TIS-B, the Radar Rebroadcast Hidden Inside Your ADS-B Traffic Display, and the Coverage Bubble That Means You Are Not Seeing Every Aircraft on Your Screen

Your ADS-B traffic display draws from two fundamentally different data sources simultaneously, and your screen treats them identically. TIS-B (Traffic Information Service-Broadcast) is the second source - a system that takes radar returns from non-ADS-B aircraft and rebroadcasts them onto your display using the FAA’s ground station network. Understanding how TIS-B works, and where it doesn’t, directly affects how much you should trust what you see.

What Is TIS-B and Why Does It Exist?

The January 2020 ADS-B Out mandate required aircraft operating in Class A, B, and C airspace and within the Mode C veil to carry an ADS-B Out transponder broadcasting GPS-derived position on either 978 MHz (UAT) or 1090 MHz (1090ES). The FAA had been signaling the requirement since 2010 - a full decade of lead time.

But the mandate left significant gaps. Class D, Class E, and Class G airspace were excluded. Aircraft operating exclusively below 10,000 feet MSL and outside the Mode C veil had no obligation to install ADS-B Out. Ultralights, experimentals, older trainers flying the pattern at uncontrolled fields, and many light sport aircraft still carry no ADS-B Out today - legally.

That left the FAA with a problem. The ADS-B In traffic picture had been sold as meaningful situational awareness, yet tens of thousands of aircraft operating on legacy Mode C transponders would never appear on an ADS-B In display without an additional solution.

TIS-B is that solution. Rather than require every aircraft to upgrade, the FAA built a bridge: the existing ATC radar network feeds aircraft positions to ADS-B ground stations, which rebroadcast those positions in ADS-B format. A Cessna 172 with a Mode C transponder but no ADS-B Out can appear on your traffic display - not because it’s transmitting its own position, but because an FAA ground station saw it on radar and sent the position on its behalf.

How the Ground Station Network Builds Your Traffic Bubble

TIS-B is not a blanket broadcast. The FAA deliberately targeted TIS-B transmissions rather than flood every receiver in the country with every radar return simultaneously.

The ground station reads your own ADS-B Out position and constructs a bubble around you - roughly 15 nautical miles in radius and approximately 3,500 feet above and below your reported altitude. Every radar-derived target inside that bubble gets rebroadcast specifically for you.

This architecture has a critical dependency: you must be transmitting ADS-B Out for the ground station to know where to build your bubble. Flying with ADS-B In but no ADS-B Out - still legal in uncontrolled airspace - means you’ll receive direct broadcasts from ADS-B Out-equipped aircraft on the common frequencies, but you will receive no TIS-B traffic. The ground system has no position reference from which to build your picture.

ADS-B In and ADS-B Out are not independent capabilities. They are engineered to work together, and TIS-B degrades precisely when the Out side is absent.

TIS-B can only rebroadcast what radar can see. Radar has terrain-driven blind spots that predate ADS-B by decades - the Rocky Mountain West, the Appalachians, valleys, and ridgelines throughout much of the country. Below radar coverage, TIS-B has nothing to rebroadcast.

The most operationally significant scenario is a small uncontrolled airport in a valley. Pattern altitude might be 1,200 feet AGL. Surrounding terrain masks the pattern from radar until aircraft climb well above the ridgeline. A pilot descending toward that field may have no TIS-B coverage for the most critical phase of the approach - not because the system failed, but because it’s working exactly as designed.

A quiet traffic display in this environment is not the same as a clear sky.

How Update Rate Affects TIS-B Target Accuracy

ADS-B Out aircraft transmit GPS-derived positions at approximately one update per second. TIS-B targets update on radar time.

Terminal radar sweeps every 4 to 5 seconds. En route radar can be as slow as 12 seconds between sweeps. After radar detects a return, that position must traverse the network, be formatted as a TIS-B message, and broadcast from the ground station. By the time a TIS-B target appears on your display, the position data may be 8 to 15 seconds old.

At 120 knots, that’s roughly 400 feet of positional uncertainty before accounting for display rendering latency. For general traffic awareness this is manageable - you’re building a picture, not flying formation. But TIS-B targets represent where aircraft were recently, not where they are now.

UAT vs. 1090 MHz: Which Gets Better TIS-B Coverage?

The U.S. ADS-B system runs on two frequencies: 1090 MHz (1090ES), the international standard used by commercial aviation worldwide, and 978 MHz (UAT), a domestic-only format developed specifically for the U.S. general aviation market. UAT also delivers free FIS-B weather products - radar summaries, METARs, TAFs, NOTAMs, and PIREPs - to any UAT receiver within range.

TIS-B is broadcast on both frequencies. Which frequency the ground station uses for your bubble depends on which frequency your own ADS-B Out uses. The system matches the data to your receiver.

For low-altitude GA operations, there is a meaningful practical difference. The UAT ground station network was engineered specifically for dense low-altitude coverage in the GA environment. The 1090 MHz network skews toward higher altitudes where commercial traffic operates. In practice, UAT-equipped aircraft often see better TIS-B coverage below 3,000 feet and in the pattern. The 1090 MHz advantage lies in international operations and higher-altitude airspace where the international standard dominates.

Why Your Display Shows All Traffic the Same Way

Your EFB traffic screen may show six orange diamonds simultaneously. Three might be direct ADS-B Out transmissions from GPS-equipped aircraft with near-real-time position accuracy. Three might be TIS-B rebroadcasts derived from radar, with terrain-dependent coverage and up to 15 seconds of latency. The screen renders all six identically.

Some products have improved this. Garmin avionics label TIS-B targets differently from direct ADS-B targets if you look for it. ForeFlight on compatible hardware distinguishes data source. But many pilots never examine the distinction - they see a diamond, note the altitude, and move on.

The FAA is explicit: TIS-B is situational awareness, not a separation service. Its own operational guidance states clearly that an aircraft not appearing on your display is not guaranteed to be absent. It means the system didn’t detect it, didn’t have a position to build a bubble around, or couldn’t reach you with a broadcast.

How to Calibrate Your Confidence by Airspace Environment

The practical guidance TIS-B knowledge yields is not to distrust the display - it’s to calibrate confidence based on where you are.

In Class B airspace at 7,000 feet over a major metro area, the TIS-B picture is likely excellent. Almost every aircraft there is either ADS-B Out-equipped or clearly visible on decades-old radar infrastructure. Latency is manageable and gaps are small.

In the pattern at a small uncontrolled field in a valley, assume aircraft are present that aren’t on your display. Make position calls. Fly the visual scan. A clear traffic display in that environment reflects TIS-B’s thinnest coverage and the highest concentration of non-ADS-B aircraft - exactly the combination that makes the display least reliable.

Where TIS-B’s Bridging Concept Is Heading

The FAA Remote ID mandate for drones, effective 2023, mirrors TIS-B’s underlying architecture challenge at a new scale. Small drones operating beyond visual line of sight must now broadcast position and operator location in real time. But the drone surveillance picture and the manned aircraft ADS-B picture currently exist in separate systems.

As unmanned traffic management (UTM) matures, the integration question becomes pressing: how do you build a unified picture that includes a Cessna at 3,000 feet, a cargo drone at 400 feet, and an air taxi at 1,000 feet - each using different surveillance architectures? TIS-B’s bridging concept of using ground infrastructure to stitch together sources that don’t natively communicate is directly relevant to that answer.

Meanwhile, ADS-B equipage in controlled airspace has climbed steadily since the 2020 mandate. ADS-B-equipped aircraft now account for the vast majority of operations in Class B and Class C environments, per FAA performance reports. As that percentage rises, TIS-B becomes less a primary data source and more a legacy backstop for the remaining Mode C-only traffic.

The gap in uncontrolled, low-altitude, and backcountry airspace is a different story. Aircraft operating there never needed ADS-B Out under the mandate. That gap won’t close through regulation - it will close, if it closes at all, through avionics costs falling far enough to reach the simplest aircraft in the fleet.

Key Takeaways

TIS-B rebroadcasts radar-derived positions of non-ADS-B aircraft through FAA ground stations, extending traffic awareness beyond what ADS-B Out alone covers.
TIS-B requires your own ADS-B Out to function. Without it, the ground station can’t locate you to build a traffic bubble, and TIS-B targets won’t appear on your display.
Radar coverage gaps are TIS-B blind spots. Valleys, terrain masking, and low-altitude backcountry environments can eliminate TIS-B coverage entirely - often in the pattern at exactly the fields where non-ADS-B aircraft are most common.
TIS-B targets can be 8–15 seconds out of date, reflecting radar sweep rates rather than GPS update rates; at 120 knots that’s roughly 400 feet of positional uncertainty.
A quiet traffic display is not a clear sky in low-altitude, uncontrolled, terrain-masked environments. Position calls and the visual scan are not backup procedures - they remain primary.