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38 Radar The third is radar. ‘Due Regard’ air-to- air radar technology has been adapted from fighter aircraft, and uses an active electronically steerable array (AESA) to track multiple aircraft simultaneously while continuing to look for other aircraft. The AESA is a set of antennas that can ‘look’ in different directions by changing the phase of the outgoing signals, which can also reduce the power consumption. These systems can currently track objects beyond eight miles and will pick up an aircraft such as a King Air at a distance of 12 miles. There is a growing focus on reducing the size and weight of radar systems for CA. Synthetic aperture (STAR), a single antenna with separate transmit and receive channels, can be optimised for lower weight and power by using a duty cycle that is much longer than the current 10% active time, which provides better gain and signal-to-noise ratio. Prototype systems that weigh less than 454 g (1 lb) and use less than 30 W rather than kilowatts of power are already being tested on sub-30 kg UAVs. They are suitable for small UAVs that don’t have the power to handle complex algorithms. Also, they have a modular design, which allows them to be tailored to a given platform. For example, some platform makers want a 210 º viewing angle in front of the craft, while others want the full 360 º , and the needs of a fixed-wing craft are different from those of rotor craft. This can be for a number of reasons. Fixed-wing craft are generally moving faster and need more time to change direction than a rotary craft, and the shape of the craft can require the antennae to be placed in different locations. Typically with a fixed-wing design the antenna is a vee shape on the front that can see in front, and one mounted on the back of the craft to cover the rear. In contrast, a rotor craft may have antennas around the body of the aircraft as there is no front as such. Other craft might have only one revolving antenna, mounted above or below the airframe, to provide the radar data. These small radar systems are intended to meet the regulatory requirements for a ‘sufficient’ distance to see and avoid other craft. They need to provide a detection distance of at least a mile, but that can depend on the airspace and the speed of the aircraft – for faster, fixed-wing craft a longer range may be necessary, for example. Implementing such systems requires a combination of technologies. Off-the- shelf antennas with a combination of signal generation, data acquisition on the receive path and processing are all tightly coupled to reduce the size and weight but still provide the ability to sense other aircraft and predict any potential collision. These systems are now being tested and are expected to be on the market towards the end of 2016. Standardisation To standardise systems that rely on radar, the UAV community is working closely with the RTCA, which develops the technical standards for aviation systems, and within the RTCA the SC-228 committee is developing the minimum operational performance standards for unmanned aircraft systems. This has two working groups, for detect and avoid (DA) and command and control (CNC). The radar subgroup within the DA group is defining the minimum requirements for the radar systems, which translates to the smallest target of roughly one square metre. This is an IFR-based approach that uses a standard called TCAS II for collision avoidance for larger unmanned aircraft. TCAS itself was designed for transport-type aircraft rather than UAVs, and uses simplified radar to monitor the airspace around an aircraft for other aircraft equipped with a corresponding active transponder that is independent of ATC. It has to be fitted to all aircraft with a maximum take-off mass of more than 5700 kg (12,600 lb) or carrying more than 19 passengers, but the SC-228 group believes it will be suitable for larger UAVs. One challenge here is that these platforms are not designed for high- Dec 2015/Jan 2016 | Unmanned Systems Technology Radar systems will be packed in handheld units for UAV systems in 2016 (Courtesy of IMSAR) Passive sense and avoid using image sensors and machine vision algorithms is seen as the next step, but the challenge is the processing power