Unmanned Systems Technology 023 I Milrem Multiscope I Wireless charging I Logistics insight I InterGeo, CUAV London & USA show reports I VideoRay Defender I OS Engines GR400U-FI I Ultrabeam Hydrographic Ultra-2 I IMUs

57 “The Defender has six degrees of freedom control,” he says. “It can pitch, lift heavy loads and it’s very fast. That comes from the design of the thrusters developed for the frame.” Many more components are needed in this modular platform, however, so the company, based in Pennsylvania, US, teamed up with Nortek in Norway and Greensea Systems in the US as part of the system’s design. Greensea developed the Opensea software that includes the inertial navigation software and the autonomous control system. This provides the navigation, control, payload management, mission planning and diagnostics in a single software platform, but has also been designed to be easy to integrate with the hardware. “Modularity allows a product to be adaptable while remaining robust,” says Ben Kinnaman, CEO of Greensea. “We began building the operating platform with a modular design because we foresaw the need for flexible software to support the varied needs of the marine industry.” The Defender also uses the DVL1000 doppler velocity log from Nortek to provide a more accurate way of controlling the craft. Small UUVs tend to rely on dead- reckoning navigation using a DVL and surface GPS for a heading reference. This estimates the position of the vehicle based on its previous position and its course and speed over a known interval of time. However, dead reckoning is subject to significant errors since both speed and direction must be accurately known at all times for the position to be determined accurately. Working underwater adds other challenges that do not occur on land or in the air. Currents in the water can cause ‘crabbing’ and heave, both of which push a craft off course, and dead- reckoning systems cannot compensate for these effects. The problem can be addressed by integrating an inertial sensor, but that requires tighter integration with the inertial control system that is part of the attitude and heading reference system (AHRS). The AHRS provides feedback on the vehicle’s orientation, magnetic heading and attitude, as well as a pressure sensor to determine the vehicle’s depth. Greensea developed the AHRS with a low-cost fibre-optic gyroscope (FOG) from KVH. Called the INSpect GS4, it uses KVH’s 1750 inertial measurement unit with Greensea’s proprietary algorithms and software for processing large amounts of data including inertial data, position, attitude, pitch, roll, heave, speed and heading. The unit combines very low noise accelerometers with the FOG, which is the smallest such gyroscope available, at 45.7 mm in diameter by 22.9 mm high. Data fusion It can program the rate of data capture up to 1 kHz, and the Opensea software fuses the data from available navigation- aiding sensors with the high input rate 1750 to produce an accurate and stable ‘state of the system’ estimate. The INSpect GS4 supports up to eight sensors, and can be configured through the Greensea user interface application. The DVL1000 is also the smallest such device, being 158 mm in height and weighing 1.3 kg, and works alongside the AHRS. It uses sonar with a doppler analysis to track the position of the Defender relative to the seabed at a range of 50 m down to 20 cm. It also operates down to a depth of 300 m. “The size and weight of the Nortek DVL really fit perfectly into the Defender,” says Goldstein. “I was impressed with how easily and how well Greensea integrated its software with our MSS architecture,” he adds. “This is a big advance for ease of VideoRay Defender | Digest We began with a modular design because we foresaw the need for flexible software to support the marine industry Unmanned Systems Technology | December/January 2019 The Opensea operating system combines the sensor fusion, navigation and control functions in a modular architecture (Courtesy of Greensea Systems)