The Surveillance Challenge

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. 

Today´s availability of low-cost technology and public technical standards allow for anyone to easily buy or even build receivers and have immediate access to broadcasted aircraft information. It has even become a commercial business to provide a webbased air picture based on this information.

The military soon identified this as a serious security risk and therefore the use of legacy SSR Mode has recently been increasing, so as to minimise the online dissemination of military or state aircraft position and other parameters.

GNSS vulnerable to interference

While satellite-based technology was considered as the silver bullet for a long time, it is based upon satellite signal availability, and it is at risk from harmful interferences. There has been growing concern about this and the current conflict in Ukraine has clearly proved that dependency on satellite-based positioning bears a serious risk for civil aviation, in particular close to areas of conflict. It needs to be highlighted that both state and non-state actors can cause significant disruption of aviation by simple and inexpensive means of interference.

The Ukraine crisis also forced aviation to avoid airspace close to the conflict area and had a channelling effect on traffic flows, with local high densities. The presence of many military systems also interrogating this traffic with high rates of repetition, to detect potential hostile activities as early as possible, has stressed the secondary surveillance system.

“While Satellite-Based Technology Was Considered As The Silver Bullet For A Long Time, It Is Based Upon Satellite Signal Availability, And It Is At Risk From Harmful Interferences “

This conflict has also seen the large-scale usage of new entrants (such as remotely piloted devices) relying on surveillance infrastructure for a wide variety of purposes: reconnaissance, attack, logistics, etc.

The need for a better solution

This is a very fragile situation, with increasing SSR usage by military aircraft coming along with its negative effects, increasing the risk for safety related incidents (some have already been reported). High aviation traffic densities and/or a high number of interrogators in small areas causing many interrogations result in high 1030/1090 MHz usage that could result in a denial of service. New and uncrewed entrants as “not visible” air space users in all sizes and flight levels – and, finally, satellite services being unavailable as a result of interference – all add up to make the surveillance challenge especially complex.

It will be crucial for aviation as a whole to find mitigations for these security and infrastructure congestion dilemmas in the near future and solutions in the long term. The military´s need to conceal flights from the public to ensure mission effectiveness and maximum protection for the crews, and the safe separation requirement when operating in the same airspace as civil traffic, must both be accommodated.

Surveillance data sharing between civil and military entities with a high level of integrity and trust can help in the short term. This requires close cooperation, mutual understanding and recognition, as well as common standards. Common procedures must be agreed in related contractual arrangements.

The use of legacy SSR- Modes (e.g. Mode A) must not be seen as a final solution due to its possible negative impacts on aviation safety, namely garbled transmission and over interrogation. Instead, the surveillance technology to be identified in the long run has to be limited in access to trusted entities (like the military IFF Mode 5 Level 2 broadcast) or use ADS-B enhanced mode Phase Modulation Overlay to integrate authentication mechanisms to increase integrity and availability.

Aviation’s dependency on satellite services has to be addressed, with alternative and secure means for positioning/ navigation added.

The 1030/1090MHz frequencies need to be protected and therefore used only (as far as possible) for surveillance purposes, maintaining the right level of accessibility to this application. Interrogator activities, including those of military units, need to be well coordinated and limited to the minimum necessary at regional level.

Finally, any new technology must be affordable and fit in all airframes and systems, including those remotely piloted.

A joint Civil-Military approach is the key to success and to get back on track. For this, discussions need to start as soon as possible to implement temporary mitigations and initiate the necessary research and development to deliver efficient and permanent solutions.