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73 keep them a safe distance from each other. The third, Mode S, is the most recent. In addition to supporting modes A and C, it further enhances safety by using selective interrogation (hence the ‘S’) instead of the interrogating station having to elicit replies from all aircraft within range, thus avoiding the risk of overwhelming aircraft transponders or ground operators. Typically, two pulses are sent out per interrogation to determine an aircraft’s range and bearing, with subsequent interrogations providing updates. Replies from a Mode S transponder include the aircraft’s call sign and/or a 24-bit address issued by the International Civil Aviation Organisation (ICAO) in Montreal that is unique to that transponder. In the military sector, the highest standard at the moment for transponder modes across NATO members and invited countries is Mode 5 (although non-NATO IFF transponders do exist), which uses similar principles to Mode S except in a few key respects. For example, Mode 5 is encrypted, as would be expected for military aircraft carrying out potentially sensitive operations. Without a crypto-computer or a crypto- applique, an interrogator cannot communicate with them. Mode 5 also has two levels. Level 1 is the required interrogation response mode for identification data, and Level 2 is the preferred mode owing to its mandatory inclusion of GPS-based broadcasts, the standards for which will be set and published soon. Including GNSS information in the air traffic system, whether under the Mode 5 Level 2 standard or other formats, comes as the result of the implementation of Automatic Dependent Surveillance- Broadcast (ADS-B) transponders. ADS-B The ADS-B format is rapidly forming the core of modern ATC systems. Rather than using radar, it is based on the regular broadcast of GNSS position information, typically over the same 1090 MHz band as Mode A systems, although this frequency is not used universally – ADS-B may also squawk over 978 MHz. This sharing of satellite navigation coordinates is the primary attraction of ‘ADS-B Out’, which along with data on critical factors such as identification, altitude and velocity enables improved accuracy and thus safety in the increasingly congested global airspace compared with traditional radar-based systems. It is therefore widely anticipated that the integration of ADS-B Out transponders will be a legal requirement when unmanned systems finally receive approval to enter and operate in controlled airspace. Such transponders may or may not also carry ‘ADS-B In’ functionality, which will allow the operator of an ‘In-equipped’ UAV or manned aircraft to sense all other aircraft traffic with pertinent information on positioning, heading, distance and so on – provided these vehicles are broadcasting on the ADS-B Out standard. The ‘In’ protocol also enables the user to receive meteorological information, for safer operation. Given the relative lack of standardisation in sense-and-avoid methods that UAVs might use, ADS-B In is viewed by industry stakeholders as a key component of ‘detect-and-avoid’, particularly in operations beyond visual line of sight, although the limitations of depending on ADS-B should not be forgotten – the success of the system relies on all certified aircraft using it, as any non-cooperative aircraft will only be detectable by radar. Also, not all airspace is expected to require ADS-B transponder integration. Lower altitudes (below 10,000 ft) may become increasingly congested with unmanned systems that may or may not opt to feature transponders, and will therefore require radar, vision or other sensors to enable them to be detected and avoided. There are two approaches to integrating ADS-B Out functionality – using a 978 MHz universal access transceiver, or a 1090 MHz Mode S Extended Squitter (or ‘ES’, the ADS-B extension to the 1090 MHz protocol, which adds GNSS information to the Mode S transponder’s replies and its broadcasts). Mode S ES-equipped Transponders | Focus Unmanned Systems Technology | December/January 2018 Military transponders conforming to the Mode 5 Level 2 standard will include ADS-B Out’s GPS broadcast functionality in their encrypted transmissions (Courtesy of Thales)