TIS-B, the Fifteen-Nautical-Mile Traffic Bubble, and the ADS-B Traffic Picture Most Pilots Assume Is Complete
Your ADS-B traffic display is not showing you everything - here's exactly how TIS-B works, where its coverage fails, and what your screen cannot show.
The traffic picture on your panel or tablet is not complete. Most pilots assume it is, and that gap between assumption and reality has real safety consequences. Understanding exactly how TIS-B works - and where it stops working - changes how you use your avionics and where you point your eyes.
How ADS-B Out Creates the Foundation
When the FAA designed the ADS-B network, they were solving two separate problems. The first was position broadcasting: get every aircraft transmitting its own GPS-derived position, altitude, velocity, and call sign continuously to every nearby aircraft. That is ADS-B Out.
Every aircraft with an ADS-B Out transponder broadcasts its position once per second, air to air, with no ground infrastructure involved. The resulting data on your display is less than two seconds old. That target moves in real time - as current as any instrument you have.
That is the elegant part. The harder problem was everything else: aircraft without ADS-B Out, targets beyond your direct radio range, traffic on the other side of terrain you cannot see over.
What TIS-B Actually Does
The FAA’s answer was the Traffic Information Service Broadcast - TIS-B. The FAA built a network of approximately 700 ground stations across the continental United States. Those stations receive ADS-B Out broadcasts from overhead aircraft and simultaneously pull in data from the existing secondary surveillance radar infrastructure - en route radars, terminal area radars, and all facility types covering the national airspace.
All of that data gets combined into a composite traffic picture at the ground level. Based on your own ADS-B Out broadcast, the nearest ground station identifies your position and generates a tailored traffic package specifically for your aircraft - then broadcasts it back up to you.
The Fifteen-Nautical-Mile Bubble: What It Covers
That package covers a defined volume of airspace centered on your position: roughly 15 nautical miles in every horizontal direction and roughly 3,500 feet above and below your altitude. That is the TIS-B service volume - the bubble.
Inside it, the system attempts to give you a composite picture of all detected traffic. The keyword is “attempts.” Not all traffic inside that bubble is equal.
Why TIS-B Targets Are Not the Same as Direct ADS-B Contacts
Direct ADS-B targets arrive with very low latency. The position is precise to GPS standards, and the data is fresh. TIS-B targets are fundamentally different - those are radar-painted aircraft being relayed through the ground network.
Radar has a scan rate. Depending on which type of radar covers a given area, update rates range from about 4 seconds on a fast terminal radar to 12 seconds or more on en route radar. That data then travels through the ground network to the broadcast station, then back up to you. By the time a TIS-B target’s position arrives on your display, you could be looking at information that is 10 to 15 seconds old.
At a combined closing speed of 250 knots between two aircraft, 15 seconds represents more than a mile of closure that has already occurred since that position was recorded. Your traffic display may simultaneously show two fundamentally different qualities of information, displayed with essentially the same icon, without an obvious visual distinction telling you which is which.
How to Tell a TIS-B Target from a Direct ADS-B Contact
Some avionics suites do differentiate. Garmin displays use different symbology - a direct ADS-B contact gets a solid symbol, while a TIS-B target gets a different shape or fill indicating it came from the ground relay. ForeFlight and other EFB applications do the same.
Most pilots look at the display without knowing what that distinction means because they never read that section of the manual. This is less a pilot problem than a systems design communication problem: the display presents a unified traffic picture, and the engineering behind the different quality levels is not communicated clearly at the interface level.
There is also a behavioral cue to watch for. A direct ADS-B target moves smoothly in real time. A TIS-B target steps - it appears to freeze, then jumps to a new position. That distinction matters when you are evaluating whether a target is a factor and projecting where it is going.
Where TIS-B Has No Coverage At All
Beyond data latency, entire categories of traffic fall completely outside the TIS-B picture.
Terrain-masked radar coverage is the biggest gap. TIS-B depends on radar seeing its targets, and ground-based radar is line of sight. Mountains block it. Valleys hide from it. Aircraft flying at low altitude in mountainous terrain - threading a pass, following a river canyon - may be operating entirely below the radar horizon. Those aircraft are invisible to the radar network, and therefore invisible to TIS-B.
The FAA acknowledges this openly. The system was designed for airways and instrument operations at cruise altitudes, where radar coverage is solid. Low-altitude VFR, mountain flying, and operations near significant terrain are environments where TIS-B coverage has real gaps.
Pattern work at non-towered airports is the gap that hits closest to home for most GA pilots. At low altitude, potentially near terrain, outside any meaningful radar coverage - this is exactly the environment where TIS-B is least effective. The traffic you care most about in the pattern, the other aircraft working the same pattern at your home field, may not be appearing on TIS-B at all. What you receive there is essentially direct ADS-B contacts only. Any aircraft flying without a transponder, with older equipment lacking ADS-B Out, or with malfunctioning avionics is invisible - not flagged as a gap, just absent from the display.
The Frequency Split Most Pilots Don’t Know About
ADS-B Out operates on one of two frequencies. Aircraft below 18,000 feet have the option to use 978 MHz, called the Universal Access Transceiver or UAT. Aircraft operating above 18,000 feet or internationally use 1090 MHz extended squitter - 1090 ES.
Here is the critical point: TIS-B and FIS-B weather are only broadcast on 978 MHz UAT. They are not broadcast on 1090 ES.
If you installed a 1090 ES transponder to meet the mandate and your ADS-B In receiver only listens on 1090, you are receiving direct air-to-air traffic from other 1090 ES aircraft - but you are not receiving TIS-B, and you are not receiving FIS-B datalinked weather. You have the air-to-air layer of the system only, not the ground relay layer.
The solution is a dual-frequency receiver that listens on both 978 and 1090 simultaneously. The Garmin GDL 50, GDL 52, Stratus 3, and ForeFlight Sentry Plus all receive both frequencies and deliver the complete system. If your panel has a certified ADS-B In receiver that is 1090-only, supplementing it with a portable dual-frequency unit gives you the TIS-B and weather layers on top.
You Have to Be Seen to Be Served
There is one more condition the TIS-B bubble requires, and it surprises pilots when they encounter it. The service volume is generated for you because a ground station knows you are there - it knows because it received your ADS-B Out broadcast. If you are flying through a ground station coverage gap where no station can hear your Out signal, no station generates your personalized TIS-B package. The bubble does not form. You receive direct air-to-air contacts only.
The completeness of your traffic picture depends on the completeness of your ADS-B Out coverage, not just your In coverage.
Where TIS-B Works Well
The system performs as designed in the airspace it was built for. Flying in Class B or Class C airspace at cruise altitude, in solid radar coverage, on a properly equipped aircraft - the traffic picture is nearly complete. Airlines, business aviation, and IFR GA aircraft are all represented. Compliance rates in controlled airspace since the January 2020 ADS-B Out mandate have been very high, and traffic conflict statistics in high-density airspace have improved substantially as a result.
The gaps are at the margins: low altitude, complex terrain, non-controlled airports, and unequipped aircraft.
Which Aircraft Can Legally Fly Without ADS-B Out
The mandate covers Class B, Class C, and Class D airspace, and Class E airspace above 10,000 feet MSL. It does not cover the vast majority of uncontrolled low-altitude airspace.
Agricultural operators, recreational aircraft under certain conditions, gliders, balloons, and vintage aircraft operating with FAA exemptions can all legally fly without ADS-B Out across significant portions of the national airspace. In areas where those aircraft are common, your traffic display has holes that are not visible from the display itself.
What This Means for How You Fly
The value of understanding ADS-B In is not just knowing what it shows - it is knowing what it cannot show. That knowledge changes where you point your eyes.
At cruise in controlled airspace with solid radar coverage, your picture is about as complete as it gets. In the pattern at an uncontrolled airport, especially near terrain, treat the traffic display as supplemental and scan outside. The see-and-avoid requirement under FAR Part 91 does not get suspended because you have a traffic display.
Know whether your receiver is 978-capable. Know what your display’s symbology means for different target types. Read the section in your avionics or EFB manual that explains traffic target differentiation. This is not background information. It is understanding your instrument.
Key Takeaways
- TIS-B targets can be 10–15 seconds old. At a 250-knot combined closing speed, that represents more than a mile of untracked closure before the data reaches your display.
- TIS-B and FIS-B only broadcast on 978 MHz UAT, not 1090 ES. A 1090-only receiver does not receive either service, regardless of what your panel shows.
- The TIS-B bubble only forms if a ground station can hear your ADS-B Out signal. Flying through a coverage gap eliminates the service entirely.
- Pattern work at non-towered airports is among the weakest TIS-B environments - low altitude, possible terrain masking, low radar coverage, and legally unequipped traffic combine to create a picture with significant holes.
- The traffic display shows you where to look. Your eyes find the aircraft. A clean display is not clearance to stop scanning.
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