Aircraft Transponder


  • The transponder developed as a simple means for an aircraft to communicate its position with air traffic control
  • Consisting of only a few components, the transponder system can operate in the air and on the ground
  • Equipment deviations may be authorized by ATC, depending on the operation flown
  • As a component of the air traffic control system, transponders must comply with inspections and testing to validate their accuracy
  • Pilots should be aware that proper application of transponder and ADS-B operating procedures will provide both VFR and IFR aircraft with a higher degree of safety while operating on the ground and airborne
    • Transponder/ADS-B panel designs differ; therefore, a pilot should be thoroughly familiar with the operation of their particular equipment to maximize its full potential
    • ADS-B Out and transponders with altitude reporting mode turned ON (Mode C or S) substantially increase the capability of surveillance systems to see an aircraft
    • This provides air traffic controllers, as well as pilots of suitably equipped aircraft (TCAS and ADS-B In), increased situational awareness and the ability to identify potential traffic conflicts
    • Even VFR pilots who are not in contact with ATC will be afforded greater protection from IFR aircraft and VFR aircraft that are receiving traffic advisories
    • Nevertheless, pilots should never relax their visual scanning vigilance for other aircraft, and should include the ADS-B In display (if equipped) in their normal traffic scan

Transponder History:

  • Bendix/King KT-74 ADS-B Mode S Transponder
    Amazon, Bendix/King KT-74 ADS-B Mode S Transponder
  • Transponders came about during World War II when the British developed a transceiver which was capable of identifying friendly aircraft
  • This system was called "Parrot" which led to terms like "squawk your parrot" (turn on your transponder), and "strangle your parrot" (turn your transponder off)
  • The U.S. adopted this same system and instead called it "Identification of Friend or Foe," or IFF, a term still utilized by the military today
  • Like the term IFF making it to modern day operations, squawk stuck around too which is why we are assigned a squawk code to allow ATC to see where we are
  • Bendix/King KT-74 ADS-B Mode S Transponder
    Amazon, Bendix/King KT-74 ADS-B Mode S Transponder

Transponder Components:

  • Transponder Unit:

    • The actual transponder unit is a unit which installs into the dashboard of an aircraft's cockpit
    • Some aircraft (like those with glass cockpits) do not have a transponder as shown in [Figure 1] however, the computational unit remains
    • Switchology:

      • OFF: disconnects power to the unit
      • SBY: standby, applies electrical power but does not emit a signal
      • ON: operation in Mode 3/A (identification only)
        • Should be delayed as long as possible prior to takeoff
      • ALT: altitude, sets Mode 3 and Mode C
      • TST: test, confirms operation of the transponder
  • Transponder Antenna:

    Ted Transponder Antenna/Stub/Bnc 104-12
    Amazon, Ted Transponder Antenna/Stub/Bnc 104-12
    Comant CI-105 DME/Transponder Antenna
    Amazon, Comant CI-105 DME/Transponder Antenna
    • Transponders have two different types of antenna's, varying by aircraft
    • These include the straight metal antenna with a spherical tip [Figure 2] or a fin blade [Figure 3] which simply houses the transponder antenna, ruggedizing the installation

Transponder Operation:

  • Transponder Code Designation:

    • For ATC to utilize one of the 4096 discrete codes, a four-digit code designation will be used; for example, code 2102 will be expressed as "TWO ONE ZERO TWO"
      • Circumstances may occasionally require ATC to assign a non-discrete code; i.e., a code ending in "00"
    • All codes are allocated in accordance with the National Beacon Code Allocation Plan (NBCAP)
    • Military/Civil Code Equivalency:

      • Air Traffic Control Radar Beacon System (ATCRBS) is similar to and compatible with military coded radar beacon equipment
        • Civil Mode A is identical to military Mode 3
      • Military aircraft operating VFR or IFR in restricted/warning areas or VFR on VR routes will adjust their transponders to reply on Code 4000 unless another code has been assigned by ATC or coordinated, if possible, with ATC
  • Automatic Altitude Reporting:

    • Most transponders (Modes C and S) and all ADS-B Out systems are capable of automatic altitude reporting
      • This system converts aircraft altitude in 100-foot increments to coded digital information that is transmitted to the appropriate surveillance facility as well as to ADS-B In and TCAS systems
    • Adjust the transponder/ADS-B to reply on the Mode 3/A code specified by ATC and with altitude reporting enabled, unless otherwise directed by ATC or unless the altitude reporting equipment has not been tested and calibrated as required by 14 CFR Section 91.217
      • If deactivation is required by ATC, turn off the altitude reporting feature of your transponder/ADS-B
      • An instruction by ATC to "Stop altitude squawk, altitude differs [Number of Feet] Feet,," may be an indication that the transmitted altitude information is incorrect, or that the aircraft's altimeter setting is incorrect
      • While an incorrect altimeter setting has no effect on the transmitted altitude information, it will cause the aircraft to fly at a true altitude different from the assigned altitude
      • When a controller indicates that an altitude readout is invalid, the pilot should verify that the aircraft altimeter is set correctly
      • Altitude encoders are preset at standard atmospheric pressure
        • Local altimeter correction is applied by the surveillance facility before the altitude information is presented to ATC
    • Pilots should report exact altitude or flight level to the nearest hundred foot increment when establishing initial contact with an ATC facility
      • Exact altitude or flight level reports on initial contact provide ATC with information that is required prior to using automatically reported altitude information for separation purposes
      • This will significantly reduce altitude verification requests
    • Mode C validation:

      • ATC must verify (by regulation) the altitude they see on their radar display that corresponds to the altitude a pilot reports when he checks in. The controller handbook says the Mode C information must be within 300 feet of the pilot's report to be considered valid and legally useable
        • Controller must validate Mode C at least once in each facility's airspace
        • The controller may request the pilot to say "say altitude" to prompt the report and satisfy the requirement
  • IDENT Feature:

    • The transponder ident feature allows for an aircraft to be identified by air traffic control
    • The transponder/ADS-B Out equipment must be operated only as specified by ATC
    • Activate the "IDENT" feature only upon request of the ATC controller
  • Code Changes:

    • When making routine code changes, pilots should avoid inadvertent selection of Codes 7500, 7600 or 7700 thereby causing momentary false alarms at automated ground facilities
      • For example, when switching from Code 2700 to Code 7200, switch first to 2200 then to 7200, NOT to 7700 and then 7200
      • This procedure applies to non-discrete Code 7500 and all discrete codes in the 7600 and 7700 series (i.e., 7600-7677, 7700-7777) which will trigger special indicators in automated facilities, see more special codes below
      • Only non-discrete Code 7500 will be decoded as the hijack code
    • Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777
      • This code is reserved for military interceptor operations
    • Military pilots operating VFR or IFR within restricted/warning areas should adjust their trans- ponders to Code 4000 unless another code has been assigned by ATC
    • When changing codes, pilots should do so while the transponder is active (i.e., don't go to standby during the change)
  • Mode C Transponder Requirements:

    • In general, the CFRs require aircraft to be equipped with an operable Mode C transponder and ADS-B Out when operating:
      • In Class A, Class B, or Class C airspace areas;
      • Above the ceiling and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet MSL;
      • Class E airspace at and above 10,000 feet MSL within the 48 contiguous states and the District of Columbia, excluding the airspace at and below 2,500 feet AGL;
      • Within 30 miles of a Class B airspace primary airport, below 10,000 feet MSL (commonly referred to as the "Mode C Veil");
      • For ADS-B Out: Class E airspace at and above 3,000 feet MSL over the Gulf of Mexico from the coastline of the United States out to 12 nautical miles
        • The airspace described in (e) above is specified in 14 CFR 91.225 for ADS-B Out requirements. However, 14 CFR 91.215 does not include this airspace for ATC transponder requirements
      • Transponder and ADS-B Out requirements do not apply to any aircraft that was not originally certificated with an electrical system, or that has not subsequently been certified with such a system installed, including balloons and gliders. These aircraft may conduct operations without a transponder or ADS-B Out when operating:
        • Outside any Class B or Class C airspace area; and
        • Below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport, or 10,000 feet MSL, whichever is lower
    • 14 CFR Section 99.13 requires all aircraft flying into, within, or across the contiguous U.S. ADIZ be equipped with a Mode C or Mode S transponder. Balloons, gliders and aircraft not equipped with an engine-driven electrical system are excepted from this requirement
    • Pilots must ensure that their aircraft transponder/ADS-B is operating on an appropriate ATC-assigned VFR/IFR code with altitude reporting enabled when operating in such airspace. If in doubt about the operational status of either feature of your transponder while airborne, contact the nearest ATC facility or FSS and they will advise you what facility you should contact for determining the status of your equipment
  • Cooperative Surveillance Phraseology:

    • Air traffic controllers, both civil and military, will use the following phraseology when referring to operation of cooperative ATC surveillance equipment
    • Except as noted, the following ATC instructions do not apply to military transponders operating in other than Mode 3/A/C/S
      1. SQUAWK (number):

        Operate radar beacon transponder/ADS-B on designated code with altitude reporting enabled
      2. IDENT:

        Engage the "IDENT" feature (military I/P) of the transponder/ADS-B
        • ATC: "[Callsign], IDENT"
        • ATC: "[Callsign], radar contact, altimeter 29.92"
        • You may sometimes hear pilots respond to a call to ident by saying "with the flash"
      3. SQUAWK (number) and IDENT:

        Operate transponder/ADS-B on specified code with altitude reporting enabled, and engage the "IDENT" (military I/P) feature

        Switch transponder/ADS-B to standby position

        Resume normal transponder/ADS-B operation on previously assigned code. (Used after "SQUAWK STANDBY," or by military after specific transponder tests)

        Activate Mode C with automatic altitude reporting

        Turn off automatic altitude reporting
      8. STOP SQUAWK (mode in use):

        Stop transponder and ADS-B Out transmissions, or switch off only specified mode of the aircraft transponder (military)

        Operate transponder/ADS-B in the emergency position (Mode A Code 7700 for civil transponder
        • Mode 3 Code 7700 and emergency feature for military transponder)
      10. SQUAWK VFR:

        Operate radar beacon transponder/ADS-B on Code 1200 in the Mode A/3, or other appropriate VFR code, with altitude reporting enabled

Transponder Use:

  • Transponders should be operated in the altitude report mode any time an aircraft is on the movement area of an airport in accordance with Safety Alerts and Information for Operators (SAFO) 15006
    • The Federal Aviation Administration (FAA) uses airport surface surveillance capabilities at some of the busiest airports in the U.S. to determine aircraft and vehicle location when they are operating on an airport movement area
    • Runway safety systems, such as Airport Surface Detection EquipmentModel X (ASDE-X) and Advanced Surface Movement Guidance and Control System (A-SMGCS), use data from surface movement radar and aircraft transponders to obtain accurate aircraft and vehicle locations, thereby increasing airport surface safety and efficiency
  • Distinguishes aircraft from one another on secondary surveillance radar
    • Air Traffic Control Radar Beacon System (ATCRBS) tracks transponders
  • If entering a U.S. Offshore Airspace Area from outside the U.S., the pilot should advise on first radio contact with a U.S. Radar ATC facility that such equipment is available by adding "transponder" to the aircraft identification
  • Limited to "line of sight"
  • Transfers altitude in 100' increments
  • If deactivation is required, ATC will report "Stop altitude squawk, altitude differs (number of feet) feet"
  • Transponders are preset to 29.92; having the wrong altimeter setting will not affect altitude readout
  • Two Modes:
    • Mode 3/A, identification only (known as mode 3 for military, A for civilian)
      • Currently ATC automation relies on the Mode 3/A code to identify aircraft under radar surveillance and correlate the target to a flight plan. The mode 3/A code is a four digit number ranging from 0000 to 7777
    • Mode C, identification, and altitude

Mode C requires an encoder to broadcast altitude to ATC. The encoder may be either a separate unit ("blind encoder") or integrated into the aircraft altimeter ("encoding altimeter")

  • Broadcasts aircraft's pressure altitude (29.92) when operating in Mode C
    • Understand your system! Some aircraft will read altitude based off your pitot system which, in a failure, may be inaccurate
  • Uses 4 codes (0 through 7) to set a code that is manually selected by the pilot, assigned by ATC
  • Replies to ATC radar with up to 4096 discrete codes

Reserved Codes:

Transponder Codes
Transponder Codes

Required to be used when:

  • At or above 10,000' MSL over the 48 contiguous states or the District of Columbia, excluding that airspace below 2,500' AGL;
  • Within 30 miles of a Class B airspace primary airport, below 10,000' MSL
    • Balloons, gliders, and aircraft not equipped with an engine driven electrical system are excepted from the above requirements when operating below the floor of Class Alpha airspace and/or outside of a Class Bravo airspace and below the ceiling of the Class Bravo airspace (or 10,000' MSL, whichever is lower);
  • Within and above all Class C airspace, up to 10,000' MSL;
  • Within 10 miles of certain designated airports, excluding that airspace which is both outside the Class Delta surface area and below 1,200' AGL
  • Balloons, gliders, and aircraft not equipped with an engine driven electrical system are excepted from this requirement

Aircraft Conflict/Mode C Intruder Alert:

  • Issued if the controller observes another aircraft which places it in an unsafe proximity
  • When feasible, the controller will offer the pilot an alternative course of action

Emergency Operation:

  • Triggers an alarm or special indicator at all control positions
  • Understand you may not be in radar coverage; therefore continue squawking and establish radio communications as soon as possible

Altitude Reporting:

  • No person may operate an automatic pressure altitude reporting equipment if:
    • When deactivation of that equipment is directed by ATC;
    • Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or
    • Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively

Transponder and ADS-B Equipment Deviations:

  • In-flight requests for "immediate" deviation from the transponder requirement may be approved by controllers only for failed equipment, and only when the flight will continue IFR or when weather conditions prevent VFR descent and continued VFR flight in airspace not affected by the CFRs
  • In-flight requests for "immediate" deviation from the ADS-B Out requirements may be approved by ATC only for failed equipment, and may be accommodated based on workload, alternate surveillance availability, or other factors. All other requests for deviation must be made at least 1 hour before the proposed operation, following the procedures contained in Advisory Circular (AC) 90-114, Automatic Dependent Surveillance-Broadcast Operations
  • Non-immediate requests for deviation should be made at least 1 hour before the proposed operation by contacting the nearest Flight Service or Air Traffic facility in person or by telephone
  • The nearest ARTCC will normally be the controlling agency and is responsible for coordinating requests involving deviations in other ARTCC areas
  • Examples of non-immediate deviations that may be requested include:
    • With an operating transponder, but without Mode C, requested any time
    • With an inoperative transponder to the airport of ultimate destination, including any intermediate stops, or to proceed to a place where suitable repairs can be made or both, requested any time
    • That is not equipped with a transponder; requests must be made at least 1 hour before proposed operation

Inspections & Tests:

  • Transponder inspections and tests are governed by Federal Aviation Regulation 91.413
  • Must be tested and inspected within the previous 24 months or after installation or maintenance where data correspondence error could be introduced
  • Tests conducted by:
    1. A certificated repair station holding:
      • A radio rating, class III
      • A limited radio rating appropriate to the transponder
      • A limited rating appropriate to the test to be performed
    2. A holder of a continuous airworthiness maintenance program as provided in Part 121 or 135.411
    3. The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer

Transponder and ADS-B Operations:

  • Transponder and ADS-B Operations on the Ground:

    • Civil and military aircraft should operate with the transponder in the altitude reporting mode (consult the aircraft's flight manual to determine the specific transponder position to enable altitude reporting) and ADS-B Out transmissions enabled at all airports, any time the aircraft is positioned on any portion of the airport movement area
    • This includes all defined taxiways and runways. Pilots must pay particular attention to ATIS and airport diagram notations, General Notes (included on airport charts), and comply with directions pertaining to transponder and ADS-B usage
    • Generally, these directions are:
      • Departures:

        • Select the transponder mode which allows altitude reporting and enable ADS-B during pushback or taxi-out from parking spot. Select TA or TA/RA (if equipped with TCAS) when taking the active runway
      • Arrivals:

        • If TCAS equipped, deselect TA or TA/RA upon leaving the active runway, but continue transponder and ADS-B transmissions in the altitude reporting mode. Select STBY or OFF for transponder and ADS-B upon arriving at the aircraft's parking spot or gate
    • Transponders are all effective at integrating traffic into systems like the Airport Surface Detection Equipment Model X (ASDE-X) and Advanced Surface Movement Guidance and Control System (A-SMGCS)
  • Transponder and ADS-B Operations While Airborne:

    • Unless otherwise requested by ATC, aircraft equipped with an ATC transponder maintained in accordance with 14 CFR Section 91.413 MUST operate with this equipment on the appropriate Mode 3/A code, or other code as assigned by ATC, and with altitude reporting enabled whenever in controlled airspace. If practicable, aircraft SHOULD operate with the transponder enabled in uncontrolled airspace
    • Aircraft equipped with ADS-B Out MUST operate with this equipment in the transmit mode at all times, unless otherwise requested by ATC
  • Transponder and ADS-B Operation Under Visual Flight Rules (VFR):

    • Unless otherwise instructed by an ATC facility, adjust transponder/ADS-B to reply on Mode 3/A Code 1200 regardless of altitude
    • When required to operate their transponder/ADS-B, pilots must always operate that equipment with altitude reporting enabled unless otherwise instructed by ATC or unless the installed equipment has not been tested and calibrated as required by 14 CFR Section 91.217
      • If deactivation is required, turn off altitude reporting
    • When participating in a VFR standard formation flight that is not receiving ATC services, only the lead aircraft should operate its transponder and ADS-B Out and squawk code 1203
      • Once established in formation, all other aircraft should squawk standby and disable ADS-B transmissions
    • Note that:
      • If the formation flight is receiving ATC services, pilots can expect ATC to direct all non-lead aircraft to STOP Squawk, and should not do so until instructed
      • Firefighting aircraft not in contact with ATC may squawk 1255 in lieu of 1200 while en route to, from, or within the designated firefighting area(s)
      • VFR aircraft flying authorized SAR missions for the USAF or USCG may be advised to squawk 1277 in lieu of 1200 while en route to, from, or within the designated search area
      • Since gliders may fly an unpredictable flight path, varying altitude and speed, VFR pilots not in contact with ATC should squawk 1202
  • A pilot on an IFR flight who elects to cancel the IFR flight plan prior to reaching their destination, should adjust the transponder/ADS-B according to VFR operations
  • If entering a U.S. OFFSHORE AIRSPACE AREA from outside the U.S., the pilot should advise on first radio contact with a U.S. radar ATC facility that such equipment is available by adding "transponder" or "ADS-B" (if equipped) to the aircraft identification
  • It should be noted by all users of ATC transponders and ADS-B Out systems that the surveillance coverage they can expect is limited to "line of sight" with ground radar and ADS-B radio sites
    • Low altitude or aircraft antenna shielding by the aircraft itself may result in reduced range or loss of aircraft contact
    • Though ADS-B often provides superior reception at low altitudes, poor coverage from any surveillance system can be improved by climbing to a higher altitude
    • Pilots should refer to AIM, paragraph 4-5-7, Automatic Dependent Surveillance - Broadcast (ADS-B) Services, for a complete description of operating limitations and procedures
Advanced Navigation & Positioning Corporation, Transponder Landing System Ground Equipment
Advanced Navigation & Positioning Corporation, Transponder Landing System Ground Equipment

Remote Identification of Unmanned Aircraft:

  • Remote Identification, or RID is the ability of a UAS in flight to provide identification and location information that can be received by other parties
  • RID allows the FAA, national security agencies, law enforcement, and others to distinguish compliant airspace users from those potentially posing a safety or security risk
    • It helps these agencies find the control station when a UAS appears to be flying unsafely or where it is prohibited
  • 14 CFR Part 89, Remote Identification (RID) of Unmanned Aircraft, will require most drones operating in U.S. airspace to have RID capability. UAS not equipped with RID capability will be limited to operating in specific FAA-approved geographic locations, such as FRIA
  • There are three ways drone pilots will be able to meet the identification requirements of the RID rule:
    • Standard RID
    • RID Broadcast Module
    • FRIAs
  • Standard RID:

    • Only standard RID drones may be manufactured after the September 16, 2022, rule effective date. Unmanned aircraft broadcast the RID message elements directly from the unmanned aircraft from takeoff to shutdown. Message elements include: (1) A unique identifier to establish the identity of the unmanned aircraft; (2) an indication of the unmanned aircraft latitude, longitude, geometric altitude, and velocity; (3) an indication of the control station latitude, longitude, and geometric altitude; (4) a time mark; and (5) an emergency status indication. Operators may choose whether to use the serial number of the unmanned aircraft or a session ID (e.g., an alternative form of identification that provides additional privacy to the operator) as the unique identifier
  • RID Broadcast Module:

    • An unmanned aircraft can be equipped with a Remote ID broadcast module that broadcasts message elements from takeoff to shutdown. Message elements include: (1) The serial number of the broadcast module assigned by the producer; (2) an indication of the latitude, longitude, geometric altitude, and velocity of the unmanned aircraft; (3) an indication of the latitude, longitude, and geometric altitude of the unmanned aircraft takeoff location; and (4) a time mark
  • FAA-Recognize Identification Area:

    • An FAA-recognized identification area (FRIA) is a defined geographic area where persons can operate UAS without remote identification, provided they maintain visual line of sight. Organizations eligible to request establishment of a FRIA include CBOs recognized by the FAA and educational institutions. The latter group includes primary and secondary educational institutions, trade schools, colleges, and universities
    • To operate in a FRIA according to the 14 CFR Part 89, RID of unmanned aircraft, operators must be physically located within the boundaries of the FRIA, must only operate drones within those boundaries, and must operate within VLOS at all times. UAS equipped with RID broadcast capability must broadcast continuously even while operating within or transiting a FRIA
  • RID Paths to Compliance
    RID Paths to Compliance


  • Transponders have a variety of uses that extend beyond their original design of identification to ATC
    • These include collision avoidance systems and the aforementioned transponder landing system
  • If transponders are inoperable and the aircraft must be flown to another location for maintenance, but must be flown through transponder required airspace, consider reaching out to ATC to verify if a waiver is possible
  • Specific details concerning requirements to carry and operate Mode C transponders and ADS-B Out, as well as exceptions and ATC authorized deviations from those requirements, are found in 14 CFR Sections 91.215, 91.225, and 99.13
  • Some transponder operating modes can transmit additional data (i.e., mode-s can broadcast callsigns)
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