TIS-B, the Radar Relay Hiding Inside Your ADS-B Traffic Display, and the Low-Altitude Coverage Gap Pilots Mistake for Empty Sky

An empty ADS-B traffic display does not mean the sky around you is empty. Two aircraft can pass within five miles of each other with one appearing on the other’s screen and the other completely invisible - not because of equipment failure, but because of how the surveillance system was designed. Understanding the difference between direct ADS-B traffic and TIS-B relay traffic explains why, and what to do about it.

What Are the Two Types of Traffic on Your ADS-B Display?

Most pilots treat all traffic targets as the same category. They’re not. Every target on your EFB is one of two fundamentally different things.

Direct ADS-B targets come from other aircraft broadcasting their own GPS position, altitude, velocity, and identification over radio frequency. Your receiver picks up that signal with no middleman. Position updates arrive approximately once per second, representing the aircraft’s actual location within about one second of real time.

TIS-B targets - Traffic Information Service Broadcast - work differently. An FAA ground station watches radar, captures returns from aircraft that do not have ADS-B Out, and rebroadcasts those radar returns as a downlink to equipped aircraft in the area. Your receiver picks these up the same way it picks up direct targets. They appear on the same display, often with similar-looking symbols. Many pilots treat them identically.

They are not identical.

How Stale Is TIS-B Traffic Data?

Radar refreshes on a sweep cycle, typically four to five seconds per revolution. That position is already several seconds old the moment the sweep completes. The ground station then processes the return, formats it as a TIS-B message, and broadcasts it on the uplink frequency. By the time your receiver decodes it and your EFB renders it, you may be looking at traffic data that is 12 to 15 seconds stale.

For a light aircraft doing 100 knots, 15 seconds of travel is roughly 400 feet. A business jet at 220 knots could have moved nearly half a mile from the position shown on your screen.

That doesn’t make TIS-B useless. Before ADS-B In existed, pilots had essentially zero in-cockpit traffic awareness. TIS-B extends the picture to non-ADS-B aircraft showing up on radar - the vintage Cessna 152, the glider squawking a transponder code, the helicopter doing touch-and-goes. Those contacts would be completely invisible without TIS-B. The latency matters, but the picture is still a genuine safety benefit.

Why Does Your ADS-B Traffic Display Go Blank at Low Altitude?

The FAA built a network of approximately 700 ADS-B ground stations across the continental United States as part of the NextGen modernization program. These stations receive ADS-B Out signals from aircraft and relay them to ATC. They also transmit downlinks back to ADS-B In-equipped aircraft - which is where TIS-B traffic and FIS-B weather originate.

The key word is down. Ground stations transmit downward to you, and for that signal to reach your aircraft, you must be within radio line of sight of the station.

Ground stations are mounted at fixed heights on towers, buildings, and elevated terrain. Their signals propagate outward in roughly a cone shape - solid coverage at altitude, sharply degraded near the surface. Mountain ranges, ridgelines, and deep valley geography create a patchwork of blind spots at low altitude that the NextGen buildout never fully solved.

The FAA publishes coverage analysis data showing the minimum altitude at which you can expect to receive a ground station signal by geographic area. In the flat Midwest and Southeast, coverage floors are typically around 3,000 to 4,000 feet above sea level. In the mountain West and northern Rockies, those floors can reach 8,000 to 10,000 feet or higher. A pilot flying an approach into a mountain airport at 6,000 feet elevation may be completely outside any ground station’s reach throughout the entire approach.

The subtle danger: your screen doesn’t alarm when coverage ends. It doesn’t flag itself as degraded. It just shows nothing. An empty traffic display in a coverage gap looks identical to an empty display when there genuinely is no traffic nearby.

ForeFlight includes an ADS-B status indicator within its traffic layer that shows when your device is actively receiving a ground station uplink and changes state when coverage drops. The feature is worth knowing about and actively checking - most pilots don’t.

What Happens to Weather Data When TIS-B Coverage Fails?

TIS-B and FIS-B (Flight Information Service Broadcast - the in-cockpit weather service) ride the same uplink channel. They fail together.

When a ground station can’t reach your aircraft, you lose both services simultaneously. If your METARs stop updating in flight, that’s a signal worth acting on: when FIS-B goes dark, TIS-B goes with it. If you’re flying through a mountain valley and weather data has stopped refreshing, treat your traffic screen as showing only direct ADS-B targets. The TIS-B relay picture of non-ADS-B aircraft is gone.

The uplink is also ownship-biased. Because you’re transmitting your position via ADS-B Out, the ground station knows approximately where you are and tailors the TIS-B uplink to traffic within roughly 15 nautical miles of your position. This reduces frequency congestion and keeps the picture relevant. But it also means the entire service - weather and traffic relay both - depends on that ground station reaching your aircraft.

Does Your Receiver Actually Get TIS-B?

This is where specific equipment matters in a way many pilots don’t realize.

The FAA designed ADS-B Out around two frequencies. 1090 MHz Extended Squitter is the global standard, required for aircraft operating above 10,000 feet MSL or within the Mode C veils around Class B and Class C airspace. 978 MHz UAT (Universal Access Transceiver) was designed specifically for general aviation operating below 18,000 feet in United States airspace.

The TIS-B architecture was primarily designed to serve UAT-equipped aircraft. A 1090 MHz-only installation does not receive TIS-B uplinks from the FAA ground network. It receives direct aircraft-to-aircraft ADS-B traffic from other 1090 MHz aircraft, but not the radar-relay picture of non-ADS-B traffic that TIS-B provides.

UAT offers more uplink data bandwidth than 1090 MHz, enabling richer TIS-B and FIS-B content. However, 978 MHz is a United States-only standard - cross into Canada or Mexico and the UAT side goes dark. Over time, many pilots chose 1090 MHz for global compatibility, even at the cost of losing TIS-B service.

Today the GA fleet is a mix:

• UAT-only installations receive full TIS-B and FIS-B uplinks within US airspace
• 1090 MHz-only installations receive direct ADS-B traffic only - no TIS-B relay
• Dual-link receivers like the Garmin GDL 50 and GDL 84 series receive both 978 MHz and 1090 MHz simultaneously, providing the most complete traffic picture when in ground station coverage

If you’re not certain which configuration your aircraft has, check your avionics installation paperwork or call your shop. The answer determines whether TIS-B service is available to you at all.

Which Aircraft Won’t Appear Even When TIS-B Is Working?

TIS-B extends the traffic picture to transponder-visible non-ADS-B aircraft - but only to those. An aircraft must be transponder-equipped and actively squawking for radar to capture it and generate a TIS-B target.

• Ultralights with no transponder generate no radar return
• Experimental aircraft not squawking are invisible to radar
• Gliders without radio equipment won’t appear

These aircraft will not show on your traffic display regardless of your equipment or coverage status. This is not a malfunction. It’s a structural limit of radar-based surveillance.

The distinction matters particularly near non-towered airports, where gliders, ultralights, and vintage aircraft without modern transponders are most common - which is also where TIS-B ground station coverage tends to be thinnest.

How Is This Different from TCAS on Airliners?

Traffic Collision Avoidance System (TCAS), used on larger commercial aircraft, is a completely separate active interrogation system. TCAS sends its own interrogations outward, receives direct transponder replies from nearby aircraft, and calculates collision geometry independent of any ground infrastructure.

An airliner at FL350 is not dependent on TIS-B. Its TCAS interrogates the sky directly. The low-altitude coverage gap that affects a Cessna 172 on an EFB does not apply to the airliner’s collision avoidance in the same way.

Most general aviation aircraft do not have TCAS. ADS-B In is the traffic awareness tool for the GA fleet, and understanding its actual limits is what separates an informed crew from one who trusts a tool they don’t fully understand.

What Should Pilots Do With This Information?

Know your equipment. Determine whether your ADS-B installation includes a UAT receiver or is 1090 MHz-only. That single answer determines whether TIS-B service is available to you at all.

Check FAA coverage maps before low-altitude legs. The FAA’s ADS-B ground station coverage analysis is publicly available. If you’re planning a mountain cross-country or any extended leg below 5,000 feet in complex terrain, review the coverage floor for your route before departure.

Watch weather timestamps in flight. If FIS-B METARs stop updating, assume TIS-B coverage has failed as well. Adjust your traffic awareness assumptions accordingly and increase your visual scan rate.

Keep eyes outside at pattern altitude. Near non-towered airports especially, the visual scan remains the primary traffic tool. The FAA states it explicitly: ADS-B traffic is a supplement, not a replacement for see-and-avoid. The combination of thin TIS-B coverage, non-transponder aircraft, and high traffic density makes the downwind entry one of the highest-risk moments in any flight - and the moment when the traffic screen is least reliable.

The long-term path to closing low-altitude coverage gaps runs through space-based surveillance - receiving ADS-B signals from orbit where line-of-sight limitations disappear entirely. That capability operates today for oceanic and remote airspace. Integrating it more deeply into the domestic low-altitude picture is a longer-term effort, not an imminent fix.

Key Takeaways

• TIS-B traffic carries 12–15 seconds of latency - a business jet target may be nearly half a mile from its displayed position
• FAA ground station coverage floors are 3,000–4,000 feet MSL in flat terrain and 8,000–10,000+ feet in the mountain West; below those floors, TIS-B and FIS-B both go silent without any cockpit warning
• 1090 MHz-only receivers do not receive TIS-B - only dual-link or UAT-equipped aircraft get the radar-relay traffic picture
• TIS-B and FIS-B weather share the same uplink channel - stale METARs in flight mean TIS-B coverage has likely failed too
• Aircraft without active transponders - ultralights, unequipped gliders, some experimentals - are invisible to TIS-B regardless of your receiver or coverage status