IFF – the inspiration for SSR
Surveillance, as a means to ensure safe separation and identification of flights, remains one of the essential elements for modern aviation. Throughout its history defence applications were an important driver for its innovation and development. Secondary Surveillance (also known as Cooperative Surveillance) became necessary to differentiate between friendly and enemy aircraft and was therefore designated as Identification Friend or Foe (IFF). This cooperative application got picked up by civil aviation as Secondary Surveillance RADAR (SSR) – as it offered better coverage and enabled identification of flights for correlation with flight plans. To disguise military operations the encrypted standard Mode 4 was later developed.
With increasing traffic this technology soon reached its limits (a maximum number of 4096 unique codes). Mode S was introduced to solve this problem, also allowing for a unique identification of aircraft with 16 million code permutations and the possibility to downlink additional information. However, initially all position calculations continued to be made on the ground.
With the progressive introduction of satellite-based positioning and navigation it was a logical step to use the downlinked signal to share a more accurate aircraft position, using an enhanced version of Mode S. As technology advanced, this dependent and cooperative surveillance standard became the backbone of modern Air Traffic Management. This next iteration, known as ADS-B, is still being deployed (quite rapidly!); this enables the use of relatively simple and cheap ground-based receivers/antennas instead of complex, expensive and rotating RADAR installations, while still sharing extensive aircraft information.
For military applications, the successor of IFF Mode 4 was developed as Mode 5, again using encryption to allow secure identification and, similar to Mode S, the downlink of additional information. However, it was designed specifically to avoid any interferences or adverse impacts on existing secondary surveillance services.
Mode S/ADS-B widely used but not secure
The technical standards for Mode S and ADS-B are widely available and security features remain very basic. Also, the operational usage of Mode S and ADS-B as backbone for Air Traffic Management (and therefore global aviation) relies still on two frequencies (1030 MHz and 1090 MHz); this represents a serious constraint and a point of vulnerability.
It is clear that all the flights conducted under General Air Traffic (GAT) rules (regardless of what type of airspace user) need to be identified and visible for Air Traffic Management, in order to ensure adequate levels of safety. That applies also to military GAT flights and this is the reason for the use of Mode S and ADS-B, except for training and operational missions.