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88 Focus | Lidar sense & avoid February/March 2020 | Unmanned Systems Technology has already started to become available. This uses a laser chip that is designed to provide an array of thousands of individual light sources to produce collimated beams with a wavelength of about 900 nm. With a power source of 10-12 W the chip can illuminate an area up to 200 m away and detect an object with a reflectivity of 10%, making it more effective than flash architectures. Rather than using PIN or avalanche diodes to sense the reflected light, this approach uses a custom CMOS camera sensor tuned to the laser frequency. This can measure the ToF of the laser light to give the range per beam, as well as the intensity. A bandpass filter on the receiver also allows it to output a black & white image that is inherently fused with the point cloud produced by the Lidar laser data. This can be used to detect signs on the road as well as some at the side. The laser chip and camera sensor can be built on standard, high-volume chip- making processes that have the levels of reliability required for automotive designs, and the combination has no moving parts. The laser chip requires a specialist wafer substrate, and the receiver chip is built in a custom CMOS process. The architecture is modular, so system developers can chose a long-range option with a narrow FoV – for example 30 º horizontal and 15 º vertical, with an angular resolution of 0.1 x 0.2 º – and combine it with a short-range module of 50 m with an FoV of 60 x 30 º and an angular resolution of 0.3 x 03 º, for applications such as cocooning a vehicle. Signal processing ToF FW Lidars use narrow pulses that are 3-5 ns long, resulting in a digitisation bandwidth of more than 200 MHz. This requires the ADCs to sample at several GHz to obtain sufficient accuracy on the digitised pulse, limit noise aliasing and relax the requirements of the anti-alias filter before the ADC. Discrete multi-channel ADCs with multi- GHz sampling rates and 10-12 bits of resolution consume over 1 W per ADC channel, and are also limited to fewer than four ADC channels per chip owing to heat dissipation that requires additional cooling. Some ADCs have been designed with such requirements in mind, providing four channels of sampling at 3 GHz while achieving 60 dB SNR at 250 MHz bandwidth. High tolerance of clock jitter allows low-power, compact ring oscillator base phase-locked loops to be used for sampling clock generation and distribution. Aircraft A Lidar developed for UAV applications uses a mirror that is 2 mm in diameter and electromechanically driven to create an x-y scan using a 1550 nm laser pulsed at 400 kHz. It is currently used for a mapping Lidar with a 400 ft (100 m) range and using diodes as photodetectors, but the next stage is to design a Lidar for sense & avoid applications with a range of several hundred metres using the same 12-bit ADC to capture the digital data and a mainstream field programmable gate array for the processing. The longer range would be achieved using avalanche photodiodes. The rise and fall time of the detection signal is longer for avalanche, but the fall time is key to ensuring that the second part of the return pulse is captured. For sense & avoid, the resolution requirements are very different from a mapping or imaging application. For imaging, the Lidar system can detect objects less than 30 cm apart. If two objects are that close in the sky though then it is likely to be a debris field. This means the resolution requirement is considerably more than 1 m, making avalanche receivers appropriate. The same technology can be used for undersea applications using a 532 nm laser. This can be used for imaging underwater, as the return pulses can be filtered, or gated, depending on the range. This eliminates the near backscatter that blinds the sensor – called the snow noise – so the overall signal noise goes down and the image quality is higher than with optical or sonar systems. Small Lidar sensors are also being used for ground operations for UAVs. Acknowledging that long-distance Lidar sensors are still too heavy and power- hungry for sense & avoid applications in the air, the smaller sensors can be used with larger UAVs that need to manoeuvre safely from the apron to the runway without human intervention. A small system with a 50 m range and limited forward field of view is sufficient. However, that requires an interface function between the point cloud output of the sensor and the autopilot. This can be achieved with a processing board to convert the point cloud into a message for the autopilot. A modular Lidar system combines a laser chip that produces an array of collimated beams with a dedicated CMOS camera sensor that fuses the laser and image data shown here (Courtesy of Xenomatix)