Aviation transponder interrogation modes
Encyclopedia
An aviation transponder interrogation mode is the format of a sequence of pulses from an interrogating SSR
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

 radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, or similar system, and reply format usually referred to as a "Code" from a transponder
Transponder (aviation)
A transponder is an electronic device that produces a response when it receives a radio-frequency interrogation...

, used to determine detailed information from a suitably equipped aircraft.

In its simplest form, a "Mode" or interrogation type, is generally determined by pulse spacing between two or more interrogation pulses. Various modes exist from Mode 1 to 5 for military use, to Mode A, B, C and D and Mode S for civilian use.

Interrogation modes

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 5 – provides a cryptographically
    Cryptography
    Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

     secured version of Mode S and ADS-B
    Automatic dependent surveillance-broadcast
    Automatic Dependent Surveillance-Broadcast is a surveillance technology for tracking aircraft as part of the Next Generation Air Transportation System ...

     GPS position. (military only)
  • Mode C – provides 4-digit octal code for aircraft's pressure altitude
    Pressure altitude
    In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...

    . (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's 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. Pressure altitude is obtained from an altitude encoder, a separate self-contained unit mounted in the aircraft. The altitude information is passed to the transponder using a modified form of the Gray code called a Gillham Code
Gillham Code
Gillham code is a digital code using an eleven-wire interface that is used to transmit uncorrected barometric altitude between an encoding altimeter or analog air data computer and a transponder...

.

Mode 3A and C are used to help air traffic controllers to identify the aircraft and to maintain separation.

Mode S

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
Airborne Collision Avoidance System
An airborne collision avoidance system is an aircraft system that operates independently of ground-based equipment and air traffic control in warning pilots of the presence of other aircraft that may present a threat of collision. If the risk of collision is imminent, the system indicates a...

) and the ADS-B (Automatic dependent surveillance-broadcast
Automatic dependent surveillance-broadcast
Automatic Dependent Surveillance-Broadcast is a surveillance technology for tracking aircraft as part of the Next Generation Air Transportation System ...

) 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

Upon interrogation, Mode S transponders transmit information about the aircraft to the Secondary Surveillance Radar
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

 (SSR) system, TCAS receivers on board aircraft and to the ADS-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
Aircraft registration
An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile...

. 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 (224-2) unique ICAO 24-bit addresses (hex codes) available. The ICAO 24-bit address can be represented in three digital formats: hexadecimal
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

, octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...

, and binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...

. These addresses can be decoded and converted amongst each other online using tools such as those at Airframes.org and Kloth.net to obtain the aircraft's tail number
Aircraft registration
An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile...

. 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)

(These all correlate to the same aircraft registration
Aircraft registration
An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile...

, N905NA
Shuttle Carrier Aircraft
The Shuttle Carrier Aircraft are two extensively modified Boeing 747 airliners that NASA uses to transport Space Shuttle orbiters...

.)
Other features

Mode S TIS Secondary surveillance radar (SSR)
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

, or Traffic Information Service, allows a radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 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
Garmin G1000
The Garmin G1000 is an integrated flight instrument system manufactured by Garmin typically composed of two display units, one serving as a primary flight display, and one as a multi-function display...

 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 for ADS-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
Call sign
In broadcasting and radio communications, a call sign is a unique designation for a transmitting station. In North America they are used as names for broadcasting stations...

 into their Mode S transponders. In this case, the capabilities of ACAS II and Mode S SSR
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...

can be degraded.
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