What does a hollow symbol on my ADS-B traffic display mean?

A hollow or outline symbol indicates a TIS-B or ADS-R contact - traffic provided through the FAA relay chain rather than a direct air-to-air ADS-B transmission. These targets carry more latency and potentially less position accuracy than solid symbols, which represent direct ADS-B Out contacts updated approximately every second.

How old is the position data on a TIS-B target?

TIS-B targets can be 10 to 20 seconds old by the time they appear on your display, due to radar sweep intervals of 4 to 12 seconds, FAA network processing time, and broadcast timing. At 220 knots, that represents approximately 3 nautical miles of potential position difference in a converging scenario.

Why don't gliders and ultralights show up on my traffic display?

TIS-B relies on secondary surveillance radar, which interrogates transponders. Aircraft with no operating transponder - including many gliders, ultralights, balloons, and some experimentals - produce no radar return and therefore no TIS-B target. They are structurally invisible to the system regardless of how good your coverage is.

What is ADS-R and how is it different from TIS-B?

ADS-R (ADS-B Rebroadcast) bridges the two ADS-B frequencies used in the U.S.: 978 MHz UAT (used mostly by GA aircraft below 10,000 feet) and 1090 MHz ES (used by airline and most international traffic). Ground stations receive both and rebroadcast each to the other, so UAT-equipped aircraft can see 1090 ES traffic and vice versa. It uses the same infrastructure as TIS-B and carries similar latency and coverage characteristics.

Does TIS-B work reliably at low altitude?

TIS-B coverage degrades significantly at low altitudes, particularly in mountainous or hilly terrain. Both ADS-B ground station reception and radar coverage are line-of-sight systems, and terrain can block either or both. In mountain valleys, on backcountry approaches, or near ridge lines, TIS-B may be incomplete or entirely absent - often in exactly the environments where traffic awareness matters most.

The Hollow Diamond: What TIS-B Is, How It Works, and Why Your Traffic Display Has Blind Spots

Your ADS-B traffic display shows more than just ADS-B Out aircraft - but less than the full picture. TIS-B (Traffic Information Service-Broadcast) uses FAA radar data to populate your traffic display with non-ADS-B aircraft, yet it carries inherent latency, coverage limitations, and structural gaps that every pilot using an ADS-B In receiver needs to understand.

What Is TIS-B and Why Does It Exist?

The ADS-B Out mandate that took effect January 1, 2020 required equipage in Class A, Class B, Class C airspace, the shelves above Class B and C, and above 10,000 feet MSL outside Class A. That covered the high-density, high-altitude routes where collision risk with airline traffic is highest.

What it did not do was put ADS-B Out on every aircraft in the sky. Six years after the mandate, large numbers of aircraft operate legally in uncontrolled airspace without ADS-B Out: agricultural aircraft, gliders and sailplanes, balloons, ultralights under Part 103, powered parachutes, weight-shift light sport, older trainers, vintage aircraft, and experimentals with minimal avionics.

None of those aircraft appear on your traffic display through direct air-to-air ADS-B contact.

TIS-B is the FAA’s architectural answer to that gap. FAA ground stations are connected to the en route radar network and TRACON (Terminal Radar Approach Control) radar systems. When a ground station knows an ADS-B-equipped aircraft is operating in its coverage area, it takes radar return data on nearby non-ADS-B aircraft - aircraft secondary surveillance radar can see because they have transponders - and broadcasts that radar data back down to the ADS-B In receiver. Your display populates with traffic that secondary radar can paint, not only traffic broadcasting ADS-B Out.

How Direct ADS-B Contact Differs From TIS-B

When two aircraft both have ADS-B Out, they communicate directly, air-to-air, with no ground infrastructure involved. Position updates arrive approximately once per second, GPS-derived, with full heading, altitude, and speed data. This is the contact that appears as a solid symbol on your avionics display.

TIS-B targets travel a relay chain: radar interrogation → position processing → FAA network transmission → ground station packaging → ADS-B downlink frequency → your receiver. Every step in that chain adds delay.

The Latency Problem: How Stale Is That Target?

Most TRACON radars complete one sweep every 4 to 6 seconds. Older en route radars covering large swaths of high-altitude airspace sweep every 12 seconds. Each position fix then travels through the FAA data network, gets packaged into a TIS-B broadcast, and transmits to your receiver.

By the time a TIS-B target appears on your display, you may be looking at data that is 10 to 20 seconds old.

At 220 knots - a reasonable cruise speed for a fast single or light twin - an aircraft covers roughly 3 nautical miles in 20 seconds. If that aircraft is converging on your position, your display may show it at your 10 o’clock, 2 miles, when it has already moved to your 11 o’clock, 1.5 miles. The geometry is worse than the symbol suggests.

A direct ADS-B Out contact updates every second. The difference between a 1-second update rate and a 20-second update rate, when two aircraft are converging, is not a footnote. It is a meaningful operational gap.

Why TIS-B Target Accuracy Varies

Transponders come in two types: Mode C and Mode S. Mode C transponders encode altitude but provide limited position information - the radar derives position geometrically from the angle and range of the interrogation return, which is less precise than GPS. Mode S transponders are more capable and allow richer data exchange.

The quality of a TIS-B target depends on which transponder type the aircraft is using and how well the radar track has been resolved. Your display does not always distinguish between a TIS-B target with solid position data and one based only on a rough estimate.

When a heading cannot be determined from the radar track, the target appears as a non-directional symbol - a blob at an altitude with no indication of which way it is moving. Combined with latency, you have a target whose position is somewhat stale and whose direction is unknown. That aircraft might be flying parallel to you, or crossing your path at a 90-degree angle. The symbol does not tell you.

Where TIS-B Coverage Breaks Down

TIS-B requires two independent coverage layers to function. When either degrades, your traffic picture degrades with it.

Layer 1: ADS-B ground station coverage. These are line-of-sight radio systems. Terrain blocks them. At 2,000 feet AGL over flat midwestern farmland, coverage is typically solid. At 2,000 feet AGL in the Rockies or Appalachians, you may have coverage on one side of a ridge and nothing on the other.

Layer 2: Radar coverage. Even where ADS-B ground station coverage is good, mountains and terrain block radar beams. The radar coverage floor at many locations sits higher than the ADS-B coverage floor. Your ADS-B Out may be reaching the ground station, but the TIS-B data being sent back is incomplete because radar isn’t painting all targets in your vicinity.

Some avionics installations display a TIS-B service status indicator. A degraded or “not in service” message means the relay chain is incomplete. Most pilots have never noticed this indication - it doesn’t alert audibly. It quietly tells you the picture has a hole in it.

What TIS-B Cannot Show Regardless of Coverage

TIS-B is built on secondary surveillance radar, which interrogates transponders. No transponder, no return. No return, no TIS-B target. That aircraft is invisible to your display no matter how good your coverage is.

Aircraft with no operating transponder are structurally absent from your traffic picture:

Ultralights operating under Part 103
Gliders and sailplanes, particularly older fiberglass ships with no electrical system
Balloons required to squawk only when operating in specific airspace
Some experimental aircraft with period-correct, pre-transponder avionics
Paragliders and hang gliders that occasionally thermal up to altitudes shared with general aviation

The geographic areas where these aircraft are most common - soaring fields, ultralight parks, gliderports, backcountry strips - are frequently the same areas with the worst TIS-B coverage.

Understanding ADS-R: The Frequency Bridge

A related system, ADS-B Rebroadcast (ADS-R), addresses a separate architectural gap that affects what your display shows even for equipped aircraft.

ADS-B in the United States operates on two separate frequencies. 978 MHz (UAT - Universal Access Transceiver) is used primarily by general aviation aircraft below 10,000 feet. 1090 MHz Extended Squitter (1090 ES) is used by airline aircraft and by most of the world outside the United States. UAT and 1090 ES equipment cannot hear each other directly - the radios operate on different frequencies.

ADS-R bridges that gap using the same ground station infrastructure. Ground stations receive transmissions on both frequencies and rebroadcast targets from one onto the other. If you’re flying a Cessna 172 with a UAT-based ADS-B installation at 5,000 feet, the regional jets on final approach using 1090 ES appear on your display through ADS-R - not direct air-to-air contact. The latency and coverage characteristics are similar to TIS-B.

How to Read Your Traffic Display Intelligently

On any given flight, your traffic display is showing a mix of contact types:

Solid symbols - direct ADS-B Out contacts, updated every second, GPS-precise, full heading and altitude data
Hollow or outline symbols - TIS-B targets, radar-derived, potentially 15 to 20 seconds stale, variable position accuracy, possibly no heading
ADS-R rebroadcasts - bridging the 978 MHz and 1090 MHz frequency gap, similar latency and coverage characteristics to TIS-B
No symbol at all - the percentage of actual traffic around you that is not on the display

The exact symbol shapes vary by manufacturer and software version, but the underlying principle is consistent: solid means the aircraft is talking directly to your receiver; hollow or modified means the FAA relay chain is involved.

Know your specific avionics. The differentiation between direct ADS-B, TIS-B, and ADS-R contacts is documented in every major manufacturer’s pilot guide - including Garmin’s published guides for their GDL-series receivers. If you don’t know what each symbol type on your display means, that’s the first thing to look up.

Practical Habits That Change How You Fly

The traffic display is a genuinely useful layer of situational awareness. Flying cross-country with ADS-B In today is a fundamentally different experience than flying the same route before the system existed. That matters.

The risk is not the system. The risk is the false confidence floor: glancing at a quiet traffic display and concluding the sky is clear, when what the display is actually confirming is that no transponder-equipped aircraft with functional TIS-B relay coverage is currently painting on your system. Those are not the same statement.

Specific habits that close the gap:

Check TIS-B service status on installations that display it. A degraded indication is real operational information, not a nuisance message.
Expect your traffic picture to degrade at low altitude, in mountainous terrain, and in rural airspace with limited radar coverage. A quiet display near the ground is not confirmation of a clear sky.
Know the non-transponder population in your operating environment. Near a gliderport, soaring school, or backcountry strip on a weekend afternoon, gliders, ultralights, and vintage aircraft are likely active and will not appear on your display.
Maintain the visual scan outside the windshield. The traffic display supplements the scan; it does not replace it.

Key Takeaways

TIS-B (Traffic Information Service-Broadcast) relays FAA radar data to ADS-B In receivers, extending the traffic picture beyond ADS-B Out-equipped aircraft - but the relay chain introduces 10 to 20 seconds of latency.
At 220 knots, 20 seconds of latency equals roughly 3 nautical miles of potential position difference in a converging traffic scenario.
TIS-B requires two coverage layers: ADS-B ground station line-of-sight and radar coverage. Both degrade at low altitude and in mountainous or hilly terrain.
Aircraft without operating transponders - ultralights, many gliders, balloons, some experimentals - are structurally invisible to TIS-B regardless of coverage quality.
ADS-R bridges the 978 MHz UAT and 1090 MHz ES frequency gap using the same ground station infrastructure, with similar latency and coverage characteristics.
Solid symbols indicate direct ADS-B Out contact; hollow or modified symbols indicate TIS-B or ADS-R relay contacts with reduced accuracy and higher latency.