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Transponder

Introduction:

  • A transponder is a simple means for an aircraft to communicate its position with air traffic control
  • 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

Transponder History:

  • 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

Transponder Components:

  • Transponder Unit:

    Bendix/King KT-74 ADS-B Mode S Transponder
    Bendix/King KT-74 ADS-B Mode S Transponder
    Bendix/King KT-74 ADS-B Mode S Transponder
    Bendix/King KT-74 ADS-B Mode S Transponder
    • 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
    Ted Transponder Antenna/Stub/Bnc 104-12
    Comant CI-105 DME/Transponder Antenna
    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:

  • Beacon Codes:

    • 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
  • 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)
  • 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
  • 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
      • All other 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
    • 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
  • Transponder 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
      • Firefighting aircraft not in contact with ATC may squawk 1255 in lieu of 1200 while en route to, from, or within the designated fire fighting 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
      • Gliders not in contact with ATC should squawk 1202 in lieu of 1200
    • 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 formation flight that is not receiving ATC services, only the lead aircraft should operate their transponder and ADS-B Out
      • All other aircraft should disable transponder and ADS-B transmissions once established within the formation
      • 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
  • 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
      4. SQUAWK STANDBY:

        Switch transponder/ADS-B to standby position
      5. SQUAWK NORMAL:

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

        Activate Mode C with automatic altitude reporting
      7. STOP ALTITUDE SQUAWK:

        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)
      9. SQUAWK MAYDAY:

        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:

  • 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

NOTE:
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

Deviations:

  • Requests must be made to the ATC facility having jurisdiction over that airspace for operation of an aircraft:
    • 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 Landing System (TLS):

  • The TLS is designed to provide approach guidance utilizing existing airborne ILS localizer, glide slope, and transponder equipment
  • The TLS detects the aircraft's position by interrogating its transponder
  • Next, it then broadcasts ILS frequency signals to guide the aircraft along the desired approach path
  • Ground equipment consists of: [Figure 4]
    • Transponder interrogator,
    • Sensor arrays to detect lateral and vertical position, and
    • ILS frequency transmitters
  • TLS instrument approach procedures are designated Special Instrument Approach Procedures
    • Special aircrew training is required
  • TLS ground equipment provides approach guidance for only one aircraft at a time
  • Even though the TLS signal is received using the ILS receiver, no fixed course or glidepath is generated
  • The concept of operation is very similar to an air traffic controller providing radar vectors, and just as with radar vectors, the guidance is valid only for the intended aircraft
  • The TLS ground equipment tracks one aircraft, based on its transponder code, and provides correction signals to course and glidepath based on the position of the tracked aircraft
  • Flying the TLS corrections computed for another aircraft will not provide guidance relative to the approach; therefore, aircrews must not use the TLS signal for navigation unless they have received approach clearance and completed the required coordination with the TLS ground equipment operator
  • Navigation fixes based on conventional NAVAIDs or GPS are provided in the special instrument approach procedure to allow aircrews to verify the TLS guidance

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
  • 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
    • When participating in a VFR formation flight that is not receiving ATC services, only the lead aircraft should operate their transponder and ADS-B Out. All other aircraft should disable transponder and ADS-B transmissions once established within the formation
      • 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
  • 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 that the 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
  • NOTE:
    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

Conclusion:

  • 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
  • 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

References: