The EFB Traffic Display, the TIS-B Rebroadcast Delay Hidden in the Open Diamond, and the Coverage Floor Where Your Surveillance Picture Goes Dark

The traffic symbols on your EFB look like radar, but they aren’t. Your cockpit traffic display combines two fundamentally different data streams - one nearly real-time, one potentially 12 to 30 seconds old - and the display provides no warning when it may be showing you an incomplete picture.

How Did the FAA Build ADS-B, and Why Does the Architecture Matter?

The FAA’s NextGen modernization program established Automatic Dependent Surveillance-Broadcast (ADS-B) as the backbone of next-generation airspace surveillance. Rather than painting aircraft with radar beams, ADS-B-equipped aircraft determine their own GPS position and broadcast it continuously. Ground infrastructure just listens.

The January 2020 mandate required ADS-B Out equipment for aircraft operating in certain controlled airspace. ATC now receives GPS-accurate position data updated every second - a sharper picture than secondary radar alone could provide.

But the design choices made to get general aviation to equip quickly and affordably created an architecture with real operational limitations. Those limitations live inside the traffic display on your tablet.

What Is the Difference Between Direct ADS-B and TIS-B Traffic?

Every traffic symbol on your display falls into one of two categories, and they are not equivalent.

Direct ADS-B is aircraft-to-aircraft. Your receiver picks up another aircraft’s ADS-B Out transmission directly, with no ground station in the loop. The update rate is approximately once per second. For practical purposes, you are seeing where that aircraft is right now.

Traffic Information Service-Broadcast (TIS-B) is a different data path entirely. TIS-B exists because even after the 2020 mandate, many aircraft operate legally without ADS-B Out - agricultural operators below 400 feet, gliders, balloons, and certain experimental and legacy aircraft in uncontrolled airspace. Without TIS-B, your display would show nothing for these targets.

The TIS-B chain works like this: ATC radar sweeps and detects a transponder-equipped target - Mode C or Mode S, it doesn’t matter. That return gets processed, validated, and packaged by the ATC system. Then the FAA’s network of approximately 700 ground-based transceivers across the contiguous United States rebroadcasts that radar-derived position data on ADS-B frequencies to equipped aircraft nearby. Your receiver picks it up. Your display draws a symbol.

On most avionics and tablet apps, you can tell the difference. A filled, solid symbol is direct ADS-B. A hollow or open symbol - often the hollow blue diamond - is TIS-B.

How Much Latency Is Hidden Inside a TIS-B Symbol?

There is a latency chain buried in the TIS-B process, and it matters at the speeds pilots fly.

Radar is not instantaneous. A typical en route radar rotates every 5 to 12 seconds. Terminal radar is faster - approximately 4 to 5 seconds per revolution. After each sweep, the system must process the return, validate it, package it for broadcast, transmit it from a ground station, and your avionics must decode and render it.

By the time that open diamond appears on your screen, the aircraft it represents was at that position somewhere between 12 and 30 seconds ago.

Consider the geometry. Two aircraft converging head-on - a Cessna 172 and a Cherokee - have a combined closure rate of roughly 220 knots. In 20 seconds, they’ve closed more than a mile and a quarter. The hollow diamond showing traffic slightly north at your altitude may actually be merging with your position by the time you’ve processed the symbol and started a visual scan.

TIS-B is genuinely useful - it provides advance warning of traffic that would otherwise be completely invisible. But it is not the real-time picture that direct ADS-B provides, and treating them as equivalent is a meaningful cognitive error.

Where Does Your ADS-B Traffic Picture Go Dark?

The FAA designed the ground station network to guarantee ADS-B coverage at 3,500 feet AGL and above across the contiguous United States. Below that floor, coverage becomes inconsistent and entirely location-dependent.

In flat, open terrain, reliable coverage may extend down to 1,000 feet or lower. In mountainous terrain, valleys, behind ridgelines, and near certain coastal geometries, you can be well above the surface and still be in a frequency shadow cast by terrain features.

This creates two compounding problems.

First, if you’re below the coverage floor, your own ADS-B Out transmissions aren’t being received by the ground infrastructure. ATC cannot see you. That is a significant operational fact on its own.

Second, TIS-B is personalized to your reported position. Ground stations broadcast traffic data within a service volume roughly 15 nautical miles in radius and 3,500 feet vertically above and below your reported position. If the network doesn’t know where you are, you don’t receive TIS-B. The system cannot serve a position it doesn’t know about.

The obvious cases are mountainous approaches - the Rockies, Cascades, and Appalachians at lower altitudes. But the less obvious cases are flying the pattern at a non-towered airport in rolling terrain, descending on a visual approach into a valley airport, or flying low over coastal hills. In all of these situations, your traffic picture may be incomplete with no indication on screen that anything is missing. The display doesn’t warn you. It simply shows what it knows.

The FAA publishes ADS-B coverage maps showing expected coverage at various altitude bands across the country. They are publicly available, updated periodically, and worth consulting before flying into unfamiliar terrain.

Why Can’t a UAT Receiver Hear All the Traffic Around Me?

The ADS-B architecture runs on two separate radio frequencies, and this adds another layer to what your display actually shows.

1090 MHz Extended Squitter is the international standard - the same frequency band as Mode S transponders, used by every airliner operating ADS-B worldwide. 978 MHz Universal Access Transceiver (UAT) is a US-only frequency designated for general aviation aircraft operating below Flight Level 180.

UAT was created to give GA a cheaper path to ADS-B compliance, and it succeeded on price. Hundreds of thousands of GA aircraft installed UAT systems between roughly 2015 and 2020 to meet the mandate deadline.

The trade-off: a UAT-only receiver cannot directly hear 1090 MHz transmitters. The regional jet climbing through your altitude is transmitting on 1090 MHz. A UAT-only installation cannot receive its position data directly.

TIS-B bridges this gap when coverage is favorable. The ground stations receive both frequencies simultaneously, process them together, and rebroadcast a combined picture on both bands. But when coverage breaks down - at low altitude, in terrain, away from ground stations - that bridging doesn’t happen. UAT-only pilots in a coverage shadow may be receiving direct traffic from only other UAT-equipped aircraft, with nothing filling in the rest of the picture. In some regions of the country, that is a very small slice of actual traffic.

A dual-band receiver - one that simultaneously listens to both 1090 MHz and 978 MHz - removes this dependency. Direct reception of both traffic types, no reliance on TIS-B as the bridge between frequency worlds. For pilots considering their next avionics upgrade, that is worth factoring into the decision.

The long-term future of the 978 MHz UAT network is also worth considering. Internationally, 1090 Extended Squitter is the standard; UAT has no equivalent outside the United States. As drone traffic integration demands common standards and as the FAA eventually evaluates aging ground infrastructure, the future of the 978 network is genuinely unsettled. If you are making avionics choices for an aircraft you plan to fly for the next 15 to 20 years, that is a relevant data point.

How Do I Read My EFB Traffic Display Correctly?

Filled, solid symbol: Direct ADS-B. That aircraft’s GPS position is coming straight to your receiver, updated approximately every second. The picture is current.

Open or hollow symbol: TIS-B rebroadcast. That aircraft was detected by radar and processed through the ATC chain before reaching your screen. There is latency in that path - potentially up to 30 seconds. The symbol shows where that aircraft was, not necessarily where it is now. Factor that into your geometry reading.

No symbols: Not the same as no traffic. Before concluding the sky is clear, consider whether you are below the coverage floor, whether terrain is blocking your nearest ground station, or whether nearby traffic is operating on a frequency your receiver can’t hear directly and TIS-B isn’t available to compensate.

In any environment below approximately 3,500 feet AGL - particularly in terrain, on approaches to valley airports, and in the pattern at hilly non-towered fields - your traffic display is supplemental information layered on top of your eyes. The visual scan was the primary collision avoidance mode before ADS-B. In the coverage gaps, it still is.

Key Takeaways

• Filled symbols are direct ADS-B - near-real-time, GPS-derived, updated approximately every second. Open or hollow symbols are TIS-B - radar-derived and potentially 12 to 30 seconds old.
• At a combined head-on closure rate of 220 knots, 20 seconds of TIS-B latency represents more than a mile and a quarter of additional closure distance.
• The FAA guarantees ADS-B coverage only at 3,500 feet AGL and above. Below that floor - especially in mountainous or hilly terrain - your traffic picture and ATC’s picture of you may both be incomplete.
• UAT-only receivers (978 MHz) cannot directly hear 1090 MHz traffic, including most commercial aircraft. TIS-B bridges this gap only when coverage exists; a dual-band receiver removes the dependency entirely.
• The display gives no indication when it is showing an incomplete picture. Understanding what the system cannot show you is part of using it correctly.