Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

38 Focus | Sense & avoid systems avoidance system (TCAS), ADS-B, EO and IR systems, and radar. TCAS and ADS-B provide a suitable means of sensing other aircraft with transponders but of course lack the ability to detect aircraft that do not have a transponder. All this has driven the need for ground- based SAA systems that can identify aircraft in the sky, track the motion and alert neighbouring aircraft if there is a risk of a collision. Ground-based radar systems All of this can potentially be managed in the ATM system, which is becoming increasingly sophisticated as a result. Machine learning algorithms in these ground-based systems can identify the position of the aircraft and the risks, and identify the higher-risk locations. However, integrating an SAA detection system into a UAV is increasingly necessary to avoid aircraft that do not conform to ATM systems. These may not be rogue aircraft but ones that have strayed unexpectedly into an area. This is a complex requirement, as a complete SAA system in an aircraft consists of sensors and associated trackers, collision detection algorithms and a collision avoidance planner. The main role of the sensor and tracker is to detect air traffic and track the motion of detected aircraft to gain sufficient confidence that the detection is valid and that an approaching aircraft is on a potential collision course. Once detected, the collision avoidance system has to plan an evasive manoeuvre, which can be internal to the UAV or via a ground- based ATM system. One drawback to using a ground- based SAA system though is that it provides only a static coverage volume of the airspace, which might be less than the operating range of a UAV. Also, using ground-based SAA introduces the issue of maintaining a reliable and efficient data link to the ground. The local terrain could also reduce the surveillance volume and introduce noise into the data. One ground-based SAA system is the mobile aircraft tracking system (MATS) for RPASs that are large enough to host a radar and ADS-B transponder. MATS consists of a 2D primary radar, which provides range and azimuth information about targets, an ADS-B receiver and a combined transponder/interrogator. The primary radar of the MATS has a peak output power of 25 kW and provides two modes of instrumented range: 54 nautical miles with a resolution of 180 m, or 27 nautical miles at a resolution of 45 m. The main function of the MATS is to detect and track intruding aircraft and provide this information to a remote pilot located at the ground control station. Its performance was examined as part of the Smart Skies project using a specially equipped Cessna 172R as the target. Another example is the Thales Star 2000 air traffic control radar, which has a peak output of 28 kW, a range of 100 nautical miles and a resolution of 230 m. However, these are 2D systems that have a relatively slow scanning rate of a few seconds, and are large and power- hungry. As a result they tend to be used to protect restricted airspace such as airports. Although miniature-scale versions of current air traffic radars exist, they are not viable for small UAVs because they provide only 2D sensing, whereas UAV sensing requires 3D spatial localisation. Gimballed radar systems not only have blind-spot issues overhead, but the update rate is also of the order of a few seconds. For short-range radar systems, aircraft could travel most of the way through the field of view in a single update interval. An alternative approach is to use an electronically steered array (ESA) of antennas. An ESA or phased-array antenna has the same narrow beam and high resolution as a mechanically steered antenna, but it can be steered with a wider field of view and has a much higher update rate. Radar inherently has high resolution in range but poor resolution in angle, which is why it requires a phased-array antenna to provide that information. Antennas arranged in a regular pattern use the propagation path difference between elements as a way of determining the direction of arrival of a target. A phased array forms a beam in a particular direction by multiplying the antenna outputs by pre-calculated weights and summing the outputs to produce a single result that simulates an antenna pointed in that direction. In systems with costly receivers, this beamforming occurs directly at the antenna output and is referred to as analogue beamforming. The beamforming occurs after the signal has been digitised, and is referred to as digital beamforming. Analogue December/January 2022 | Unmanned Systems Technology Doosan is adding a visual sense & avoid system to its hydrogen fuel cell-powered UAV (Courtesy of Iris Automation/Doosan)