TIS-B, the Rebroadcast Layer Behind Your ADS-B Traffic Display, and the Low-Altitude Coverage Hole Your Avionics Panel Does Not Warn You About

Your ADS-B traffic display paints a picture of the airspace around you, but a significant portion of that picture depends on a ground-based rebroadcast service called TIS-B - and that service stops working at the altitudes where mid-air collision risk is highest. Understanding how TIS-B works, and where it doesn’t, is essential to using your traffic display correctly.

What Is TIS-B and Why Does It Exist?

The January 2020 ADS-B Out mandate requires equipped aircraft in most controlled airspace to broadcast GPS-derived position, altitude, velocity, and identification once per second. But the mandate covers specific airspace classes and altitude floors - not every aircraft operating legally in shared airspace. Older aircraft, ultralights, gliders, and aircraft with FAA exemptions can legally fly without ADS-B Out.

An ADS-B In receiver only hears aircraft that are transmitting. A non-equipped aircraft simply doesn’t appear on your display.

Traffic Information Service-Broadcast (TIS-B) was the FAA’s solution to this gap. The concept has roots in an older technology called TIS (Traffic Information Service), which delivered traffic alerts through Mode S transponders to specific aircraft in terminal radar environments. When the national ADS-B architecture was being designed in the 2000s, TIS-B extended that concept to cover the entire airspace system.

How TIS-B Turns Radar Returns Into Traffic Targets

The FAA operates a network of approximately 700 ground stations in the contiguous United States, with additional installations in Alaska, Hawaii, Puerto Rico, and Guam. Each station functions as both a receiver and a transmitter.

On the receive side, a ground station picks up ADS-B Out broadcasts from equipped aircraft. It also connects to the existing secondary surveillance radar infrastructure - the backbone of ATC separation services for decades. Secondary surveillance radar interrogates any transponder-equipped aircraft and extracts altitude and identity regardless of ADS-B equipage. A Mode C transponder, required in most controlled airspace since the early 1990s, replies to radar interrogation and transmits altitude back.

The ground station takes that radar return from a non-ADS-B aircraft, packages the position and altitude data, and rebroadcasts it on ADS-B frequencies. Your ADS-B In receiver picks it up and paints the target. Most avionics implementations use a different icon shape or color to differentiate TIS-B targets from direct ADS-B contacts, but the target appears on your display as real traffic.

How Accurate Is TIS-B Compared to Direct ADS-B?

Direct ADS-B targets update once per second with GPS-derived position accurate to roughly 30 feet under normal conditions. That is close to real-time position data.

A TIS-B target travels a longer chain: transponder reply → radar sweep → ground station processing → broadcast → your receiver. That chain introduces latency. Depending on the radar site’s antenna rotation rate and ground station processing time, a TIS-B target may be 12 to 15 seconds old by the time it appears on your screen.

Position accuracy also differs. TIS-B position is radar-derived, and several hundred feet of position uncertainty is typical. That is still useful situational awareness, but it carries different weight in close-proximity traffic situations than a direct ADS-B contact does.

Why TIS-B Coverage Fails Below 3,000 Feet AGL

For TIS-B to reach your aircraft, two conditions must be true simultaneously: you must be within radio range of a ground station on your frequency, and that station must have radar data covering your area.

Radio frequency propagation is line-of-sight. The curvature of the earth limits how far a ground station signal can travel at low altitudes. At 500 feet AGL in a rural area, you may be geometrically invisible to every ground station in the network.

The FAA designed the ground station network to provide reliable ADS-B surveillance coverage above roughly 3,000 feet AGL across most of the contiguous United States. Below that floor, coverage becomes inconsistent - and in many areas, nonexistent.

The problem is compounded by where that coverage gap falls in the flight profile. Traffic pattern operations occur between 800 and 1,000 feet AGL. Arrivals and departures at uncontrolled airports happen below the 3,000-foot coverage floor. Low-altitude VFR cross-country flying in Class G airspace, practice areas around training fields - these are environments with high traffic density, often with mixed-equipage aircraft, and unreliable or absent TIS-B coverage.

Why Your Avionics Won’t Tell You When TIS-B Drops Out

This is the critical operational point. Your traffic display does not tell you when TIS-B coverage has dropped out. There is no standard annunciation on most implementations that reads “TIS-B unavailable.” The display simply shows fewer targets.

If you descend from 8,000 feet into the pattern and targets disappear from your screen, the natural interpretation is that those aircraft turned away or landed. The correct interpretation - that you have descended below TIS-B coverage and your display went partially blind - requires active knowledge to even consider.

Some newer avionics include coverage status indicators. Recent Garmin implementations display traffic data link age. FreeFlight receivers have status outputs. EFB applications like ForeFlight that ingest ADS-B data often show a ground station acquisition indicator. Checking the pilot operating handbook for your specific avionics to understand whether it displays TIS-B coverage status will change how you interpret your traffic picture for the rest of your flying career.

How TIS-B Bridges the UAT and 1090 ES Divide

The FAA built the US ADS-B system around two separate data link standards. 1090 MHz Extended Squitter (1090 ES) is the international standard, required above Flight Level 180. Universal Access Transceiver (UAT), operating on 978 MHz, was designed for general aviation below FL180 and bundles weather data with the equipage decision.

A UAT-equipped aircraft and a 1090 ES-equipped aircraft cannot directly receive each other’s ADS-B Out broadcasts - they transmit on different frequencies. Neither aircraft sees the other in direct peer-to-peer ADS-B.

TIS-B at the ground station handles the translation. When a UAT-equipped aircraft below FL180 sees airline traffic or a business jet on their display, those contacts are arriving via TIS-B rebroadcast on 978 MHz, not direct reception. The ground station heard the 1090 ES transmission, converted it, and sent it back on the UAT frequency. TIS-B is not just filling in non-equipped aircraft - it is the interoperability layer that makes two incompatible standards share the same traffic picture.

One important consequence: FIS-B (Flight Information Service-Broadcast), which delivers Nexrad radar, METARs, TAFs, winds aloft, and NOTAMs alongside the traffic picture, operates on 978 MHz only. A 1090 ES-only installation receives TIS-B traffic but no FIS-B weather from the ADS-B datalink. A separate subscription weather receiver is required for in-cockpit weather on a 1090 ES-only system.

Where Multilateration (MLAT) Fits Into the Picture

Multilateration (MLAT) is a related surveillance technology that feeds the same TIS-B pipeline. Where radar uses a single interrogation point to derive position, MLAT uses multiple ground stations receiving the same transponder transmission simultaneously and calculates position from the time difference of arrival at each station.

MLAT requires no radar sweep and can deliver position updates several times per second with higher accuracy than traditional radar, particularly at lower altitudes in dense terminal environments. Several major airports use MLAT as a primary surface and terminal surveillance tool.

Because MLAT data can feed into TIS-B just as radar data does, TIS-B around a busy Class B airport often looks better than TIS-B 20 miles out at low altitude. More stations, better geometry, and more data sources all feed the rebroadcast pipeline in busy terminal areas.

How to Use Your Traffic Display with This Knowledge

The useful mental model is altitude-dependent fidelity.

Above 5,000 feet in most of the continental United States, your traffic picture is reliable and layered: direct ADS-B contacts from equipped aircraft, TIS-B contacts from transponder-equipped non-ADS-B aircraft, and ground station translation bridging UAT and 1090 ES traffic from each side. When both ADS-B Out and In are installed, the ground station knows your position and serves up a targeted traffic bubble centered on roughly 30 to 40 nautical miles. The system works well up there and was designed to work well up there.

As you descend below 3,000 feet, TIS-B coverage becomes progressively less reliable - and your display does not annotate that change. You carry that knowledge yourself.

In the traffic pattern, away from controlled airspace, transponder-less aircraft are invisible to all traffic displays regardless of altitude or coverage. An ultralight, a glider, or an aircraft on a straight-in without a transponder will never appear on any ADS-B display under any circumstances. See-and-avoid is not a backup in those environments - it is the primary system.

The ADS-B transition genuinely improved the surveillance picture in controlled airspace above the mandate altitude floors. That was the goal, and the data shows it worked. But the system was never designed to provide complete low-altitude traffic awareness. Understanding where the design intended to deliver - and where it was never meant to go - is how you use it correctly.

Key Takeaways

• TIS-B (Traffic Information Service-Broadcast) rebroadcasts radar-tracked, non-ADS-B aircraft to your display, filling in transponder-equipped traffic that isn’t broadcasting ADS-B Out.
• TIS-B targets carry 12–15 seconds of latency and several hundred feet of position uncertainty, compared to approximately 30-foot GPS accuracy and 1-second updates for direct ADS-B contacts.
• FAA ground station coverage becomes unreliable below 3,000 feet AGL - exactly the altitudes where traffic pattern and low-altitude operations occur.
• Your traffic display does not warn you when TIS-B drops out. Fewer targets on descent may mean aircraft moved away, or it may mean your picture just went partially blind.
• TIS-B is also the interoperability bridge between UAT (978 MHz) and 1090 ES aircraft, which cannot directly receive each other’s broadcasts.
• Transponder-less aircraft - ultralights, gliders, exempted aircraft - are invisible to all traffic displays regardless of altitude or TIS-B coverage.