UAT Nine Seventy-Eight and One Zero Nine Zero Extended Squitter, the Two ADS-B Frequencies That Cannot Hear Each Other Directly, and the Rebroadcast Gap That Opens When You Leave Ground Station Coverage

Your ADS-B traffic display can show a converging aircraft and then drop it - not because that aircraft turned, but because your two planes are broadcasting on completely different frequencies that cannot receive each other without a FAA ground station in the middle. When that ground station coverage ends, the display goes dark. This is not a malfunction. It is a documented limitation of a dual-frequency architecture that most pilots were never explicitly taught.

Two Frequencies, One Airspace

The FAA’s ADS-B mandate, which took effect January 1, 2020, requires aircraft operating in most controlled airspace to carry ADS-B Out - a transmitter broadcasting position, altitude, speed, and identification continuously. The intent was to replace radar interrogation with self-reported positions and give every pilot and controller the same real-time picture simultaneously.

The complication is that the mandate allows two completely incompatible frequencies to satisfy that requirement.

1090 MHz Extended Squitter (1090 ES) is the international standard. Every commercial airliner uses it. It operates through Mode S transponders and works at all altitudes. Above 18,000 feet MSL in the United States, 1090 ES is the only legal option. The same broadcasts are receivable in Europe, Australia, and Japan - any country with ADS-B infrastructure.

978 MHz Universal Access Transceiver (UAT) is a United States-only standard. It operates below 18,000 feet and was designed specifically for general aviation. UAT infrastructure does not exist in Europe. Cross-border compatibility with Canada is limited. UAT is a domestic system built for the Skyhawks, Cherokees, and Mooneys that make up the backbone of GA.

These two frequencies do not cross-receive. A UAT receiver scanning 978 MHz cannot receive a 1090 ES broadcast. A 1090 ES receiver cannot receive a UAT broadcast. A Cessna 172 on UAT and a TBM 960 on 1090 ES - both ADS-B Out equipped, both fully compliant - are invisible to each other’s traffic receivers without an external bridge.

Why the FAA Created Two Incompatible Systems

The dual-frequency architecture was finalized in approximately 2010–2011 through the RTCA standards process. Two factors drove the decision.

The first was channel congestion. The 1090 MHz frequency was already heavily loaded - Mode S transponders, TCAS interrogations, and existing extended squitter broadcasts competing for bandwidth in busy terminal airspace. Adding thousands of light GA aircraft transmitting every second risked serious congestion around Class B airports in Atlanta, Chicago, and Los Angeles.

The second was cost. UAT hardware was significantly cheaper to manufacture than 1090 ES hardware in that era. The FAA needed high equipage rates across the GA fleet to build a complete surveillance picture. If hardware cost drove pilots to defer equipping, the system would remain incomplete. UAT was the accessible path.

UAT also carried something 1090 ES does not. The 978 MHz uplink supports FIS-B - the Flight Information Service Broadcast - delivering cockpit weather continuously and without a subscription: NEXRAD radar imagery, METARs, TAFs, and special use airspace status. The 978 uplink also carries TIS-B, the Traffic Information Service Broadcast, which will be addressed below.

The architecture appeared reasonable on paper: commercial aviation on 1090 ES, GA below 18,000 feet on UAT, everyone broadcasting, everyone receiving. The seam came from one problem the design could not avoid on its own.

How ADS-B Rebroadcast Bridges the Frequency Gap

The FAA’s solution to cross-frequency blindness is ADS-B Rebroadcast, abbreviated ADS-R. The same FAA ground station network that supports FIS-B and TIS-B operates receivers on both frequencies simultaneously.

When a UAT aircraft broadcasts on 978 MHz, the ground station captures that position and re-transmits it on 1090 ES - making the UAT aircraft visible to 1090 ES receivers. When a 1090 ES aircraft broadcasts, the ground station re-transmits on 978 - making it visible to UAT receivers. The bridge is automatic and continuous.

The FAA built efficiency logic into ADS-R. Rather than flooding every cockpit with regional traffic data, the ground station uses each aircraft’s own transmissions to estimate its position and delivers only relevant re-broadcasts - generally traffic within approximately 15 nautical miles. The system tailors what it sends based on where it calculates you to be.

Combined with TIS-B - which uplinks radar-derived positions of aircraft carrying no ADS-B Out at all - the design intent is a complete traffic picture: everything ATC sees, plus all ADS-B-equipped aircraft across both frequencies, re-bridged and delivered to every equipped cockpit. When ground coverage is active, it works as designed.

Where ADS-R Coverage Breaks Down

ADS-R depends entirely on FAA ground station coverage. The network is extensive, but it does not cover all airspace at all altitudes.

At approximately 4,000 feet AGL and above, coverage across the contiguous United States is nearly universal. Ground station density and radio line-of-sight geometry work at altitude. ADS-R and TIS-B are active, and the cross-frequency bridge operates as intended.

The gaps open at low altitude:

Below 4,000 feet AGL, coverage becomes inconsistent in mountainous terrain, valleys, and remote western areas
Below 2,000 feet AGL, gaps are significant
Below 1,000 feet AGL, coverage is absent in many parts of the country

This directly affects specific operations: backcountry flying in Idaho or Colorado, low-altitude pipeline and powerline patrol, agricultural aviation, helicopter EMS, balloon and parachute operations, and gliders below pattern altitude. These missions combine low altitude with terrain that frequently places pilots outside ground station radio range.

Coverage is not guaranteed even at altitude. Ground stations have maintenance downtime. Coverage at the margins of a station’s radio horizon is inconsistent. Mountainous terrain can shadow an uplink even above 4,000 feet. When an ADS-R uplink drops, cross-frequency traffic disappears from the display - and the problem is symmetric. If you’re on UAT and ADS-R drops, the 1090 ES aircraft loses your position from their display as well.

TIS-B Traffic and Its Accuracy Limitations

TIS-B uplinks radar-derived positions of aircraft carrying no ADS-B Out. A Piper Warrior with a Mode C transponder will appear on a properly equipped display as TIS-B traffic, provided FAA radar is tracking it and the uplink is active. TIS-B targets are typically displayed differently from direct ADS-B traffic - often a distinct symbol or color - indicating the data came from a secondary source.

The accuracy of a TIS-B target depends entirely on the radar that generated it. Secondary surveillance radar with Mode C provides position and altitude. Primary radar - a skin-paint return - provides position only, with no altitude tag. That aircraft may display at unknown altitude. Older radar systems have update rates between 1 and 12 seconds depending on the installation. At a 12-second update rate, an aircraft flying at 250 knots has moved nearly a full nautical mile since the last position fix. That is a significant error when assessing traffic separation.

TIS-B is valuable. It is not equivalent to direct ADS-B reception, and knowing the difference matters during traffic avoidance.

The Policy Question: Will UAT Eventually Be Sunset?

The logical endpoint of this architecture is consolidation onto a single frequency. If every aircraft operated on 1090 ES, ADS-R becomes unnecessary. Aircraft hear each other directly. Ground station dependency for cross-frequency translation disappears. International interoperability is automatic.

The argument gains traction because 1090 ES avionics costs have dropped substantially since 2010–2011, weakening the original economic rationale for UAT. The dual-frequency complexity now exists primarily to protect a cost advantage that has largely eroded.

The counterargument is the installed base. Tens of thousands of UAT-equipped aircraft are flying today. Mandating re-equipage to 1090 ES imposes real costs on aircraft owners and disrupts the GA fleet. Channel congestion at busy Class B airports remains a genuine concern. And sunsetting 978 MHz would eliminate the FIS-B weather uplink for transitioning aircraft, requiring a replacement weather data channel to be established.

As of now, the FAA has not moved toward sunsetting UAT. The dual-frequency system is current policy. Pilots who want to track this issue should monitor the RTCA standards process and FAA NextGen program documentation.

Practical Option: Dual-Frequency Portable Receivers

Portable ADS-B receivers such as Stratux receive both frequencies simultaneously. These devices can receive direct 1090 ES broadcasts without depending on an ADS-R uplink, in addition to UAT 978 broadcasts. In coverage-limited environments, this partially closes the gap from the reception side.

The limitation is asymmetric. A pilot on UAT with a dual-frequency receiver can see 1090 ES traffic within direct radio range, even when ADS-R is unavailable. But a 1090 ES aircraft without ADS-R uplink still cannot see that UAT pilot’s broadcasts. Reception improves; transmission visibility does not. It is an improvement on a pure UAT-only setup, not a full solution.

What the Performance Data Shows

FAA system-level performance reports show that at altitude and within coverage, ADS-B delivers extraordinary position accuracy and update rates. The mandate has measurably improved surveillance in airspace that radar struggled to serve well. In the environments the system was designed for, it performs as designed.

The known limitations are at the edges: low altitude, coverage gaps, the boundary between frequencies, and the lag in radar-derived TIS-B targets. These are documented in the FAA’s own materials - not surprises, but predictable constraints that are manageable once understood.

The FAA’s official guidance on ADS-B traffic has not changed since the mandate took effect. It supplements situational awareness. It does not replace the see-and-avoid requirement. A clear display is not confirmation of a clear sky.

Key Takeaways

UAT (978 MHz) and 1090 ES cannot hear each other directly - ever, by design. Cross-frequency visibility requires an active FAA ground station in the middle.
ADS-R is the bridge, not the baseline. Traffic display completeness depends entirely on ground station coverage being active at your altitude and location.
Coverage gaps are real and predictable. Below 4,000 feet AGL - especially in mountainous or remote terrain - ADS-R may simply not be present.
TIS-B carries accuracy caveats. Radar-derived positions can lag by up to 12 seconds and may carry no altitude data for primary-only radar returns.
A blank traffic display is not a clear-sky guarantee. ADS-B supplements situational awareness; see-and-avoid remains the pilot’s legal and practical responsibility.