Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

77 Fortunately though there are also now much clearer guidelines on how transponders for medium-to- high altitude UAVs must be designed and react to interrogator signals that come from ground or aerial sources. Transponder manufacturers are therefore releasing new products that not only meet these requirements but exceed them in various ways. Mode and band evolution New transponder regulations have not brought with them the need for new technologies or transmission architectures, or the use of any newly defined portion of the RF spectrum – 1090 MHz continues to be the band used for commercial transponder downlinks, and will be so for the foreseeable future. Existing transponder modes for civilian airspace, as defined by the International Civilian Airspace Organisation (ICAO), have also remained unchanged. These are Mode A, in which transponders transmit an identification code in response to an interrogation request; Mode C which adds pressure altitude information; and Mode S which transmits the aircraft’s 24-bit ICAO code (with or without the information typical of Modes A and C), as well as ADS-B information about its GNSS position as per the ICAO’s 2009 ‘Extended Squitter’ update to Mode S. Military transponder standards have however received some small but significant changes in recent months. For example, Military Mode 5 – which is largely similar to the civilian Mode S but with added encryption – has been adopted as the standard for Identification Friend-or-Foe (IFF) transponder systems as of July 2020, with NATO forces being required to make all the necessary modifications to their equipment to meet this requirement. The standard’s exact guidelines have been written by the US Department of Defense’s AIMS Program Office, so the new requirements are laid out in the DoD AIMS 17-1000 Mark XIIB specification, which replaces the previous DoD AIMS 03-1000 Mark XIIA spec. NATO meanwhile has adopted the new specification and standardised it internally as STANAG 4193. Mark XIIB removes all the requirements for Military Mode 4, and replaces them with new ones that include having transponders broadcast using stronger encryption, different response schedules and priorities in the transmitter, and ADS-B Out information. Meeting these means that a critical need for hundreds of thousands of new Mark XIIB-compliant systems, as well as drop-in replacements for Mode S Extended Squitter for civilian operations, has arisen among UAV manufacturers in the past year. Fortunately, beyond the clear minimum requirements on encryption, response prioritisation and ruggedness – with Mil-Std-461 and -810 being explicit targets for military testing, and DO- 160G for civilian airspace testing – there is considerable freedom in which transponder manufacturers can engineer their new systems to meet the much stricter SWaP-C requirements that UAVs have over manned aircraft. The first Mark XIIB-compliant UAS transponders have already been unveiled, at sizes smaller than a deck of cards and weights below 200 g. That makes them around one-fourteenth the size and one- sixth the weight of the previous generation of certified transponders. These new transponders will transmit information in Military Modes 1, 2, 3 and 5 while incorporating far improved capabilities over those of their predecessors. They are also designed with a higher degree of modularity and interoperability with other onboard systems, to enable them to work more like ‘platforms’ than the fixed, factory-set units of older transponders. For example, while the new specifications only require UAV transponders to work using a single antenna, the new high-end models are broadly being manufactured with dual-antenna outputs. This enables such systems to communicate with both ground and aerial interrogators, as one antenna can be set to facing downwards and the other to be placed atop the fuselage or wings. And although the specifications also cap the transmission power of UAV transponders at 500 W, broader adoption of dynamic RF front ends allow intelligent control and power management strategies. This could enable calculated reductions in the power of radio emissions as and when needed, potentially down to 0.1 W, be it to save onboard energy or reduce the saturation of the RF band in places such as cities, harbours or airports, or when operating a swarm via a mesh network. Transponders | Focus Unmanned Systems Technology | April/May 2021 Unmanned traffic management will in future combine both ground-based and airborne sensors, to avoid collisions with cooperative and non-cooperative aircraft (Courtesy of L3Harris)