ADS-B IN, the TIS-B Rebroadcast Network, and the Traffic Picture Every Cockpit Display Looks Complete but Isn't

ADS-B IN traffic displays give pilots unprecedented situational awareness, but the picture is incomplete by design. The system relies on two incompatible radio frequencies bridged by a ground relay network with geographic limits, activates only when ADS-B Out equipped aircraft are present, and cannot display any aircraft operating without a transponder. Understanding these structural limits is what turns ADS-B IN from a tool you trust completely into one you trust correctly.

Why Does ADS-B Use Two Different Frequencies?

ADS-B stands for Automatic Dependent Surveillance – Broadcast. Each word is precise. “Automatic” means the aircraft broadcasts continuously without being interrogated by a ground station or another aircraft. “Dependent” means the system depends on the aircraft knowing its own GPS position. “Broadcast” means the transmission is omnidirectional - going out to any receiver within radio range.

The FAA authorized two separate frequencies for ADS-B Out - the transmitting side of the system. Above 18,000 feet MSL, all aircraft must use 1090 MHz extended squitter (1090 ES), a protocol built on the existing Mode S transponder standard with GPS-derived position and velocity added. Airlines, turbines, and everything operating at altitude uses 1090 ES without exception.

Below 18,000 feet, the FAA authorized 978 MHz Universal Access Transceiver (UAT) as a lower-cost alternative for general aviation. A UAT installation can be completed for meaningfully less than a 1090 ES upgrade - a real consideration for aging Cessna 172s and Piper Cherokees where avionics budgets are tight.

The engineering consequence is fundamental: a 1090 ES aircraft and a 978 UAT aircraft broadcast on completely different radio frequencies and cannot hear each other directly. A jet on 1090 and a Cessna on UAT sharing the same airspace are invisible to each other’s traffic receivers. Not because the system is malfunctioning - because they’re on different radio channels.

What Is TIS-B and How Does It Bridge the Frequency Gap?

The FAA anticipated this incompatibility and built a solution into the system from the start: TIS-B (Traffic Information Service – Broadcast).

A network of dedicated ground stations - positioned at airports, on towers, and on elevated terrain across the continental United States - listens on both frequencies simultaneously. Each station captures 1090 ES broadcasts from jets, 978 UAT broadcasts from GA aircraft, and position data from traditional radar returns (Mode C and Mode S transponder returns that have fed ATC computers for decades). That data is fused into a unified traffic picture, then rebroadcast back to aircraft on both frequencies.

The result: a UAT-equipped aircraft’s ADS-B IN receiver picks up the ground station rebroadcast, which includes position data for the 1090 ES jet that was invisible at the direct broadcast level. The ground network acts as translator and relay between two incompatible radio systems, performing double duty as both a collection infrastructure and a rebroadcast infrastructure.

Why Does Your Own ADS-B Out Transponder Activate Your Traffic Picture?

TIS-B is not a continuous broadcast to all aircraft. A TIS-B broadcast is generated only when the FAA ground network detects an ADS-B Out equipped aircraft in a given area. That broadcast is focused on a volume of airspace centered roughly on that aircraft - approximately 15 nautical miles in radius and ±3,500 feet in altitude.

The implication is direct: your ADS-B Out transponder is the anchor for your own traffic picture. Your presence triggers the ground station to generate a TIS-B broadcast for your location, which your ADS-B IN receiver then picks up.

Without ADS-B Out - as some pilots installed ADS-B IN receivers without adding the transmitter during the mandate period - you receive no TIS-B service. The ground network doesn’t know you’re there. Your electronic traffic picture is limited to direct ADS-B broadcasts from equipped aircraft within your radio line of sight, and nothing more.

Where Does ADS-B Ground Coverage Have Gaps?

Coverage is solid over most of the continental United States at 3,000 feet AGL and above. The FAA publishes ADS-B coverage maps available through official FAA resources and moving map applications showing signal coverage at that altitude. But genuine gaps exist:

• Mountain West terrain that blocks ground station line of sight
• Parts of Alaska where station density is low
• Some coastal and island areas with limited infrastructure
• Low altitude everywhere - below roughly 1,000 feet AGL, ground station line-of-sight constraints produce larger and more frequent gaps

The practical difference matters. Flying a canyon in the Rockies at 500 feet above the canyon floor, ADS-B IN may be receiving little to no TIS-B data from the ground network. The traffic picture there is fundamentally different from what the same receiver shows on departure from a busy Class Bravo airport surrounded by dense ground station infrastructure.

How Old Is the Traffic Data on My ADS-B IN Display?

TIS-B carries relay latency. Data travels from an aircraft to a ground station, gets processed into the surveillance picture, gets repackaged, and gets transmitted back to your receiver. The design specification allows up to approximately 3 seconds of total delay - actual performance is usually better.

At higher speeds, 3 seconds matters. A turbine aircraft at 450 knots covers roughly 0.7 nautical miles in 3 seconds. In a busy terminal environment, that’s a meaningful position age.

Direct ADS-B reception - aircraft talking to aircraft on the same frequency - carries essentially no relay latency. Position data received directly from another aircraft’s broadcast is as current as their last one-second transmission. Well-designed avionics systems prioritize direct broadcast data over TIS-B relay data when both are available for the same target. The RTCA’s Minimum Operational Performance Standards for ADS-B surveillance applications address exactly this kind of data source prioritization and fusion.

What Is FIS-B and How Is It Different from TIS-B?

FIS-B (Flight Information Service – Broadcast) travels on the same ground network infrastructure as TIS-B but carries a completely separate data channel: Nexrad radar mosaics, METARs, TAFs, PIREPs, TFRs, and NOTAMs. FIS-B is transmitted exclusively on 978 MHz UAT - it is not carried on 1090 ES.

This explains why a 1090-only aircraft with a Sentry or Stratus portable receiver can still display FIS-B weather on a tablet. The portable receiver listens on 978 UAT for FIS-B content that the certified 1090 panel was never designed to carry. Two radios, two separate jobs.

Nexrad weather delivered via FIS-B carries 5 to 15 minutes of latency, depending on composite update timing and when the receiver last captured the broadcast. The familiar guidance applies: never use datalink weather as real-time convective avoidance. TIS-B latency is far shorter, but the underlying principle is identical - both are processed surveillance pictures with relay time between the real world and the cockpit display.

Why Are Some Aircraft Invisible to ADS-B IN Traffic?

Every element of the ADS-B traffic picture - TIS-B, direct ADS-B, all of it - functions only for aircraft carrying transponders.

FAR 91.215 defines where transponders are required: inside Class Bravo airspace, inside Class Charlie airspace, and within the Mode C veil - the 30 nautical mile ring around Class Bravo primary airports where Mode C altitude encoding or ADS-B Out is mandated. Within those boundaries, transponders are required. But in Class Echo below 10,000 feet MSL and in Class Golf airspace, transponders are not universally required.

That covers a substantial portion of the low-altitude flying environment, particularly in the western United States. Aircraft that may be operating legally without any transponder include:

• Ultralights under Part 103
• Gliders at soaring areas
• Experimental amateur-built aircraft without electrical systems
• Vintage aircraft preserved without modern electronics

These aircraft don’t show up as uncertain or unverified on the display. They don’t exist in the picture at all. Near a gliderport, a popular soaring site, a backcountry strip with ultralight traffic, or any environment where unequipped aircraft routinely operate, this is a genuine operational consideration - the display isn’t showing you an incomplete picture, it’s showing you a picture that structurally excludes an entire category of traffic.

What’s Next for ADS-B Technology?

The FAA has continued expanding ground station coverage since the January 2020 ADS-B Out mandate deadline, and low-altitude gap coverage has improved meaningfully since then.

Aireon’s space-based ADS-B system, built on the Iridium NEXT satellite constellation, has extended surveillance-grade ADS-B to oceanic airspace where no ground stations exist. That system serves primarily as an ATC tool rather than a pilot traffic feed, but it demonstrates that the global coverage problem is solvable.

Remote ID for unmanned aircraft became effective in 2023, requiring drones to broadcast identity and position - conceptually related to ADS-B Out, though on a different system and frequency. Integration of Remote ID data into the pilot traffic picture remains an unresolved engineering problem. As drone traffic density increases in low-altitude airspace, the pressure to solve it grows alongside it.

How Should I Use ADS-B IN Traffic in the Cockpit?

ADS-B IN traffic is most reliable for transponder-equipped aircraft in airspace with solid ground station coverage at pattern altitude and above. It is less reliable at very low altitudes, in remote terrain, and in areas with sparse ground station geometry. It provides no information, under any conditions, about aircraft without transponders.

The visual scan is not redundant with ADS-B IN traffic - it’s complementary. The scan catches what the display structurally cannot show. Used together, both tools are more capable than either one alone.

Think of ADS-B IN traffic the same way you think of Nexrad weather on a moving map. Nexrad has genuinely transformed weather awareness for GA pilots - no reasonable pilot would fly without it today. But no one uses it as a real-time lightning strike indicator, because everyone understands it’s a picture with latency. Apply the same discipline to traffic. Use it for what it does well. Understand its conditions. Cross-reference it with ATC advisories, knowledge of local traffic patterns, and your eyes outside.

The FAA’s ADS-B publications and the Aeronautical Information Manual sections on ADS-B ground infrastructure are the authoritative sources for deeper technical detail on this system.

Key Takeaways

• ADS-B uses two incompatible frequencies - 1090 MHz ES (required above 18,000 feet) and 978 MHz UAT (GA alternative below 18,000 feet) - that cannot directly hear each other; the TIS-B ground network bridges the gap.
• TIS-B only activates when the ground network detects an ADS-B Out aircraft. Without ADS-B Out, you receive no TIS-B service and rely solely on direct broadcasts from same-frequency aircraft in range.
• Ground coverage has real geographic gaps - particularly below 1,000 feet AGL, in mountain terrain, and in parts of Alaska and coastal areas.
• TIS-B relay latency runs up to approximately 3 seconds by design spec; at 450 knots, that’s roughly 0.7 nautical miles of position age per target.
• Aircraft without transponders are structurally invisible to ADS-B IN - they generate no data at all, not uncertain data; this includes legally operating ultralights, gliders, and some experimental and vintage aircraft.