The aviation transponder interrogation modes are the standard formats of pulsed sequences from an interrogating

Secondary Surveillance Radar Secondary surveillance radar (SSR)''Secondary Surveillance Radar'', Stevens M.C. Artech House, is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the de ...

(SSR) or similar

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(ADS-B) system. The reply format is usually referred to as a "code" from a

transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...

, which is 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 protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

s have been standardized for aviation transponders: Mode A and Mode C are implemented using air traffic control radar beacon system as the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechani ...

, whereas Mode S is implemented as a standalone backwards-compatible protocol. ADS-B can operate using Mode S-ES or Universal Access Transceiver as its

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end c ...

Mode A

When the transponder receives an interrogation request, it broadcasts the configured transponder code (or " squawk code"). This is referred to as "Mode 3A" or more commonly, Mode A. A separate type of response called "Ident" can be initiated from the airplane by pressing a button on the transponder control panel.

Mode A with Mode C

A Mode A transponder code response can be augmented by a pressure altitude response, which is then referred to as Mode C operation. Pressure altitude is obtained from an altitude encoder, either a separate self-contained unit mounted in the aircraft or an integral part of the transponder. The altitude information is passed to the transponder using a modified form of the modified

Gray code The reflected binary code (RBC), also known as reflected binary (RB) or Gray code after Frank Gray (researcher), Frank Gray, is an ordering of the binary numeral system such that two successive values differ in only one bit (binary digit). For ...

called a Gillham code. Mode A and C responses are used to help air traffic controllers identify a particular aircraft's position and altitude on a radar screen, in order to maintain separation.

Mode S

Another mode called Mode S (Select) 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 Mode A and Mode C

(SSR) systems. This is the type of transponder that is used for TCAS or ACAS II ( Airborne Collision Avoidance System) functions, and is required to implement the extended squitter broadcast, one means of participating in ADS-B systems. A TCAS-equipped aircraft must have a Mode S transponder, but not all Mode S transponders include TCAS. Likewise, a Mode S transponder is required to implement 1090ES extended squitter ADS-B Out, but there are other ways to implement ADS-B Out (in the U.S. and China.) The format of Mode S messages is documented in ICAO Doc 9688, ''Manual on Mode S Specific Services''.

Mode S features

Upon interrogation, Mode S transponders transmit information about the aircraft to the SSR system, to TCAS receivers on board aircraft and to the ADS-B SSR system. This information includes the

call sign In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique identifier for a transmitter station. A call sign can be formally as ...

of the aircraft and/or the aircraft's permanent ICAO 24-bit address (which is represented for human interface purposes as six hexadecimal characters.) One of the hidden features of Mode S transponders is that they are backwards compatible; an aircraft equipped with a Mode S transponder can still be used to send replies to Mode A or C interrogations. This feature can be activated by a specific type of interrogation sequence called inter-mode.

=ICAO 24-bit address

= Mode S equipped 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

Flight management system A flight management system (FMS) is a fundamental component of a modern airliner's avionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that mod ...

. 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 (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...

octal Octal (base 8) is a numeral system with eight as the base. In the decimal system, each place is a power of ten. For example: : \mathbf_ = \mathbf \times 10^1 + \mathbf \times 10^0 In the octal system, each place is a power of eight. For ex ...

, and binary. These addresses are used to provide a unique identity normally allocated to an individual aircraft or registration. As an example, following is the ICAO 24-bit address assigned to the

Shuttle Carrier Aircraft The Shuttle Carrier Aircraft (SCA) are two extensively modified Boeing 747 airliners that NASA used to transport Space Shuttle orbiters. One (N905NA) is a 747-100 model, while the other (N911NA) is a short-range 747-100SR. Both are now retired. ...

with the registration N905NA: * Hexadecimal: AC82EC * Octal: 53101354 * Binary: 101011001000001011101100 ''(Note: occasionally, spaces are added for visual clarity, thus 1010 1100 1000 0010 1110 1100 and 001 101 110 100 000 100 110 101 )'' * Decimal: 11305708 These are all the same 24-bit address of the Shuttle Carrier Aircraft, represented in different numeral systems (see above).

Issues with Mode S transponders

An issue with Mode S transponders arises when pilots enter the wrong flight identity code into the Mode S transponder. In this case, the capabilities of ACAS II and Mode S SSR can be degraded.

Extended squitter

In 2009 the ICAO published an "extended" form of Mode S with more message formats to use with ADS-B; it was further refined in 2012. Countries implementing ADS-B can require the use of either the extended squitter mode of a suitably-equipped Mode S transponder, or the UAT transponder on 978 MHz.

Use in meteorology

Mode-S data has the potential to contain the aircraft's movement vectors in relation to the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and its atmosphere. The difference between these two vectors is the wind acting on the aircraft. Deriving winds (and temperatures from the

Mach number The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. It is named after the Austrian physicist and philosopher Erns ...

and

true airspeed The true airspeed (TAS; also KTAS, for ''knots true airspeed'') of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Tra ...

) was developed simultaneously by Siebren de Haan of the KNMI and Edmund Stone of the

Met Office The Met Office, until November 2000 officially the Meteorological Office, is the United Kingdom's national weather and climate service. It is an executive agency and trading fund of the Department for Science, Innovation and Technology and ...

. Over the UK the number of aircraft observations has increased from approximately 7500 per day from AMDAR to over 10 million per day. The Met Office together with KNMI and FlightRadar24 are actively developing an expanded capability including data from every continent other than Antarctica.

References

{{Reflist, 30em Radar Microwave technology Measuring instruments Navigational equipment Air traffic control