TIS-B, the Ground Stations That Rebroadcast the Radar Picture to Your Cockpit Display, and the Hybrid Surveillance Architecture Most Pilots Have Never Thought About

Every target on your ADS-B traffic display is not created equal. Some are direct broadcasts from aircraft transponders - fast, GPS-accurate, nearly real-time. Others were derived from ground radar and rebroadcast by an FAA ground station most pilots have never thought about. That second category is Traffic Information Service-Broadcast, or TIS-B, and understanding the difference changes how accurately you interpret what your display is actually telling you.

Why ADS-B Alone Leaves Gaps in the Traffic Picture

ADS-B Out - mandated for Class B, Class C, and above 10,000 feet MSL in the contiguous United States as of January 1, 2020 - works by having each equipped aircraft continuously broadcast its GPS-derived position, altitude, and velocity. No radar interrogation required. Any receiver listening on the right frequency picks it up.

But not every aircraft has ADS-B Out. Experimental homebuilts in certain configurations, gliders, balloons, older aircraft operating legally outside the mandate airspace, and military traffic under exemptions all remain unequipped. A Cherokee on a cross-country at 6,000 feet, below Class B and outside Class C, has no legal obligation to carry ADS-B Out. Without a solution, those aircraft simply wouldn’t appear on anyone’s cockpit display.

How TIS-B Works: Radar Speaks for Unequipped Aircraft

The FAA built a nationwide network of ground-based transceivers (GBTs) - hundreds of stations across the country - and connected them directly to the existing ATC radar infrastructure. Secondary surveillance radar, primary radar, Mode C, Mode S - the same systems controllers have relied on for decades.

When radar detects an aircraft, even one with no ADS-B Out at all, the ground station receives that position data and rebroadcasts it on the ADS-B datalink frequency. Your ADS-B In receiver picks up the signal and renders a traffic target on your display. That aircraft never transmitted to you directly. The ground station spoke for it. That is TIS-B.

The Two-Frequency Architecture and Why It Matters

ADS-B in the United States operates on two radio frequencies. 1090 MHz Extended Squitter is the international standard used by airliners, business jets, and any aircraft operating internationally. 978 MHz UAT (Universal Access Transceiver) is a domestic-only system built primarily for general aviation, with lower equipment costs and a built-in weather uplink channel.

TIS-B is broadcast on both frequencies by the ground network. A GA pilot with only a 978 MHz UAT receiver cannot directly receive an airliner’s 1090 MHz ADS-B Out signal - the frequencies don’t match. But if that pilot is within range of a ground station, TIS-B translates the airliner’s 1090 signal and rebroadcasts it on 978. The GA pilot sees the airliner. The ground station is a frequency bridge.

The “Hockey Puck”: How TIS-B Traffic Is Delivered to You

TIS-B doesn’t flood the datalink with every radar target in a ground station’s coverage area. When a ground station detects your ADS-B Out signal, it knows your position and builds a virtual cylinder - the ownship-centric broadcast - centered on you: roughly 15 nautical miles in radius and about 3,500 feet above and below your altitude. Only traffic within that cylinder gets rebroadcast specifically to your receiver. As you move through the airspace, the cylinder moves with you.

The critical implication: you must have ADS-B Out to receive TIS-B. The ground station needs your position to build the cylinder around you. ADS-B In without ADS-B Out means the ground station doesn’t know where you are and won’t send you a tailored traffic picture. This is a primary reason the FAA’s mandate targeted Out over In - the systemic safety benefit depends on the ground knowing your position first.

TIS-B Data Quality vs. Direct ADS-B

Not all traffic targets carry the same accuracy, and the difference is meaningful.

A direct ADS-B Out target comes from GPS. Update rate is typically once per second, with position accuracy within a few meters under normal conditions. What you see on screen is very close to where that aircraft actually is right now.

A TIS-B target is radar-derived. Secondary surveillance radar typically updates every four to five seconds, depending on antenna rotation rate; some older systems are slower. The displayed position is an extrapolation based on the last radar fix and last known velocity. By the time it reaches your screen, the actual aircraft could be several hundred feet from where the symbol sits.

For situational awareness, TIS-B is extremely valuable. For last-second traffic avoidance decisions, it carries more uncertainty than a direct ADS-B target. This is a designed tradeoff - radar-derived data is dramatically better than nothing - but pilots who understand the architecture use the information more accurately.

FIS-B: The Other Service on the Same Infrastructure

The same GBT network that delivers TIS-B also pushes FIS-B (Flight Information Service-Broadcast) - the weather and aeronautical information most GA pilots use every flight. NEXRAD composite mosaics, METARs, TAFs, PIREPs, TFRs, and NOTAMs all arrive via the same ground-based transceivers. Two services, one infrastructure.

One critical caveat: NEXRAD imagery on FIS-B can be five to fifteen minutes old by the time it renders on your display. The data is timestamped, and a quality display will show you the age of the image. FIS-B weather is strategic context - not real-time tactical data. Treating it as current when working near developing convection introduces serious risk.

Where TIS-B Coverage Works - and Where It Doesn’t

The GBT network provides solid coverage across most of the contiguous United States airspace where ADS-B is required - Class B and Class C primary airports, and cruise altitudes in populated and controlled areas.

Below 3,000 feet AGL in remote regions, coverage thins significantly. Mountain flying, low-level operations in less-served areas, and large portions of Alaska can place you beyond line-of-sight range of any ground station. Your ADS-B In receiver will still pick up direct broadcasts from nearby equipped aircraft, but the radar-derived rebroadcast won’t be there. In those environments, the traffic on your display is not necessarily all the traffic that’s there. The FAA publishes coverage maps for the GBT network for pilots who want to check specific routes.

The Hybrid Architecture - and Where It’s Headed

Today’s surveillance system is a hybrid: direct ADS-B Out broadcasts from equipped aircraft stitched together with radar-derived TIS-B rebroadcasts from ground stations, presented as a unified display. Compliance with the 2020 mandate was high, and the overwhelming majority of aircraft operating in covered airspace have ADS-B Out.

The long-term direction under the FAA’s NextGen program is a shift in surveillance dependence - from radar as the primary source to ADS-B as the primary source, with radar as backup rather than foundation. As the existing radar infrastructure ages and eventually gets retired, TIS-B loses the radar data it rebroadcasts. At that point, every aircraft either has its own ADS-B Out or it won’t be visible to anyone. No safety net.

The FAA has not published a hard transition date for radar retirement. The hybrid system continues for the foreseeable future. But the direction is unambiguous: the destination is an airspace where every aircraft is its own broadcast node and the radar-derived TIS-B bridge becomes unnecessary because there are no longer meaningful numbers of unequipped aircraft to compensate for.

What This Means When You Look at Your Traffic Display

Learn your display’s target symbology. Many ADS-B receivers distinguish between direct ADS-B targets and TIS-B-derived targets in the raw data stream. If your display uses different symbols for each, know what they mean. The direct target is more accurate and more current.

Keep your transponder healthy. A failed ADS-B Out doesn’t just eliminate your broadcast - it eliminates your TIS-B service. The ground station can’t build the hockey puck around you if it can’t see you.

Don’t assume coverage below the floor. Low and slow in remote terrain means your traffic picture may be partial. Factor that into your situational awareness rather than assume the display is showing everything.

No system sees everything. An aircraft with a dead transponder, or one legally operating outside the mandate, will not appear anywhere on your display. TIS-B bridges an enormous gap. It doesn’t close every gap. Looking out the window remains non-optional.

Key Takeaways

TIS-B rebroadcasts radar-derived traffic to your ADS-B In display, making unequipped aircraft visible by having the ground station speak for them
You must have ADS-B Out to receive TIS-B - the ground network builds a targeted cylinder of roughly 15 nm radius / 3,500 feet vertical around your position, and it requires your position to do so
TIS-B targets carry more positional uncertainty than direct ADS-B - radar updates every 4–5 seconds versus approximately once per second for GPS-based ADS-B Out
FIS-B weather on the same ground network can be 5–15 minutes old - treat NEXRAD imagery as strategic context, not real-time tactical data
Coverage degrades below 3,000 feet AGL in remote areas - in those environments, your traffic picture may be incomplete, and you should fly accordingly
The long-term trajectory is full ADS-B dependence - as the FAA’s radar infrastructure ages and retires under NextGen, TIS-B goes with it, and the gap-filling safety net disappears