Several different RF communication protocols have been standardized for aviation transponders:
§ Mode 1 – provides 2-digit 5-bit mission code. (military only – cockpit selectable)
§ Mode 2 – provides 4-digit octal unit code. (military only – set on ground for fighters, can be changed in flight by transport aircraft)
§ Mode 3/A – provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller. (military and civilian)
§ Mode 4 – provides a 3-pulse reply to crypto coded challenge. (military only)
§ Mode C – provides 4-digit octal code for aircraft's pressure altitude. (military and civilian)
§ Mode S – provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian)
Mode A and Mode C
When the transponder receives a radar signal it sends back a transponder code (or "squawk code"). This is referred to as "Mode 3A" or more commonly "Mode A". A transponder code can be paired with pressure altitude information, which is called "Mode C".
Mode 3A and C are used to help air traffic controllers to identify the aircraft and to maintain separation.
Another mode called Mode S (Selective) is designed to help avoiding overinterrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are compatible with Modes A & C. This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B (Automatic dependent surveillance-broadcast) systems function.
Mode S is mandatory in controlled airspace in many countries. Some countries require that all aircraft be equipped with Mode S, even in uncontrolled airspace. However in the field of general aviation, there have been objections to these moves, because of the cost, size, limited benefit to the users in uncontrolled airspace, and, in the case of balloons and gliders, the power requirements for these aircraft that have limited electrical power.
Mode S features
Mode S transponders broadcast information about the aircraft to the Secondary Surveillance Radar (SSR) system, TCAS receivers on board aircraft and to theADS-B SSR system. This information includes the call sign of the aircraft and/or the transponder's permanent ICAO 24-bit address in the form of a hex code.
ICAO 24-bit address
All modern aircraft are assigned a unique ICAO 24-bit address or (informally) Mode-S "hex code" upon national registration and this address becomes a part of the aircraft's Certificate of Registration. Normally, the address is never changed, however, the transponders are reprogrammable and, occasionally, are moved from one aircraft to another (presumably for operational or cost purposes), either by maintenance or by changing the appropriate entry in the aircraft's FMS system.
There are 16,777,214 unique ICAO 24-bit addresses (hex codes) available. The ICAO 24-bit address can be represented in three digital formats:hexadecimal, octal, and binary. These addresses can be decoded and converted amongst each other online using tools such as those at Airframes.org andKloth.net to obtain the aircraft's tail number. Whichever format is used, the same information is carried through the signal.
Example of an ICAO 24-bit address:
§ Hexadecimal: AC82EC
§ Decimal: 11305708 (Note: rarely used format)
§ Octal: 53101354
§ Binary: 101011001000001011101100 (Note: occasionally, spaces are added for visual clarity, thus 10101100 10000010 11101100)
Mode S TIS Secondary surveillance radar (SSR), or Traffic Information Service, allows a radar installation to send information about nearby traffic back to aircraft, which then displays it on the moving map. Mode S TIS is only available when the aircraft is within radar range of a radar installation that supports it. A Mode S TIS installation combines a Mode S transponder that sends TIS data to a display device – usually a GPS device or Multi Function Display (MFD). Examples of such pairings are a Garmin GTX330D transponder and a GNS530 GPS, and the Garmin G1000 avionics suite.
Mode S capable transponders are also a building block for next generation air traffic control systems, as they can be used to transmit location information forADS-B and potentially other air traffic control communications. Currently the FAA is deactivating several Mode S TIS equipped stations.
Issues with Mode S transponders
One major issue with Mode S transponders is that pilots have frequently been entering the wrong "flight identity" (i.e. call sign) into their Mode S transponders. In this case, the capabilities of ACAS II and Mode S SSR can be degraded
A pilot may be requested to squawk a given code by the air traffic controller via the radio, using a phrase such as "Cessna 123AB, squawk 0363". The pilot then selects the 0363 code on their transponder and the track on the radar screen of the air traffic controller will become correctly associated with their identity.
Because primary radar generally gives bearing and range position information, but lacks altitude information, mode C and mode S transponders also report pressure altitude. Around busy airspace there is often a regulatory requirement that all aircraft be equipped with an altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil. Mode S transponders are compatible with transmitting the mode C signal, and have the capability to report in 25 foot increments. Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and must rely on the altitude reported by the pilot via radio. This limitation has resulted in at least one accident. On 31 August 1986, a Piper Archer with a pilot and two passengers had inadvertently penetrated the 6,000-foot floor of controlled airspace without a clearance and collided with Aeromexico Flight 498, a DC-9 with 58 passengers and 6 crew at an altitude of 6,650 feet. The Archer had only a mode A squawk reporting capability and the air traffic controller assumed it was below the controlled airspace.
All mode A, C, and S transponders include an "ident" button, which activates a special "thirteenth" bit on the mode A reply known as Ident, short for Identify. When radar equipment receives the Ident bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot (i.e. "Cessna 123AB, squawk 0363 and ident").
Ident can also be used in case of a reported or suspected radio failure to determine if the failure is only one way and whether the pilot can still transmit or receive but not both (i.e. "Cessna 123AB, if you read, squawk ident").
Transponder codes are four digit numbers transmitted by the transponder in an aircraft in response to a secondary surveillance radar interrogation signal to assist air traffic controllers in traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to uniquely identify an aircraft. This allows easy identity of the aircraft on radar.
Squawk codes are four-digit octal numbers; the dials on a transponder read from zero to seven inclusive. Thus the lowest possible squawk is 0000 and the highest is 7777. There are 4096 combinations of these four digit codes, which is why they are often called "4096 code transponders." Because these squawks are sensitive, care must be taken not to squawk any emergency code during a code change. For example, when changing from 1200 to 6501 (an assigned ATC squawk), one might turn the second wheel to a 5 (thus 1500), and then rotate the first wheel backwards in the sequence 1-0-7-6 to get to 6. This would momentarily have the transponder squawking a hijack code (7500), which might lead to more attention than one desires. Pilots are instructed not to place the transponder in "standby mode" while changing the codes as it causes the loss of target information on the ATC radar screen, but instead to carefully change codes to avoid inadvertently selecting an emergency code. Additionally, modern digital transponders are operated by buttons to avoid this problem
There are other codes known as 'conspicuity codes' which are not necessarily unique to a particular aircraft, but may have their own meaning and are used to convey information about the aircraft to ATC, possibly when the aircraft is not in radio contact.
The use of the word "squawk" comes from the system's origin in the World War II Identification Friend or Foe (IFF) system, which was code-named "Parrot". Parrot today generally refers to IFF only. The parrot check is generally done as part of the last-chance inspection at the runway, or after becoming airborne. Parrot sweet, and parrot sour are given, and the aircraft will have to abort in a real-world mission when sour, or face being attacked by friendly forces. Modern use of the word Parrot refers to a test transponder located at a fixed location off the radar facility. The parrot verifies range and direction accuracy of the radar facility.
§ military intercept code (in the US)
§ mode C or other SSR failure (in the UK).
§ shall not be used – is a non-discrete mode A code (Europe)
§ 0001: Military code for high speed uncontrolled (non-ATC directed) flight (US)
§ 0033: Parachute dropping in progress (UK)
§ Instrument Flight Rules (IFR) flight below 18,000' when no other code has been assigned (Canada)
§ non-discrete mode A code reserved for future use in Mode S radar environment where the aircraft identification will be used to correlate the flight plan instead of the mode A code
§ 1200: Visual flight rules (VFR) flight, this is the standard squawk code used in Australian and North American airspace when no other has been assigned.
§ 1400: VFR flight above 12,500'ASL when no other code has been assigned (Canada)
§ 2000: The code to be squawked when entering a secondary surveillance radar (SSR) area from a non-SSR area (used as Uncontrolled IFR flight squawk code in ICAO countries and in Canada for uncontrolled IFR at or above 18,000')
§ 4000: Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (US)
§ VFR standard squawk code when no other code has been assigned (ICAO)
§ UK: this code does not imply VFR; 7000 is used as a general conspicuity squawk.)
§ Sudden military climb out from low-level operations (UK)
§ Used in some countries to identify VFR traffic (France, ...)
§ 7004: Aerobatic and display code in some countries.
§ 7010: VFR circuit traffic code in the UK
§ 707X: Paradrop activities in France (7070, 7071, 7072...)
§ military interception (US) ("Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.")
§ non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE). (US, Germany, Netherlands, Belgium, ...)
In Belgium following codes are assigned for VFR traffic under Flight Information Services (BXL FIC)
§ series from 0041 till 0057
In Germany the following codes have been used:
§ 0021: VFR squawk code for German airspace (5000 feet and below)
§ 0022: VFR squawk code for German airspace (above 5000 feet)
As from 15 March 2007 these have been replaced by the international 7000 code for VFR traffic.
§ 7700: General Emergency
§ 7600: Lost Communications
§ 7500: Unlawful Interference (Hijack)
Codes assigned by ATC
Most codes above can be selected by aircraft if and when the situation requires or allows it, without permission from ATC. Other codes are generally assigned by ATC units. For IFR flights, the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight. VFR flights, when in uncontrolled airspace, will "squawk VFR" (1200 in the US, 7000 in Europe). Upon contact with an ATC unit, they will be told to squawk a certain unique code. When changing frequency, for instance because the VFR flight leaves controlled airspace or changes to another ATC unit, the VFR flight will be told to "squawk VFR" again.
In order to avoid confusion over assigned squawk codes, ATC units will typically be allocated blocks of squawk codes, not overlapping with the blocks of nearby ATC units, to assign at their discretion.
Not all ATC units will use radar to identify aircraft, but they assign squawk codes nevertheless. As an example, London Information – the Flight Information Service station that covers the lower half of the UK – does not have access to radar images, but does assign squawk code 0027 to all aircraft that receive an FIS from them. This tells other radar equipped ATC units that that specific aircraft is listening on the London Information radio frequency, in case they need to get hold of that aircraft.