TIS-B, the Ghost Traffic on Your ADS-B Display, and Why That Yellow Diamond Isn't Always Where It Says It Is

TIS-B (Traffic Information Service Broadcast) is a ground uplink service that packages radar returns from non-ADS-B aircraft and broadcasts them to ADS-B In receivers in the vicinity. It looks nearly identical to a direct ADS-B target on most displays, but the position accuracy and latency are fundamentally different - and that distinction matters every time you scan for traffic that isn’t where your screen says it should be.

What ADS-B In Actually Shows You

ADS-B Out means your aircraft continuously broadcasts its GPS position, altitude, speed, and identification to any receiver in range. The FAA mandated ADS-B Out for most controlled airspace by January 1, 2020, and equipage rates have continued climbing since.

ADS-B In is the receiving side. When a nearby aircraft is transmitting ADS-B Out and within radio range, your receiver picks up that broadcast directly. Position updates arrive at roughly once per second. Positional error under normal conditions is measured in meters. That direct ADS-B target shows you approximately where that aircraft actually is, in near real time.

That is the gold standard. TIS-B is what fills the gap when the gold standard isn’t available.

Why TIS-B Exists: The Non-ADS-B Traffic Problem

Years after the mandate, not every aircraft in U.S. airspace has ADS-B Out. Gliders, some ultralights, certain agricultural aircraft with FAA exemptions, and military traffic operating under different rules all move through the same airspace without broadcasting. Radar has always been able to track these aircraft. Your ADS-B receiver cannot.

Without a solution, ADS-B In receivers would present a selective picture - some traffic, not all traffic - with no clear indication of what was missing. A display that looks complete but isn’t is arguably worse than no display at all.

The FAA’s answer was TIS-B.

How TIS-B Works: Radar Data Uplinked to Your Cockpit

The FAA operates a network of approximately 700 ADS-B ground stations distributed across the country. These stations don’t just receive ADS-B Out transmissions - they also tap into the existing radar infrastructure: Airport Surveillance Radars, Air Route Surveillance Radars, and the systems ATC has depended on for decades.

When radar tracks an aircraft without ADS-B Out, the ground network packages that radar return and uplinks it to ADS-B In equipped aircraft in the vicinity. Your receiver gets the data. Your display shows a traffic symbol - a diamond on most systems - that looks nearly identical to any other traffic target.

Position Accuracy and Latency: The Critical Difference

The position accuracy of a TIS-B target is determined by the radar that tracked it, not GPS.

Radar calculates position by timing the return of a sweep and triangulating range and bearing. It’s extraordinarily effective for ATC purposes. But positional error on a radar-derived track is typically one-tenth to several tenths of a nautical mile. Direct ADS-B Out targets have positional error measured in meters. That’s sometimes two orders of magnitude difference in precision.

Then add latency. Airport Surveillance Radars sweep at roughly 4–5 seconds per rotation. En route Air Route Surveillance Radars take 10–12 seconds per sweep. That radar data can be 5–12 seconds old before the ground station even begins packaging it for uplink. Add processing time and propagation delay, and a TIS-B target on your display may represent where an aircraft was 15 seconds ago, not where it is now.

At 120 knots of closure between two aircraft, 15 seconds of lag is roughly half a mile of uncertainty - before accounting for radar positional error. At higher closure rates, that math gets less comfortable. If the aircraft has been turning in the last 12 seconds, both the symbol’s heading and position can be significantly off from the aircraft’s actual location.

This is not a system failure. TIS-B was designed to provide situational awareness, not separation precision. Understanding that design intent is what makes the display useful rather than misleading.

Why Your TIS-B Coverage Depends on Flight Following

TIS-B doesn’t broadcast equally to all aircraft in an area. The FAA uses a service volume approach built around your correlated position in the system.

When you’re squawking an assigned transponder code and an ATC facility has your radar track correlated with your transponder return, the ground network knows where you are. It then calculates a service volume cylinder centered on your position - typically 15 nautical miles in radius and approximately 3,500 feet above and below - and uplinks traffic within that volume specifically to you.

If ATC cannot correlate your position - because you’re squawking 1200 in Class E airspace with no flight following and no radar facility has you confirmed - the system may not know where to aim the uplink. Coverage can be degraded or absent entirely. You may see fewer targets not because there’s less traffic, but because the system has no anchor point to build your service volume around.

Flight following improves your TIS-B service volume. The benefits stack: ATC radar advisories, better airspace coordination, and a more complete traffic picture on your display.

The 978 MHz vs. 1090 MHz Frequency Gap

ADS-B in the United States operates on two frequencies that don’t directly communicate with each other.

978 MHz (Universal Access Transceiver, or UAT) is used below 18,000 feet by most general aviation aircraft. UAT carries both TIS-B traffic uplinks and FIS-B weather data.

1090 MHz Extended Squitter (1090 ES) is required above 18,000 feet and is the international standard used by airline and business jet traffic.

If you’re a 978 UAT receiver, you cannot directly receive 1090 ES transmissions from airline traffic. You depend on TIS-B to show you that Airbus overhead - and whether TIS-B includes it depends on whether that aircraft has been properly correlated in the ground network’s uplink structure.

The asymmetry runs the other direction too. That airline above you can’t hear your UAT broadcast directly either. Their TCAS (Traffic Collision Avoidance System) finds you by interrogating your Mode C or Mode S transponder - a completely different surveillance method. The traffic pictures each aircraft builds are constructed differently and carry different characteristics. Don’t assume the other aircraft sees what you see.

Reading Your Display: TIS-B vs. Direct ADS-B Targets

Quality avionics systems provide visual cues to distinguish target types. On Garmin displays, a filled or solid traffic symbol typically indicates a direct ADS-B target; an outline or hollow symbol often indicates a TIS-B rebroadcast. Color variations are also used. The specific convention varies by manufacturer and even between software versions on the same hardware.

Check your avionics manual. Knowing you’re looking at a TIS-B target versus a direct ADS-B target should change how much precision you attribute to the position shown.

TIS-B duplicate targets are also worth knowing about. A non-ADS-B aircraft tracked by radar may simultaneously get interrogated by TCAS from a nearby airliner, feeding a separate track into the uplink chain. In these cases, two symbols can appear on your display representing one physical aircraft. Good avionics software filters these duplicates, but imperfect filtering occasionally lets one through. Two targets flying in extremely close formation that don’t logically belong together are a candidate for a duplicate rather than actual close traffic.

Where TIS-B Coverage Breaks Down

TIS-B requires two things working simultaneously: ADS-B ground station coverage and radar coverage. In mountainous terrain, radar shadowing is real - ridgelines block radar at low altitudes. In remote areas, ground station spacing is wider. Over ocean, neither exists usefully.

Below roughly 3,000 feet AGL in many parts of the country, you may be outside reliable TIS-B service entirely. The FAA has been transparent about this in its published ADS-B performance reports: the system works very well in the airspace it was optimized for - controlled airspace at altitude - and less reliably at low altitudes in complex terrain.

The environments where terrain makes see-and-avoid hardest are often the same environments where TIS-B coverage is spottiest.

The Trend Worth Watching

ADS-B Out equipage continues rising. The FAA’s own statistics show the large majority of IFR-active aircraft are now equipped, and VFR equipage has climbed year over year since the 2020 mandate. Around Class Bravo airports today, a substantial majority of what your display shows is likely direct ADS-B, not radar-derived TIS-B. The gap TIS-B was designed to bridge is getting narrower.

In lighter traffic areas, at lower altitudes, and around older aircraft operating under exemptions, TIS-B remains genuinely valuable. It’s the layer that catches what ADS-B hasn’t reached yet.

Key Takeaways

TIS-B is radar-derived, not GPS-derived. Positional error is typically one-tenth to several tenths of a nautical mile - sometimes two orders of magnitude less precise than a direct ADS-B target.
TIS-B targets can be up to 15 seconds old by the time they appear on your display. At 120 knots of closure, that’s roughly half a mile of uncertainty before accounting for radar positional error.
Flight following improves your TIS-B service volume. Without a correlated position in the ATC system, the ground network may not know where to aim your traffic uplink.
Know your symbology. Your avionics manual explains how your specific system distinguishes TIS-B targets from direct ADS-B targets - that distinction should inform how much confidence you place in the precise position shown.
ADS-B In is a situational awareness tool, not a separation tool. The display tells you where to look. It does not replace the requirement to look.