collapse the duplicates into one, and most modern displays are clean. But the ghost was never a classic bug - it was the honest receipt for the complexity of running two links.

The Honest Balance: Benefits and Limitations

The upside is real and large. Radar sweeps only once every several seconds and loses accuracy with distance from the antenna. ADS-B updates about once per second and is as accurate as GPS itself. In mountain valleys radar never reached, ground stations can now see previously invisible traffic. Controllers got a better picture, separation got tighter and safer, and free weather transformed light GA flying.

The limitations are equally honest:

• The system depends on your airplane telling the truth - a bad GPS source or misconfigured install can broadcast garbage with total confidence.
• The dual link leans on the ground-station network. Outside coverage - low in terrain or out over water - the relay simply is not there, and your traffic picture thins without warning.
• Receive-only gear creates a false sense of completeness.
• Privacy: you broadcast your position in the clear once a second, and anyone with a $40 receiver and an antenna can log it. Public flight-tracking sites are built on exactly this. The FAA offers rotating anonymous identity codes for some operators, but most light-aircraft pilots are on the air, named, all the time.

The Timeline and Who Is Building It

This is not future technology - it is the floor you already stand on. Since the start of 2020, ADS-B Out has been required to fly in most busy US airspace: the Mode C veil around major Class B airports, above 10,000 feet, and in and around the Class C shelves. That mandate is finished.

What is still evolving is ADS-B In - the receive side that puts traffic and weather in your cockpit. It is not required, and that is where the innovation lives.

On the airline and turbine side, the builders are the major avionics houses - Honeywell, Collins, and the transponder makers. On the light-aircraft side: Garmin, Dynon, uAvionix, and Appareo, along with the portable-receiver crowd and the experimental and homebuilt world, which as usual drove prices down and creativity up.

The mandate built the highway; the industry is still building the cars - cleaner traffic fusion, software to kill the last ghosts, and antennas that hold coverage lower into the valleys.

The one thing to carry away: your traffic screen is not radar. It is a broadcast that depends on every airplane telling the truth and on a ground network deciding you are worth talking to. Trust it, but keep your eyes outside - the best surveillance system in a light airplane is still the one behind your own forehead.

Key Takeaways

• US ADS-B uses two frequencies: 1090 ES for high-altitude airliners and turbines (the global standard) and 978 MHz UAT for general aviation, to avoid congesting the 1090 channel.
• TIS-B relays traffic between the two frequencies and FIS-B delivers free weather, both broadcast from 700+ ground stations.
• TIS-B only builds you a traffic picture if you transmit - receive-only boxes get an incomplete view that can look deceptively complete.
• Ghost targets occur when one aircraft is received through multiple paths and the software fails to reconcile the duplicate reports.
• ADS-B Out has been mandatory since the start of 2020 in most busy airspace; ADS-B In remains optional and is where current innovation is focused.