Why did a fully ADS-B-equipped airliner appear so late on my traffic display?

If your receiver is single-band UAT and TIS-B coverage was limited or absent in that area, your hardware could not hear the airliner's 1090 ES broadcast directly and received no TIS-B substitute in time. Both aircraft were legally compliant - the late appearance is a frequency incompatibility and a TIS-B coverage limitation, not an equipment malfunction.

What is TIS-B and how does it bridge the two ADS-B frequencies?

TIS-B is a FAA ground-station service that receives traffic on both 978 MHz and 1090 MHz simultaneously, then rebroadcasts a combined traffic picture targeted at specific aircraft in the coverage area. It allows a UAT receiver to see 1090 ES aircraft - and vice versa - without requiring hardware that listens on both frequencies directly.

How do I know if my ADS-B receiver is dual-band?

Check the product specification sheet for the frequencies listed. If both 978 MHz and 1090 MHz (or "1090 ES") appear, you have dual-band capability. If only 978 MHz is listed, your receiver is single-band UAT and depends entirely on TIS-B for any 1090 ES traffic.

Is ADS-B In (the traffic receiver) required by the FAA mandate?

No. FAR 91.225 requires only ADS-B Out - the transmitter that broadcasts your position. The receiver that displays surrounding traffic is entirely optional. A legally compliant aircraft can be broadcasting its own position while displaying no traffic information whatsoever.

Will a UAT transponder become non-compliant if the FAA eventually sunsets the frequency?

No decision has been announced and UAT compliance is not at risk in the near term. However, for new installations, 1090 ES warrants serious consideration given its status as the global standard and the declining price gap between the two technologies.

Nine Seventy-Eight and One Zero Nine Zero, the Two Frequencies Running ADS-B, and the Traffic Blind Spot That Exists When Both Systems Share the Sky

ADS-B operates on two separate, incompatible frequencies: 978 MHz UAT, used by most general aviation aircraft in the United States, and 1090 MHz Extended Squitter (1090 ES), the international standard used by airliners and high-altitude aircraft. Because these systems cannot hear each other directly, a bridge called TIS-B fills the gap - but that bridge has real geographic and altitude limitations. Understanding which frequencies your equipment actually monitors is the difference between a traffic display that informs and one that creates false confidence.

What Is ADS-B and How Does It Actually Work?

ADS-B stands for Automatic Dependent Surveillance Broadcast. The key word is dependent - unlike radar, the system relies on the aircraft itself to generate its own position data. GPS provides the aircraft’s location, and the avionics package that position along with altitude, airspeed, aircraft identification, and intent into an open broadcast. Any receiver within radio range picks it up - no interrogation required, no radar sweep needed.

Federal Aviation Regulation 91.225 mandated ADS-B Out for virtually all aircraft operating in transponder-required airspace. The compliance deadline was January 1, 2020. For aircraft flying in Class A, Class B, Class C, and most Class E airspace, the transition is complete. Below 10,000 feet in uncontrolled airspace, some aircraft still operate without it.

Why Does ADS-B Use Two Different Frequencies?

Below 18,000 feet in U.S. airspace, an aircraft can satisfy the ADS-B Out requirement with either frequency. That frequency-agnostic rule was a deliberate design compromise - and it created a seam in the traffic picture.

978 MHz - Universal Access Transceiver (UAT). UAT was designed specifically for general aviation operating below FL180. Early avionics hit the market at price points that made sense for a Cessna 172 owner. UAT is a U.S.-only standard - it does not exist in any other country’s airspace.

UAT also carries FIS-B (Flight Information Service Broadcast): in-cockpit weather including METARs, TAFs, NEXRAD radar imagery, TFRs, and NOTAMs, all pushed from the ground to your cockpit at no additional subscription cost. If you have ever pulled up in-flight weather on ForeFlight or Garmin Pilot without a satellite weather subscription, that data was likely arriving over UAT FIS-B.

1090 MHz Extended Squitter (1090 ES). 1090 ES is the international standard. Every commercial airliner worldwide uses it. Every aircraft operating above FL180 in the United States is required to run it. Unlike UAT, 1090 ES carries no weather data uplink - it is surveillance only.

A UAT receiver operates on 978 MHz. A 1090 ES aircraft transmits on 1090 MHz. UAT hardware cannot hear 1090 ES traffic - not because of a malfunction, but because the two systems operate on fundamentally different frequencies.

Picture a Cirrus SR22 at 8,000 feet with a UAT transponder and a portable UAT receiver feeding the EFB. A Piper Cherokee two miles off the wing on UAT shows up cleanly, updating roughly once per second. An airliner descending through 10,000 feet, transmitting 1090 ES only, is completely absent from that display. Both aircraft are fully compliant with every ADS-B Out requirement. Both are actively broadcasting. At the hardware level, they are invisible to each other.

What Is TIS-B and Does It Close the Gap?

The FAA engineered a bridge for this problem: Traffic Information Service Broadcast (TIS-B). The FAA operates several hundred ADS-B ground stations across the continental United States, with additional coverage in Alaska, Hawaii, and the Caribbean. Each ground station receives transmissions on both frequencies simultaneously and also pulls in radar data from aircraft carrying only conventional transponders.

The ground station then rebroadcasts a combined traffic picture - but not indiscriminately. It targets the uplink at specific aircraft it has positively identified in its coverage area. When the system detects a UAT aircraft in an area, it transmits TIS-B on 978 MHz carrying the positions of nearby 1090 ES aircraft that the UAT receiver could not hear directly. The same process runs in reverse for 1090 ES aircraft.

In theory, TIS-B closes the seam entirely. In practice, the data tells a more complicated story.

What Are TIS-B’s Real-World Limitations?

Coverage degrades at low altitude. TIS-B is a ground-to-air broadcast, and its useful range from any given station is limited by terrain and altitude. At 10,000 feet over flat midwestern plains, multiple stations overlap and coverage is solid. At 2,000 feet in a mountain valley in central Idaho, coverage may be nonexistent. At pattern altitude at a rural non-towered airport, it is inconsistent at best. The ground station network was designed to meet mandate requirements at specified altitudes in specific airspace classes - not to deliver full coverage at every altitude everywhere in the country.

Update rates are slower. Direct ADS-B from a 1090 ES aircraft updates approximately once per second. TIS-B updates more slowly. When TIS-B data is generated from radar rather than from a direct ADS-B broadcast, latency increases further as radar sweep rates, processing time, and transmission cycles all add delay. For a converging fast-moving jet, the difference between a one-second update and a five-second update is not trivial.

You have to be seen to receive the service. The ground station only targets aircraft it has positively identified. If you are operating at an altitude or in terrain where the ground station cannot reliably receive your ADS-B Out transmission, it will not target you for TIS-B service.

Combine all three constraints and the gap becomes visible: a UAT aircraft and a 1090 ES aircraft converging at low altitude in complex terrain. The UAT aircraft is below reliable TIS-B coverage. The ground station is not targeting it. The 1090 ES aircraft cannot hear UAT directly. Neither sees the other on a traffic display - despite both being fully and legally equipped.

Does a Dual-Band Receiver Solve the Problem?

Yes, substantially. A dual-band ADS-B receiver listens simultaneously on both 978 MHz and 1090 MHz. It does not depend on TIS-B to see 1090 ES traffic - it hears that traffic directly, with the same update rate and reliability as local UAT traffic. No intermediary, no coverage dependency, no latency from the rebroadcast chain.

Several panel-mounted and portable products now include dual-band capability. If you purchased your traffic system within the last four to five years, there is a reasonable chance it is already dual-band. If you are running an older portable from the early 2010s, check the product specification sheet. Many of those were single-band UAT only. If 978 MHz is the only frequency listed, TIS-B is your only bridge to airliner traffic - with all the limitations that entails.

What Traffic Will Never Appear on Any ADS-B Display?

TIS-B injects radar-detected aircraft carrying conventional Mode C transponders into the traffic picture, so a legacy no-ADS-B aircraft can still appear on your display if ground radar coverage exists.

Aircraft with no transponder, no radar return, and no ADS-B are absent entirely: gliders thermaling in Class E airspace, ultralights, paragliders, hot air balloons, hang gliders, and skydiving aircraft near jump altitude. These targets generate no electronic signature. This is not a problem ADS-B created - it predates the system by decades.

See-and-avoid remains the foundational rule of visual flight. What ADS-B changed is not the rule but the expectation. A pilot who spends hundreds of hours watching a display that shows most of the traffic most of the time will naturally begin trusting the display and reducing the visual scan. That is a training problem as much as a technology limitation. The display is a tool with defined parameters, not a guarantee of complete airspace awareness.

Should You Upgrade to 1090 ES or Keep Your UAT Transponder?

The United States is the only country using 978 MHz for ADS-B. Every other nation runs exclusively on 1090 ES. As global aviation integration continues, the dual-frequency system creates ongoing friction for aircraft flying internationally and for foreign aircraft transiting U.S. airspace. There is periodic industry discussion about whether UAT has a long-term future.

No FAA decision has been made. There is no announced UAT sunset timeline, and current UAT compliance is not at risk in the near term. Manufacturers who have invested in UAT avionics are not advocating for its removal.

For anyone planning an avionics upgrade or a new panel installation, the frequency question deserves careful consideration. A 1090 ES installation may offer better long-term flexibility, particularly for aircraft that regularly operate in Class B or Class C airspace where airliner traffic density makes the cross-band seam most consequential. UAT’s FIS-B weather benefit is genuine, but that same weather data is increasingly available through cellular datalink for pilots who already carry an EFB.

Key Takeaways

ADS-B operates on two incompatible frequencies: 978 MHz UAT (U.S. general aviation only) and 1090 MHz ES (international standard, all airliners). They cannot hear each other directly at the hardware level.
TIS-B bridges the gap via FAA ground stations, but coverage degrades at low altitude and in complex terrain, update rates are slower than direct ADS-B, and the ground station must be able to receive your signal to target you.
A dual-band receiver that monitors both frequencies simultaneously eliminates TIS-B dependency and delivers direct, real-time traffic from 1090 ES aircraft regardless of ground station coverage.
Aircraft with no transponder and no ADS-B - gliders, ultralights, hot air balloons, paragliders - do not appear on any traffic display. See-and-avoid is still the primary collision-avoidance mechanism in visual flight.
If you’re running an older single-band UAT portable, check the spec sheet. Those units have a meaningful blind spot for airliner traffic in areas of limited TIS-B coverage.