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Faa Modes Code Assignment

Section 2. Beacon Systems


a. General.

1. Mode 3/A is designated as the common military/civil mode for air traffic control use.

2. Make radar beacon code assignments to only Mode 3/A transponder-equipped aircraft.

b. Unless otherwise specified in a directive or a letter of agreement, make code assignments to departing, en route, and arrival aircraft in accordance with the procedures specified in this section for the radar beacon code environment in which you are providing ATC service. Give first preference to the use of discrete beacon codes.



SQUAWK (code).

A code environment is determined by an operating position's/sector's equipment capability to decode radar beacon targets using either the first and second or all four digits of a beacon code.


a. Issue discrete beacon codes assigned by the computer. Computer-assigned codes may be modified as required.

1. TERMINAL. Aircraft that will remain within the terminal facility's delegated airspace shall be assigned a code from the code subset allocated to the terminal facility.

2. TERMINAL. Unless otherwise specified in a facility directive or a letter of agreement, aircraft that will enter an adjacent ATTS facility's delegated airspace shall be assigned a beacon code assigned by the ARTCC computer.

1. This will provide the adjacent facility advance information on the aircraft and will cause auto-acquisition of the aircraft prior to handoff.

2. When an IFR aircraft, or a VFR aircraft that has been assigned a beacon code by the host computer and whose flight plan will terminate in another facility's area, cancels ATC service or does not activate the flight plan, send a remove strips (RS) message on that aircraft via host keyboard, the FDIO keyboard, or call via service F.

b. Make handoffs to other positions/sectors on the computer-assigned code.

c. Coastal facilities accepting “over” traffic that will subsequently be handed-off to an oceanic ARTCC shall reassign a new discrete beacon code to an aircraft when it first enters the receiving facility's airspace. The code reassignment shall be accomplished by inputting an appropriate message into the computer and issued to the pilot while operating in the first sector/position in the receiving facility's airspace.

Per an agreement between FAA and the Department of Defense, 17 Code subsets in the NBCAP have been reserved for exclusive military use outside NBCAP airspace. To maximize the use of these subsets, they have been allocated to ARTCC's underlying NBCAP airspace that do not abut an oceanic ARTCC's area. To preclude a potential situation where two aircraft might be in the same airspace at the same time on the same discrete code, it is necessary to reassign an aircraft another code as specified in subpara c.


a. Assign appropriate nondiscrete beacon codes from the function codes specified in para 5-2-6, Function Code Assignments.

b. Unless otherwise coordinated at the time of handoff, make handoffs to other positions/sectors on an appropriate nondiscrete function code.


a. When discrete beacon code capability does not exist in your area of responsibility, comply with the procedures specified in para 5-2-3, Nondiscrete Environment.

In a mixed code environment, a situation may exist where a discrete-equipped position/sector exchanges control of aircraft with nondiscrete-equipped facilities or vice versa.

b. When discrete beacon code capability exists in your area of responsibility:

1. Comply with the procedures specified in para 5-2-2, Discrete Environment, and

2. Unless otherwise coordinated at the time of handoff, assign aircraft that will enter the area of responsibility of a nondiscrete-equipped position/sector an appropriate nondiscrete function code from the codes specified in para 5-2-6, Function Code Assignments, prior to initiating a handoff.


Unless otherwise specified in a directive or a letter of agreement or coordinated at the time of handoff, do not request an aircraft to change from the code it was squawking in the transferring facility's area until the aircraft is within your area of responsibility.


Unless otherwise specified by a directive or a letter of agreement, make nondiscrete code assignments from the following categories:

a. Assign codes to departing IFR aircraft as follows:

1. Code 2000 to an aircraft which will climb to FL 240 or above or to an aircraft which will climb to FL 180 or above where the base of Class A airspace and the base of the operating sector are at FL 180, and for inter-facility handoff the receiving sector is also stratified at FL 180. The en route code shall not be assigned until the aircraft is established in the high altitude sector.

2. Code 1100 to an aircraft which will remain below FL 240 or below FL 180 as above.

3. For handoffs from terminal facilities when so specified in a letter of agreement as follows:

(a) Within NBCAP airspace- Code 0100 to Code 0400 inclusive or any other code authorized by the appropriate service area office.

(b) Outside NBCAP airspace- Code 1000 or one of the codes from 0100 to 0700 inclusive or any other code authorized by the appropriate service area office.

b. Assign codes to en route IFR aircraft as follows:

1. FL 180 may be used in lieu of FL 240 where the base of Class A airspace and the base of the operating sector are at FL 180, and for inter-facility handoff the receiving sector is also stratified at FL 180.

2. The provisions of subparas b2(b) and (c) may be modified by facility directive or letter of agreement when operational complexities or simplified sectorization indicate. Letters of agreement are mandatory when the operating sectors of two facilities are not stratified at identical levels. The general concept of utilizing Codes 2100 through 2500 within Class A airspace should be adhered to.

1. Aircraft operating below FL 240 or when control is transferred to a controller whose area includes the stratum involved.

(a) Code 1000 may be assigned to aircraft changing altitudes.

(b) Code 1100 to an aircraft operating at an assigned altitude below FL 240. Should an additional code be operationally desirable, Code 1300 shall be assigned.

2. Aircraft operating at or above FL 240 or when control is transferred to a controller whose area includes the stratum involved.

(a) Code2300 may be assigned to aircraft changing altitudes.

(b) Code2100 to an aircraft operating at an assigned altitude from FL 240 to FL 330 inclusive. Should an additional code be operationally desirable, Code 2200 shall be assigned.

(c) Code2400 to an aircraft operating at an assigned altitude from FL 350 to FL 600 inclusive. Should an additional code be operationally desirable, Code 2500 shall be assigned.

3. Code 4000 when aircraft are operating on a flight plan specifying frequent or rapid changes in assigned altitude in more than one stratum or other conditions of flight not compatible with a stratified code assignment.

1. Categories of flight that can be assigned Code 4000 include certain flight test aircraft, MTR missions, aerial refueling operation requiring descent involving more than one stratum, ALTRVs where continuous monitoring of ATC communications facilities is not required and frequent altitude changes are approved, and other aircraft operating on flight plans requiring special handling by ATC.

2. 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.

c. Assign the following codes to arriving IFR aircraft, except military turbojet aircraft as specified in para 4-7-4, Radio Frequency and Radar Beacon Changes for Military Aircraft:

FL 180 may be used in lieu of FL 240 where the base of Class A airspace and the base of the operating sector are at FL 180, and for inter-facility handoff the receiving sector is also stratified at FL 180.

1. Code2300 may be assigned for descents while above FL 240.

2. Code1500 may be assigned for descents into and while within the strata below FL 240, or with prior coordination the specific code utilized by the destination controller, or the code currently assigned when descent clearance is issued.

3. The applicable en route code for the holding altitude if holding is necessary before entering the terminal area and the appropriate code in subparas 1 or 2.


Assign codes to emergency aircraft as follows:

a. Code 7700 when the pilot declares an emergency and the aircraft is not radar identified.


b. After radio and radar contact have been established, you may request other than single-piloted helicopters and single-piloted turbojet aircraft to change from Code 7700 to another code appropriate for your radar beacon code environment.

1. The code change, based on pilot concurrence, the nature of the emergency, and current flight conditions will signify to other radar facilities that the aircraft in distress is identified and under ATC control.

2. Pilots of single-piloted helicopters and single-piloted turbojet aircraft may be unable to reposition transponder controls during the emergency.


c. The following shall be accomplished on a Mode C equipped VFR aircraft which is in emergency but no longer requires the assignment of Code 7700:

1. TERMINAL. Assign a beacon code that will permit terminal minimum safe altitude warning (MSAW) alarm processing.

2. EN ROUTE. An appropriate keyboard entry shall be made to ensure en route MSAW (EMSAW) alarm processing.


When you observe a Code 7600 display, apply the procedures in para 10-4-4, Communications Failure.

Should a transponder-equipped aircraft experience a loss of two-way radio communications capability, the pilot can be expected to adjust his/her transponder to Code 7600.


a. For VFR aircraft receiving radar advisories, assign an appropriate function code or computer-assigned code for the code environment in which you are providing service.

1. Para 5-2-2, Discrete Environment; para 5-2-3, Nondiscrete Environment, and para 5-2-4, Mixed Environment, specify code assignment procedures to follow for the three code environments.

2. Para 5-2-6, Function Code Assignments, specifies the function code allocation from which an appropriate code for the aircraft indicated in subpara a should be selected. In the terminal environment, additional function codes may be authorized by the appropriate service area office.

1. If the aircraft is outside of your area of responsibility and an operational benefit will be gained by retaining the aircraft on your frequency for the purpose of providing services, ensure that coordination has been effected:

(a) As soon as possible after positive identification, and

(b) Prior to issuing a control instruction or providing a service other than a safety alert/traffic advisory.

Safety alerts/traffic advisories may be issued to an aircraft prior to coordination if an imminent situation may be averted by such action. Coordination should be effected as soon as possible thereafter.

b. Instruct IFR aircraft which cancel an IFR flight plan and are not requesting radar advisory service and VFR aircraft for which radar advisory service is being terminated to squawk the VFR code.



SQUAWK 1200.

1. Aircraft not in contact with an ATC facility may squawk 1255 in lieu of 1200 while en route to/from or within the designated fire fighting area(s).

2. VFR aircraft which fly 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.

c. When an aircraft changes from VFR to IFR, the controller shall assign a beacon code to Mode C equipped aircraft that will allow MSAW alarms.


a. Mode 3/A, Code 4400, and discrete Codes 4440 through 4465 are reserved for use by R-71, F-12, U-2, B-57, pressure suit flights, and aircraft operations above FL 600.

The specific allocation of the special use codes in subset 4400 is in FAAO 7110.66, National Beacon Code Allocation Plan.

b. Ensure that aircraft remain on Code 4400 or one of the special use discrete codes in the 4400 subset if filed as part of the flight plan. Except when unforeseen events, such as weather deviations, equipment failure, etc., cause more than one aircraft with same Mode 3/A discrete beacon codes to be in the same or adjacent ARTCC's airspace at the same time, a controller may request the pilot to make a code change, squawk standby, or to stop squawk as appropriate.

Due to the inaccessibility of certain equipment to the flight crews, Code 4400 or a discrete code from the 4400 subset is preset on the ground and will be used throughout the flight profile including operations below FL 600. Controllers should be cognizant that not all aircraft may be able to accept the transponder changes identified in the exception. Emergency Code 7700, however, can be activated.



Ensure exercise FAKER aircraft remain on the exercise flight plan filed discrete beacon code.

1. NORAD will ensure exercise FAKER aircraft flight plans are filed containing discrete beacon codes from the Department of Defense code allocation specified in FAAO JO 7610.4, Special Operations, Appendix 6.

2. NORAD will ensure that those FAKER aircraft assigned the same discrete beacon code are not flight planned in the same or any adjacent ARTCC's airspace at the same time. (Simultaneous assignment of codes will only occur when operational requirements necessitate.)


You may instruct an aircraft operating on an assigned code to change transponder to “standby” or “low sensitivity” position:

National standards no longer require improved transponder to be equipped with the low sensitivity feature. Therefore, aircraft with late model transponders will be unable to respond to a request to “squawk low.”

a. When approximately 15 miles from its destination and you no longer desire operation of the transponder.

b. When necessary to reduce clutter in a multi-target area, or to reduce “ring-around” or other phenomena, provided you instruct the aircraft to return to “normal sensitivity” position as soon as possible thereafter.





Continuously monitor the Mode 3/A radar beacon codes assigned for use by aircraft operating within your area of responsibility when nonautomated beacon decoding equipment (e.g., 10-channel decoder) is used to display the target symbol.

In addition to alphanumeric and control symbology processing enhancements, the MEARTS, STARS, and the TPX-42 systems are equipped with automatic beacon decoders. Therefore, in facilities where the automatic beacon decoders are providing the control slash video, there is no requirement to have the nonautomated decoding equipment operating simultaneously.

a. This includes the appropriate IFR code actually assigned and, additionally, Code 1200, Code 1255, and Code 1277 unless your area of responsibility includes only Class A airspace. During periods when ring-around or excessive VFR target presentations derogate the separation of IFR traffic, the monitoring of VFR Code 1200, Code 1255, and Code 1277 may be temporarily discontinued.

b. Positions of operation which contain a restricted or warning area or VR route within or immediately adjacent to their area of jurisdiction shall monitor Code4000 and any other code used in lieu of 4000 within the warning/restricted area or VR route. If by local coordination with the restricted/warning area or VR route user a code other than 4000 is to be exclusively used, then this code shall be monitored.

c. If a normally assigned beacon code disappears, check for a response on the following codes in the order listed and take appropriate action:

When Codes7500 and/or 7600 have been preselected, it will be necessary for the ID-SEL-OFF switches for these codes to be left in the off position so that beacon target for an aircraft changing to one of these codes will disappear, thereby alerting the controller to make the check. This check will not be required if automatic alerting capability exists.

1. Code 7500 (hijack code).

2. Code 7600 (loss of radio communications code).


a. Inform an aircraft with an operable transponder that the assigned beacon code is not being displayed.

(Identification) RESET TRANSPONDER, SQUAWK (appropriate code).

b. Inform an aircraft when its transponder appears to be inoperative or malfunctioning.


c. Ensure that the subsequent control position in the facility or the next facility, as applicable, is notified when an aircraft transponder is malfunctioning/inoperative.


Inform aircraft concerned when the ground interrogator appears to be inoperative or malfunctioning.



Disapprove a request or withdraw previously issued approval to operate in Class A airspace with a failed transponder solely on the basis of traffic conditions or other operational factors.


Ensure that Mode C altitude readouts are valid after accepting an interfacility handoff, initial track start, track start from coast/suspend tabular list, missing, or unreasonable Mode C readouts. For TPX-42 and equivalent systems ensure that altitude readout is valid immediately after identification. (TCDD-/BANS-equipped tower cabs are not required to validate Mode C readouts after receiving interfacility handoffs from TRACONs according to the procedures in para 5-4-3, Methods, subpara a4.)

a. Consider an altitude readout valid when:

1. It varies less than 300 feet from the pilot reported altitude, or

(If aircraft is known to be operating below the lowest useable flight level),



(If aircraft is known to be operating at or above the lowest useable flight level),


2. You receive a continuous readout from an aircraft on the airport and the readout varies by less than 300 feet from the field elevation, or

A continuous readout exists only when the altitude filter limits are set to include the field elevation.

3. You have correlated the altitude information in your data block with the validated information in a data block generated in another facility (by verbally coordinating with the other controller) and your readout is exactly the same as the readout in the other data block.

b. When unable to validate the readout, do not use the Mode C altitude information for separation.

c. Whenever you observe an invalid Mode C readout below FL 180:

1. Issue the correct altimeter setting and confirm the pilot has accurately reported the altitude.

(Location) ALTIMETER (appropriate altimeter), VERIFY ALTITUDE.

2. If the altitude readout continues to be invalid:

(a) Instruct the pilot to turn off the altitude- reporting part of his/her transponder and include the reason; and

(b) Notify the operations supervisor-in-charge of the aircraft call sign.


d. Whenever you observe an invalid Mode C readout at or above FL 180, unless the aircraft is descending below Class A airspace:

1. Confirm that the pilot is using 29.92 inches of mercury as the altimeter setting and has accurately reported the altitude.


(If aircraft is known to be operating at or above the lowest useable flight level),


2. If the Mode C readout continues to be invalid:

(a) Instruct the pilot to turn off the altitude- reporting part of his/her transponder and include the reason; and

(b) Notify the operational supervisor-in-charge of the aircraft call sign.


e. Whenever possible, inhibit altitude readouts on all consoles when a malfunction of the ground equipment causes repeated invalid readouts.


Request a pilot to confirm assigned altitude on initial contact unless:

For the purpose of this paragraph, “initial contact” means a pilot's first radio contact with each sector/position.

a. The pilot states the assigned altitude, or

b. You assign a new altitude to a climbing or a descending aircraft, or

c. The Mode C readout is valid and indicates that the aircraft is established at the assigned altitude, or

d. TERMINAL. The aircraft was transferred to you from another sector/position within your facility (intrafacility).

(In level flight situations),VERIFY AT (altitude/flight level).

(In climbing/descending situations),

(if aircraft has been assigned an altitude below the lowest useable flight level),



(If aircraft has been assigned a flight level at or above the lowest useable flight level),



a. Request a pilot to confirm assigned altitude on initial contact unless:

For the purpose of this paragraph, “initial contact” means a pilot's first radio contact with each sector/position.

1. The pilot states the assigned altitude, or

2. You assign a new altitude to a climbing or a descending aircraft, or

3. TERMINAL. The aircraft was transferred to you from another sector/position within your facility (intrafacility).

(In level flight situations),VERIFY AT (altitude/flight level).

(In climbing/descending situations),VERIFY ASSIGNED ALTITUDE/FLIGHT LEVEL (altitude/flight level).

b. USA. Reconfirm all pilot altitude read backs.

(If the altitude read back is correct),

AFFIRMATIVE (altitude).

(If the altitude read back is not correct),





Inform an aircraft when you want it to turn on/off the automatic altitude reporting feature of its transponder.




Controllers should be aware that not all aircraft have a capability to disengage the altitude squawk independently from the beacon code squawk. On some aircraft both functions are controlled by the same switch.


Apply the following procedures to requests to deviate from the Mode C transponder requirement by aircraft operating in the airspace of the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL and below 18,000 feet MSL, excluding the airspace at and below 2,500 feet AGL.

1. 14 CFR Section 91.215(b) provides, in part, that all U.S. registered civil aircraft must be equipped with an operable, coded radar beacon transponder when operating in the altitude stratum listed above. Such transponders shall have a Mode 3/A 4096 code capability, replying to Mode 3/A interrogation with the code specified by ATC, or a Mode S capability, replying to Mode 3/A interrogations with the code specified by ATC. The aircraft must also be equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments.

2. The exception to 14 CFR Section 91.215 (b) is 14 CFR Section 91.215(b)(5) which states: except balloons, gliders, and aircraft without engine-driven electrical systems.

a. Except in an emergency, do not approve inflight requests for authorization to deviate from 14 CFR Section 91.215(b)(5)(i) requirements originated by aircraft without transponder equipment installed.

b. Approve or disapprove other inflight deviation requests, or withdraw approval previously issued to such flights, solely on the basis of traffic conditions and other operational factors.

c. Adhere to the following sequence of action when an inflight VFR deviation request is received from an aircraft with an inoperative transponder or Mode C, or is not Mode C equipped:

1. Suggest that the aircraft conduct its flight in airspace unaffected by the CFRs.

2. Suggest that the aircraft file an IFR flight plan.

3. Suggest that the aircraft provide a VFR route of flight and maintain radio contact with ATC.

d. Do not approve an inflight deviation unless the aircraft has filed an IFR flight plan or a VFR route of flight is provided and radio contact with ATC is maintained.

e. You may approve an inflight deviation request which includes airspace outside your jurisdiction without the prior approval of the adjacent ATC sector/facility providing a transponder/Mode C status report is forwarded prior to control transfer.

f. Approve or disapprove inflight deviation requests within a reasonable period of time or advise when approval/disapproval can be expected.


Inform an aircraft when you want it to turn off its transponder.



(For a military aircraft when you do not know if the military service requires that it continue operating on another mode),

STOP SQUAWK (mode in use).



Set altitude filters to display Mode C altitude readouts to encompass all altitudes within the controller's jurisdiction. Set the upper limits no lower than 1,000 feet above the highest altitude for which the controller is responsible. In those stratified positions, set the lower limit to 1,000 feet or more below the lowest altitude for which the controller is responsible. When the position's area of responsibility includes down to an airport field elevation, the facility will normally set the lower altitude filter limit to encompass the field elevation so that provisions of para 2-1-6, Safety Alert, and para 5-2-17, Validation of Mode C Readout, subpara a2 may be applied. Air traffic managers may authorize temporary suspension of this requirement when target clutter is excessive.

The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. ATCRBS assists ATC surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft (known as targets) and to distinguish those returns from ground clutter.

Parts of the system[edit]

The system consists of transponders, installed in aircraft, and secondary surveillance radars (SSRs), installed at air traffic control facilities. The SSR is sometimes co-located with the primary surveillance radar, or PSR. These two radar systems work in conjunction to produce a synchronized surveillance picture. The SSR transmits interrogations and listens for any replies. Transponders that receive an interrogation decode it, decide whether to reply, and then respond with the requested information when appropriate. Note that in common informal usage, the term "SSR" is sometimes used to refer to the entire ATCRBS system, however this term (as found in technical publications) properly refers only to the ground radar itself.

Ground Interrogation Equipment[edit]

An ATC ground station consists of two radar systems and their associated support components. The most prominent component is the PSR. It is also referred to as skin paint radar because it shows not synthetic or alpha-numeric target symbols, but bright (or colored) blips or areas on the radar screen produced by the RF energy reflections from the target's "skin." This is a non-cooperative process, no additional avionic devices are needed. The radar detects and displays reflective objects within the radar's operating range. Weather radar data is displayed in skin paint mode. The primary surveillance radar is subject to the radar equation that says signal strength drops off as the fourth power of distance to the target. Objects detected using the PSR are known as primary targets.

The second system is the secondary surveillance radar, or SSR, which depends on a cooperating transponder installed on the aircraft being tracked. The transponder emits a signal when it is interrogated by the secondary radar. In a transponder based system signals drop off as the inverse square of the distance to the target, instead of the fourth power in primary radars. As a result, effective range is greatly increased for a given power level. The transponder can also send encoded information about the aircraft, such as identity and altitude.

The SSR is equipped with a main antenna, and an omnidirectional "Omni" antenna at many older sites. Newer antennas (as in the adjacent picture), are grouped as a left and right antenna, and each side connects to a hybrid device which combines the signals into sum and difference channels. Still other sites have both the sum and difference antenna, and an Omni antenna. Surveillance aircraft, e.g. AWACS, have only the sum and difference antennas, but can also be space stabilized by phase shifting the beam down or up when pitched or rolled. The SSR antenna is typically fitted to the PSR antenna, so they point in the same direction as the antennas rotate. The omnidirectional antenna is mounted near and high, usually on top of the radome if equipped. Mode-S interrogators require the sum and difference channels to provide the monopulse capability to measure the off-boresight angle of the transponder reply.

The SSR repetitively transmits interrogations as the rotating radar antenna scans the sky. The interrogation specifies what type of information a replying transponder should send by using a system of modes. There have been a number of modes used historically, but four are in common use today: mode 1, mode 2, mode 3/A, and mode C. Mode 1 is used to sort military targets during phases of a mission. Mode 2 is used to identify military aircraft missions. Mode 3/A is used to identify each aircraft in the radar's coverage area. Mode C is used to request/report an aircraft's altitude.

Two other modes, mode 4 and mode S, are not considered part of the ATCRBS system, but they use the same transmit and receive hardware. Mode 4 is used by military aircraft for the Identification Friend or Foe (IFF) system. Mode S is a discrete selective interrogation, rather than a general broadcast, that facilitates TCAS for civilian aircraft. Mode S transponders ignore interrogations not addressed with their unique identity code, reducing channel congestion. At a typical SSR radar installation, ATCRBS, IFF, and mode S interrogations will all be transmitted in an interlaced fashion. Some military facilities and/or aircraft will also utilize Mode S.

Returns from both radars at the ground station are transmitted to the ATC facility using a microwave link, a coaxial link, or (with newer radars) a digitizer and a modem. Once received at the ATC facility, a computer system known as a radar data processor associates the reply information with the proper primary target and displays it next to the target on the radar scope.

Airborne Transponder Equipment[edit]

The equipment installed in the aircraft is considerably simpler, consisting of the transponder itself, usually mounted in the instrument panel or avionics rack, and a small L bandUHF antenna, mounted on the bottom of the fuselage. Many commercial aircraft also have an antenna on the top of the fuselage, and either or both antennas can be selected by the flight crew.

Typical installations also include an altitude encoder, which is a small device connected to both the transponder and the aircraft's static system. It provides the aircraft's pressure altitude to the transponder, so that it may relay the information to the ATC facility. The encoder uses 11 wires to pass altitude information to the transponder in the form of a Gillham Code, a modified binary Gray code.

The transponder has a small required set of controls and is simple to operate. It has a method to enter the four-digit transponder code, also known as a beacon code or squawk code, and a control to transmit an ident, which is done at the controller's request (see SPI pulse below). Transponders typically have 4 operating modes: Off, Standby, On (Mode-A), and Alt (Mode-C). On and Alt mode differ only in that the On mode inhibits transmitting any altitude information. Standby mode allows the unit to remain powered and warmed up but inhibits any replies, since the radar is used for searching the aircraft and exact location of aircraft.

Theory of operation[edit]

The steps involved in performing an ATCRBS interrogation are as follows: First, the ATCRBS interrogator periodically interrogates aircraft on a frequency of 1030 MHz. This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF). Interrogations are typically performed at 450 - 500 interrogations/second. Once an interrogation has been transmitted, it travels through space (at the speed of light) in the direction the antenna is pointing until an aircraft is reached.

When the aircraft receives the interrogation, the aircraft transponder will send a reply on 1090 MHz after a 3.0 μs delay indicating the requested information. The interrogator's processor will then decode the reply and identify the aircraft. The range of the aircraft is determined from the delay between the reply and the interrogation. The azimuth of the aircraft is determined from the direction the antenna is pointing when the first reply was received, until the last reply is received. This window of azimuth values is then divided by two to give the calculated "centroid" azimuth. The errors in this algorithm cause the aircraft to jitter across the controllers scope, and is referred to as "track jitter." The jitter problem makes software tracking algorithms problematic, and is the reason why monopulse was implemented.

The interrogation[edit]

Interrogations consist of three pulses, 0.8 μs in duration, referred to as P1, P2 and P3. The timing between pulses P1 and P3 determines the mode (or question) of the interrogation, and thus what the nature of the reply should be. P2 is used in side-lobe suppression, explained later.

Mode 3/A uses a P1 to P3 spacing of 8.0 μs, and is used to request the beacon code, which was assigned to the aircraft by the controller to identify it. Mode C uses a spacing of 21 μs, and requests the aircraft's pressure altitude, provided by the altitude encoder. Mode 2 uses a spacing of 5 μs and requests the aircraft to transmit its Military identification code. The latter is only assigned to Military aircraft and so only a small percentage of aircraft actually reply to a mode 2 interrogation.

The reply[edit]

Replies to interrogations consist of 15 time slots, each 1.45 μs in width. The reply is encoded by the presence or absence of a 0.45 μs pulse in each slot. These are labeled as follows:

F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 SPI

The F1 and F2 pulses are framing pulses, and are always transmitted by the aircraft transponder. They are used by the interrogator to identify legitimate replies. These are spaced 20.3 μs apart.

The A4, A2, A1, B4, B2, B1, C4, C2, C1, D4, D2, D1 pulses constitute the "information" contained in the reply. These bits are used in different ways for each interrogation mode.

For mode A, each digit in the transponder code (A, B, C, or D) may be a number from zero to seven. These octal digits are transmitted as groups of three pulses each, the A slots reserved for the first digit, B for the second, and so on.

In a mode C reply, the altitude is encoded by a Gillham interface, Gillham code, which uses Gray code. The Gillham interface is capable of representing a wide range of altitudes, in 100-foot (30 m) increments. The altitude transmitted is pressure altitude, and corrected for altimeter setting at the ATC facility. If no encoder is attached, the transponder may optionally transmit only framing pulses (most modern transponders do).

In a mode 3 reply, the information is the same as a mode A reply in that there are 4 digits transmitted between 0 and 7. The term mode 3 is utilized by the military, whereas mode A is the civilian term.

The X bit is currently only used for test targets. This bit was originally transmitted by BOMARC missiles that were used as air-launched test targets. This bit may be used by drone aircraft.

The SPI pulse is positioned 4.35μs past the F2 pulse (3 time slots) and is used as a "Special Identification Pulse". The SPI pulse is turned on by the "identity control" on the transponder in the aircraft cockpit when requested by air traffic control. The air traffic controller can request the pilot to ident, and when the identity control is activated, the SPI bit will be added to the reply for about 20 seconds (two to four rotations of the interrogator antenna) thereby highlighting the track on the controllers display.


The SSR's directional antenna is never perfect; inevitably it will "leak" lower levels of RF energy in off-axis directions. These are known as side lobes. When aircraft are close to the ground station, the side lobe signals are often strong enough to elicit a reply from their transponders when the antenna is not pointing at them. This can cause ghosting, where an aircraft's target may appear in more than one location on the radar scope. In extreme cases, an effect known as ring-around occurs, where the transponder replies to excess resulting in an arc or circle of replies centered on the radar site.

To combat these effects, side lobe suppression (SLS) is used. SLS employs a third pulse, P2, spaced 2μs after P1. This pulse is transmitted from the omnidirectional antenna (or the antenna difference channel) by the ground station, rather than from the directional antenna (or the sum channel). The power output from the omnidirectional antenna is calibrated so that, when received by an aircraft, the P2 pulse is stronger than either P1 or P3, except when the directional antenna is pointing directly at the aircraft. By comparing the relative strengths of P2 and P1, airborne transponders can determine whether or not the antenna is pointing at the aircraft when the interrogation was received. The power to the difference antenna pattern (for systems so equipped) is not adjusted from that of the P1 and P3 pulses. Algorithms are used in the ground receivers to delete replies on the edge of the two beam patterns.

To combat these effects more recently, side lobe suppression (SLS) is still used, but differently. The new and improved SLS employs a third pulse, spaced 2μs either before P3 (a new P2 position) or after P3 (which should be called P4 and appears in the Mode S radar and TCAS specifications). This pulse is transmitted from the directional antenna at the ground station, and the power output of this pulse is the same strength as the P1 and P3 pulses. The action to be taken is specified in the new and improved C74c as:

2.6 Decoding Performance.

c. Side-lobe Suppression. The transponder must be suppressed for a period of 35 ±10 microseconds following receipt of a pulse pair of proper spacing and suppression action must be capable of being reinitiated for the full duration within 2 microseconds after the end of any suppression period. The transponder must be suppressed with a 99 percent efficiency over a received signal amplitude range between 3 db above minimum triggering level and 50 db above that level and upon receipt of properly spaced interrogations when the received amplitude of P2 is equal to or in excess of the received amplitude of P1 and spaced 2.0 ±0.15 microsecond from P3.

Any requirement at the transponder to detect and act upon a P2 pulse 2μs after P1 has been removed from the new and improved TSO C74c specification.

Most "modern" transponders (manufactured since 1973) have an "SLS" circuit which suppresses reply on receipt of any two pulses in any interrogation spaced 2.0 microseconds apart that are above the MTL Minimum Triggering Level threshold of the receiver amplitude descriminator (P1->P2 or P2->P3 or P3->P4). This approach was used to comply with the original C74c and but also complies with the provisions of the new and improved C74c.

The FAA refers to the non-responsiveness of new and improved TSO C74c compliant transponders to Mode S compatible radars and TCAS as "The Terra Problem", and has issued Airworthiness Directives (ADs) against various transponder manufacturers, over the years, at various times on no predictable schedule. The ghosting and ring around problems have recurred on the more modern radars.

To combat these effects most recently, great emphasis is placed upon software solutions. It is highly likely that one of those software algorithms was the proximate cause of a mid-air collision recently, as one airplane was reported at showing its altitude as the pre-flight paper filed flight plan, and not the altitude assigned by the ATC controller (see the reports and observations contained in the below reference ATC Controlled Airplane Passenger Study of how radar worked).

See the reference section below for errors in performance standards for ATCRBS transponders in the US.

See the reference section below for FAA Technician Study of in-situ transponders.

Radar display[edit]

The beacon code and altitude were historically displayed verbatim on the radar scope next to the target, however modernization has extended the radar data processor with a flight data processor, or FDP. The FDP automatically assigns beacon codes to flight plans, and when that beacon code is received from an aircraft, the computer can associate it with flight plan information to display immediately useful data, such as aircraft callsign, the aircraft's next navigational fix, assigned and current altitude, etc. near the target in a data block. Although the ATCRBS does not display aircraft heading.[1]

Mode S[edit]

See also: Secondary surveillance radar § Mode S

Mode S, or mode select, despite also being called a mode, is actually a radically improved system intended to replace ATCRBS altogether. A few countries have mandated mode S, and many other countries, including the United States, have begun phasing out ATCRBS in favor of this system. Mode S is designed to be fully backward compatible with existing ATCRBS technology.

Mode S, despite being called a replacement transponder system for ATCRBS, is actually a data packet protocol which can be used to augment ATCRBS transponder positioning equipment (radar and TCAS).

One major improvement of Mode S is the ability to interrogate a single aircraft at a time. With old ATCRBS technology, all aircraft within the beam pattern of the interrogating station will reply. In an airspace with multiple interrogation stations, ATCRBS transponders in aircraft can be overwhelmed. By interrogating one aircraft at a time, workload on the aircraft transponder is greatly reduced.

The second major improvement is increased azimuth accuracy. With PSRs and old SSRs, azimuth of the aircraft is determined by the half split (centroid) method. The half split method is computed by recording the azimuth of the first and last replies from the aircraft, as the radar beam sweeps past its position. Then the midpoint between the start and stop azimuth is used for aircraft position. With MSSR (monopulse secondary surveillance radar) and Mode S, the radar can use the information of one reply to determine azimuth. This is calculated based on the RF phase of the aircraft reply, as determined by the sum and difference antenna elements, and is called monopulse. This monopulse method results in superior azimuth resolution, and removes target jitter from the display.

The Mode S system also includes a more robust communications protocol, for a wider variety of information exchange. At this time, this capability is becoming mandatory across Europe with some states already requiring its use.

See also[edit]


External links[edit]

The antenna system of a typical ground radar. The ladder-like top section is the SSR directional antenna, and the remainder of the assembly makes up the PSR antenna.

A light aircraft transponder
Detail of a radarscreen: a target in skin paint mode (amber) and as synthetic video (white)