TIS-B, the Radar-Derived Traffic Uplink Behind Your ADS-B In Display, and the Coverage Hockey Puck Most Pilots Don't Know Exists

Your ADS-B In traffic display combines two fundamentally different data sources - direct broadcasts from equipped aircraft and radar-derived uplinks for unequipped traffic - and the difference between them shapes what you see and what you miss. TIS-B (Traffic Information Service-Broadcast) is the radar uplink layer behind most of the unequipped traffic on your screen, and its coverage has a hard geographic boundary that most pilots have never been explicitly told about.

How Does ADS-B In Traffic Actually Work?

In its purest form, ADS-B is a direct peer-to-peer system. An equipped aircraft takes a GPS-derived position fix, combines it with pressure altitude, groundspeed, heading, and tail number, then broadcasts that package every second over a radio frequency. Any ADS-B In receiver within radio line-of-sight receives that broadcast directly - no radar, no controller, no meaningful latency. Position accuracy is in meters. Track vectors are built from one-second updates.

The problem the FAA faced during the national ADS-B rollout was fleet transition time. If the traffic picture only showed ADS-B Out-equipped aircraft, it would have been nearly useless during the transition years - half the targets in the sky would have been invisible.

TIS-B was built to bridge that gap.

What Is TIS-B and Where Does the Data Come From?

TIS-B uses the FAA’s network of Ground Based Transceivers (GBTs) - roughly 700 stations placed across the continental United States, Alaska, Hawaii, Puerto Rico, and the Gulf of Mexico. When a GBT receives your ADS-B Out transmission, it correlates your position with the radar picture from nearby Air Route Surveillance Radars and Terminal Radar Approach Control (TRACON) facilities.

Any radar target in your vicinity that is not itself transmitting ADS-B Out gets extracted from the radar feed, packaged with position, altitude, groundspeed, and track data, and uplinked to you through the GBT. On your display, it appears alongside direct ADS-B contacts - same symbol, same behavior, one apparent picture. The underlying data quality, however, is not the same.

What Is the ADS-B “Hockey Puck” Service Volume?

TIS-B is not a sector-wide broadcast. The GBT doesn’t push a regional traffic picture to every aircraft on frequency. The service is personalized to your specific position. Your ADS-B Out transmission is the trigger, and the coverage area for your uplink extends approximately 15 nautical miles laterally and 3,500 feet vertically from your aircraft.

The FAA calls this the ADS-B service volume. Pilots call it the hockey puck. You are the center of that puck, and you carry it with you everywhere you fly.

Traffic inside the puck that lacks ADS-B Out gets uplinked to you via TIS-B. Traffic outside the puck that lacks ADS-B Out does not appear on your display at all. Traffic that has ADS-B Out reaches you directly through the air regardless of the puck’s boundaries, because you’re receiving its own one-second broadcast.

Consider the operational implication: you’re flying at 5,000 feet, inbound to a Class Charlie airport. 15 nautical miles behind you, a Cessna 172 with no ADS-B Out is climbing out of a Class Delta airport at the edge of your arrival corridor. It’s in radar coverage. But it’s outside your service volume. It doesn’t show on your display. The moment it enters the puck, it appears - sometimes with a position jump, sometimes with a slight track lag. Before that, nothing.

The hockey puck was a deliberate engineering choice to manage data bandwidth on the broadcast frequencies. Uplinking every radar target in the sector to every aircraft simultaneously would have overwhelmed the available spectrum. That was a practical solution, but it has operational consequences pilots need to understand.

How Does the Two-Frequency ADS-B Architecture Affect Your Traffic Picture?

The FAA adopted two separate frequencies for ADS-B. UAT (Universal Access Transceiver) operates at 978 MHz, designed primarily for general aviation below 18,000 feet. 1090 ES (1090 Extended Squitter) is the international standard used by airliners, business jets, and most foreign-registered aircraft.

A UAT receiver cannot directly hear a 1090 ES broadcast. The frequencies don’t overlap. A pilot flying with a UAT-only box - which covers a large fraction of GA pilots who installed portable ADS-B solutions or lower-cost panel units - has no direct radio link to commercial traffic operating on 1090 ES.

The GBT network bridges this gap with a third service: ADS-B Rebroadcast (ADS-R). When a GBT receives a UAT broadcast from a GA aircraft, it rebroadcasts that traffic on 1090 ES for commercial traffic listening on that frequency. The reverse works too - 1090 ES traffic from commercial aircraft gets rebroadcast on UAT for GA receivers. On your display, rebroadcast targets look like any other traffic. But they passed through a ground station, adding latency and ground station coverage as dependencies to what would otherwise be a direct surveillance relationship.

When you’re within ground station coverage, in radar coverage, and within your service volume, ADS-R and TIS-B together produce a remarkably complete picture. The critical phrase is “when you are within.”

How Much Latency Is in TIS-B Traffic Data?

Direct ADS-B Out targets update once per second. Track vectors are built from recent, accurate data. At 150 knots closing speed, the display’s position projection is very close to where the aircraft actually is.

TIS-B targets derived from radar carry the latency of the radar sweep. Primary surveillance radars at terminal facilities typically rotate once every 4.8 seconds. Long-range en route radars can take up to 12 seconds per rotation. The position data for a non-ADS-B aircraft in your TIS-B uplink is potentially up to 12 seconds old before it even gets packaged and transmitted to you. Add uplink timing and receiver processing, and the effective age of that data can exceed 15 seconds.

At 150 knots, 15 seconds represents over half a nautical mile of movement not yet reflected on your display.

Displays handle this by extrapolating a track forward from the last known position, using the last reported heading and groundspeed from the radar return. In straight and level flight at constant speed, that extrapolation is reasonable. An aircraft that has started a turn, begun a climb, or initiated a descent will look very different from where the display projects it.

FAA Advisory Circular 90-114 addresses this directly. TIS-B is described as a situational awareness tool - not a traffic separation system, not a collision avoidance guarantee. That language was chosen deliberately.

What Aircraft Won’t Show Up on Your ADS-B In Display?

Several categories of traffic are entirely absent from both TIS-B and direct ADS-B.

Untranspondered aircraft - Part 103 ultralights have no transponder requirement under FAA regulations. Powered parachutes, weight-shift trikes, many amateur-built aircraft flying in Class G airspace, and foot-launched hang gliders are transparent to radar and the ADS-B architecture. This category is not trivial in numbers, particularly near recreational flying areas, ridge soaring sites, and rural non-towered airports.

Military aircraft on Mode 4 IFF - Aircraft operating on the Identification Friend or Foe system do not appear on civilian radar displays and do not appear in TIS-B uplinks. Military training routes and operating areas exist specifically because the aircraft there fly faster and less predictably than the civilian ADS-B picture can accommodate.

Foreign aircraft transiting U.S. airspace on 1090 ES without an ADS-B Out upgrade will appear in your TIS-B uplink only if the ground station successfully correlates the radar return and their position falls within your service volume.

What Does ADS-B Coverage Actually Look Like Today?

FAA performance monitoring data shows post-mandate equipage in controlled airspace well above 80% and climbing. In the terminal environment above 3,000 feet, the direct ADS-B picture is genuinely good. The surveillance picture available to an IFR pilot at 8,000 feet today is dramatically better than what that same pilot had in 2015.

The gap in coverage is concentrated at low altitude in uncontrolled airspace. FAA ADS-B performance maps show robust coverage at 3,000 feet AGL for most of the continental U.S. At 1,000 feet AGL, gaps exist - particularly in the mountain west, parts of Alaska, and areas distant from FAA radar facilities. The low-and-slow environment - student pilots doing pattern work, sport pilots at grass strips, agricultural operators - is where TIS-B coverage is thinnest and unequipped traffic is most prevalent.

How Should Pilots Actually Use ADS-B In Traffic Displays?

Know your receiver. A UAT-only device has no direct ADS-B link to 1090 ES commercial traffic - that coverage depends entirely on ADS-R rebroadcast through the GBT network. A dual-band receiver provides the most complete picture the current architecture offers, still subject to the hockey puck and ground station coverage requirements, but with the best available inputs.

Learn your avionics’ target symbology. Garmin units typically display direct ADS-B targets differently from TIS-B uplinked targets. The pilot guides for the GDL 90, GTX 335, and newer integrated units describe this symbology in detail. ForeFlight and Garmin Pilot differentiate target types in their traffic overlays. The information is available; most pilots don’t look it up until after a close call.

Use the display as the supplement it was designed to be. Approach’s radar is the authoritative picture. Your own eyes are the last line. ADS-B In has genuinely improved situational awareness for the GA fleet, but a tool you understand is more useful than a tool you trust uncritically.

The full ADS-B architecture - where every aircraft broadcasts its own GPS position - will be significantly more capable than the transition-era system TIS-B was engineered to support. The hockey puck will shrink in importance as fleet equipage completes. That transition is not finished. Knowing the seams in the picture is not pessimism. It is how you use the tool correctly.

Key Takeaways

• TIS-B uplinks radar-derived traffic for non-ADS-B aircraft, but only within a ~15 nautical mile / 3,500-foot “hockey puck” service volume centered on your aircraft - traffic outside that boundary is invisible to your display.
• Radar-derived TIS-B targets can carry up to 15 seconds of position lag, representing over half a nautical mile of untracked movement at 150 knots.
• UAT-only receivers have no direct link to commercial 1090 ES traffic; that coverage depends on ADS-R rebroadcast through GBT ground stations.
• Untranspondered aircraft, military IFF traffic, and low-altitude uncontrolled airspace represent real gaps that no current display technology can fill.
• Advisory Circular 90-114 classifies ADS-B In as a situational awareness tool - not a separation service. ATC radar and your own eyes remain the authoritative references.