From primary radar to digital response system
Until the 1970s, air traffic controllers worked exclusively with primary radar. This emits electromagnetic pulses that are reflected by aircraft and displayed as dots on the radar screen. Although this method allows position and movement to be recognised, it does not provide any information about the type of aircraft, altitude or registration number. It was only with the introduction of secondary radar - and thus transponders - that the identification and separation of air traffic became much more precise.
The word Transponder consists of Transmit (send) and Responder (respond) together. As soon as the ground radar transmits an interrogation signal on the 1030 MHz frequency, the aircraft responds on 1090 MHz with a data set. Depending on the transponder type, this contains information such as the set code, barometric altitude, climb or descent rate and, in the case of modern devices, even GPS position and speed. This allows air traffic control to uniquely identify each aircraft and monitor its flight situation in real time.
The different transponder modes at a glance
The development of transponder technology proceeded in several stages, which are summarised in the so-called Modes reflect:
Mode A was the first standard. Here, the aircraft only transmits the four-digit numerical code that the pilot has previously set.
Mode C supplements the altitude transmission. The aircraft also transmits its barometric altitude, which is essential for vertical staggering.
Fashion S („Selective“) is the European standard today. It allows the targeted interrogation of individual aircraft and also transmits a globally unique 24-bit address, which is assigned to the respective aircraft registration number in the ICAO database. In addition, data such as selected autopilot altitudes or vertical speeds can be transmitted.
The latest expansion is ADS-B Out (Automatic Dependent Surveillance - Broadcast). Here, a GPS receiver is added to the transponder so that it continuously transmits its own position, speed and flight direction to other participants and ground stations. This not only provides air traffic control with better information, but also allows pilots working with traffic displays or tablets to see the surrounding traffic. ADS-B also forms the technical basis of many collision warning systems (e.g. PilotAware, SkyEcho, PowerFLARM).
Standardised squawk codes - the language of airspace
Every aircraft in the monitored airspace sends a four-digit code, the so-called Squawk. This code is assigned by air traffic control or, in the case of uncontrolled flights, is set according to fixed standards. Important examples:
- 7000Standard code for VFR flights (visual flight) in Europe
- 2000Standard code for IFR flights (instrument flight) if no individual code has been assigned
- 7500: Aircraft hijacking (Seven-five - men with knife)
- 7600: Radio failure (Seven-six - I can't fix)
- 7700: General emergency (Seven-seven - close to heaven)
In addition to these emergency codes, there are special regional codes for flight schools, parachutists and gliding competitions. Since the introduction of the Mode S system, however, the classic squawk has become less important because every aircraft can be uniquely identified by its ICAO address.
Where and when transponders are mandatory
In Germany, the transponder requirement applies in several cases. It is mandatory when entering controlled airspace C and D (not CTR) and in so-called Transponder Mandatory Zones (TMZ). In addition, above 5 000 ft MSL or 3 500 ft AGL (depending on which value is greater), one transponder must always be active.
The European regulation SERA.13001 also stipulates that existing transponders must always be switched on as soon as the aircraft is ready for operation. An exception only applies if the on-board power supply is not sufficient - for example in the case of gliders without a generator or ULs with a minimal power supply.
Gliders themselves are often still flying without transponders, as they usually fly at low altitudes and away from controlled airspace. In recent years, however, the number of gliders travelling with Fashion S or FLARM/ADS-B combination systems to improve mutual visibility.
Modern transponder technology: integration and the future
Technical development continues to progress. Manufacturers such as Garmin, Trig Avionics, Spark and Becker Avionics now offer compact, lightweight transponders with a wide range of interfaces. Many new devices can be integrated directly into Glass cockpits and operate them via the central display - so-called Remote transponder.
Particularly popular is the Garmin GTX 330 ES, which supports ADS-B Out, or the Trig TT31, which was specially designed for retrofitting older aircraft with minimal effort. Another feature of modern systems is the automatic adjustment of the transmission power: depending on the flight altitude and environment, the transponder reduces the output power in order to avoid interference and save energy.
Stricter requirements for ADS-B Out will apply in European airspace in future. According to ICAO regulations, all newly authorised IFR aircraft must already be ADS-B capable. There is not yet a comprehensive obligation for general aviation, but EASA and national authorities recommend a gradual upgrade to support future traffic management systems (U-Space, drone integration, Datalink ATC).
What to consider when buying a transponder
For private pilots or owners of small aircraft, three criteria are particularly important: Authorisation, performance and integration.
- Authorisation: Only transponders with ETSO or TSO approval may be installed in EASA-regulated aircraft. For ultralight aircraft or Annex I aircraft (not EASA type-approved), non-certified devices may also be sufficient, provided they fulfil the 1090 MHz frequency standard.
- Transmission power: Commonly used are 130 W (Class 2) and 250 W (Class 1). Class 2 is usually sufficient for VFR aircraft, while IFR aircraft and high-flying aircraft require Class 1.
- ADS-B-Out: If you want to be equipped for the future, you should choose a transponder that is ADS-B-capable. This requires an external or integrated GPS receiver with a certified position signal.
- Operation and integration: Modern glass cockpits can display transponder data directly; older aircraft require a separate control panel. Remote solutions save space, but require a compatible avionics environment.
- Service and support: It is important that maintenance and inspections (e.g. every 24 months according to Part-ML) can be carried out without any problems. The availability of spare parts and software updates should be taken into account when purchasing.
Conclusion
Transponders have become an integral part of modern aviation. They increase visibility, enable safe traffic guidance and are a central component of future digital airspace concepts. For pilots, this means that only those who understand and maintain their system correctly can fly safely and visibly. Anyone considering a new purchase should pay attention to Mode S and ADS-B capability - and consider the transponder in the cockpit not as a mandatory device, but as life insurance.
Source references:
Aviation magazine