ADS-B and Transponders: What Pilots Need to Know

WHAT IS A TRANSPONDER?

A transponder (transmitter-responder) is an electronic device that receives a radar interrogation signal from ATC ground stations and automatically transmits a coded reply. This reply allows ATC to identify the aircraft on radar and, with altitude encoding capability, determine its altitude. Transponders are one of the foundational technologies of the air traffic control surveillance system.

TRANSPONDER MODES

MODE A

Mode A transponders transmit a four-digit identification code (squawk code) assigned by ATC. This allows controllers to distinguish individual aircraft on their radar display. Common squawk codes include: • 1200: VFR operations (default for VFR flights not receiving ATC services) • 7500: Hijacking • 7600: Lost communications • 7700: Emergency

MODE C

Mode C adds altitude reporting to the Mode A identification. The transponder interfaces with the aircraft's encoding altimeter (or blind encoder) to transmit pressure altitude to ATC. This altitude is reported in 100-foot increments and is based on the standard altimeter setting of 29.92 inHg. ATC computers apply the local altimeter correction to display the correct altitude.

MODE S

Mode S (Select) is an advanced transponder technology that allows discrete addressing of individual aircraft. Each Mode S transponder has a unique 24-bit address assigned by the country of registration. Mode S provides more precise altitude reporting (25-foot increments), supports data link communications, and forms the foundation for ADS-B Out transmissions.

WHERE TRANSPONDERS ARE REQUIRED

FAR 91.215 mandates an operable transponder with Mode C altitude reporting in the following airspace: • Class A airspace (18,000 to 60,000 feet MSL) • Class B airspace • Class C airspace • Within 30 NM of a Class B primary airport (the Mode C veil), from the surface to 10,000 feet MSL • Above 10,000 feet MSL (excluding airspace at or below 2,500 feet AGL) • All airspace above the ceiling of Class B or Class C up to 10,000 feet MSL

Aircraft without an operable transponder may request a deviation from ATC to operate in these areas, but approval is not guaranteed.

WHAT IS ADS-B?

Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance technology that broadcasts an aircraft's identification, position (from GPS), altitude, velocity, and other data. Unlike radar, which requires a ground interrogation signal, ADS-B transmissions are automatic and continuous. The system has two components:

ADS-B OUT

ADS-B Out is the transmitting component. The aircraft broadcasts its position and identification data, which is received by ATC ground stations and other ADS-B-equipped aircraft. ADS-B Out is mandated by regulation.

ADS-B IN

ADS-B In is the receiving component. Aircraft equipped with ADS-B In can receive and display traffic information (TIS-B) and weather data (FIS-B) in the cockpit. ADS-B In is not required by regulation but provides significant situational awareness benefits.

ADS-B OUT REQUIREMENTS

Since January 1, 2020, FAR 91.225 requires ADS-B Out in the same airspace where Mode C transponders are required (listed above). The rule applies to all aircraft, from airliners to small general aviation aircraft.

EQUIPMENT STANDARDS

ADS-B Out equipment must meet TSO-C166b (1090ES) or TSO-C154c (UAT) standards: • 1090ES (Extended Squitter): Transmits on 1090 MHz, the same frequency used by Mode S transponders. Required for operations above 18,000 feet MSL (Class A airspace). Used by commercial airlines and many GA aircraft. • UAT (Universal Access Transceiver): Transmits on 978 MHz. Permitted only below 18,000 feet MSL. Provides FIS-B weather data to ADS-B In receivers. Generally less expensive than 1090ES for GA installations.

GPS POSITION SOURCE

ADS-B Out requires a WAAS-capable GPS position source that meets TSO-C145a, TSO-C146a, or equivalent standards. The GPS must provide position accuracy meeting the Navigation Accuracy Category for Position (NACp) of 8 or higher (approximately 92.6 meters). Many older GPS units in GA aircraft do not meet this standard and must be upgraded.

ADS-B IN SERVICES

TIS-B (TRAFFIC INFORMATION SERVICE-BROADCAST)

TIS-B provides traffic data to ADS-B In-equipped aircraft. The system fills gaps by broadcasting the positions of aircraft that are tracked by ATC radar but do not have ADS-B Out. TIS-B traffic is only generated when an ADS-B Out-equipped aircraft is within range of ATC radar and other non-ADS-B traffic is nearby.

FIS-B (FLIGHT INFORMATION SERVICE-BROADCAST)

FIS-B provides free weather and aeronautical information to aircraft equipped with UAT-based ADS-B In receivers. Data includes: • NEXRAD radar imagery (composite and regional) • METARs and TAFs • PIREPs (pilot reports) • NOTAMs • Winds and temperatures aloft • AIRMETs and SIGMETs

FIS-B data is broadcast on the 978 MHz UAT frequency. Aircraft with 1090ES ADS-B Out do not receive FIS-B directly but can add a separate UAT receiver to get these services.

CHECKING ADS-B COMPLIANCE

The FAA provides a free compliance verification tool through the ADS-B Performance Monitor. Pilots can fly in ADS-B rule airspace and then check their performance reports at the FAA Public ADS-B Performance Report (PAPR) website. This report shows whether the aircraft's ADS-B Out transmissions meet all required performance parameters.

COMMON ADS-B ISSUES

• ICAO code mismatch: The 24-bit ICAO code programmed into the transponder must match the aircraft's registration. Errors cause "N-number not found" alerts. • GPS position source not compliant: Older GPS units may not meet WAAS or NACp requirements, resulting in non-compliant ADS-B Out transmissions • Antenna placement: Poor antenna location can cause signal blockage. Some installations require both top and bottom antennas for complete coverage. • Anonymous mode: Some transponders allow anonymous mode (no ICAO code broadcast). This mode does not meet ADS-B Out requirements in rule airspace.

PORTABLE ADS-B RECEIVERS

Portable ADS-B In receivers (such as the Sentry, Stratus, and ForeFlight Sentry Plus) are popular among GA pilots. These devices receive ADS-B traffic and weather data and display it on tablet-based EFB apps. However, a portable receiver does not satisfy the ADS-B Out mandate. It provides ADS-B In benefits only, and the aircraft still requires a certified ADS-B Out installation to comply with FAR 91.225.