56 to ensure consistent coverage (from the UAVs’ side). Also, their autopilots and mission planning software include autonomous behaviours for different flight patterns according to the type of inspection target, including corkscrewlike patterns for mapping around towerlike structures or long narrow patterns for corridor mapping. Also integrated is Skyfish’s Skymind computer, which provides integration and control of flight, navigation and sensor systems as well as edge computing for the gimbal and motors to ensure a stable and intelligent photography platform in all weather conditions. Both craft are battery-powered, have carbon composite hulls and propellers, and are monitored remotely – or controlled if necessary – through Panasonic Toughbook-based GCSs, which are often used to stream live video data from the UAV about the quality of the data it is capturing and its flight condition. Survey preparation Assembling and launching the UAVs tend to be straightforward. “That’s been part of the design since early on,” Pilskalns says. “There’s no point in having a great photogrammetry solution if users can’t deploy it in the field. “One key part of setting up – and disassembly – is that we didn’t want the payload sensor to have to be attached and detachedmultiple times per day, even whenmoving from site to site. So theM4 andM6 fold at their arms, fit into cases, and can be deployed within a fewminutes once the arms are folded back out and the batteries are installed. The boot-up time for all the systems to come online is then between 60 and 90 seconds.” The pre-flight planning on Skyfish’s GCS includes preparing a cache of local maps for fast and up-to-date georeferencing, as well as SRTM (Shuttle Radar Topography Mission) data for following the terrain’s elevation, and inputting points of interest alongside navigational waypoints. The waypoints can also be adjusted for altitude requirements to assist with surveying vertical structures. Naturally (as these aircraft are photogrammetry vehicles), setting up an RTK base station is a common part of survey prep. Most of the time that means setting up a tripod-type RTK unit nearby, or on a survey point by way of a plumb bob, with Skyfish’s software providing offsets where necessary. “RTK processing isn’t specifically more valuable for cell towers than other industrial photogrammetry targets,” Pilskalns says. “In terms of geolocation, cell tower operators are already quite confident that they knowwhere their towers are, it’s more the real-time corrections of modelling data that are useful. “Recently, we flew with a moving baseline RTK system on one of the UAVs. We’re offering that now, essentially as part of a ‘Generation 1.5’ version of our UASs which are to incorporate some newer technologies. But that mounted RTK system is especially useful because, out of all the systems on a UAV that will suffer problems during photogrammetry, the most severely impacted will be the magnetic compass. “Magnetic fields are easily disturbed by a lot of things, and anyone who’s flown a DJI UAV for photogrammetry will be familiar with the constant warning pings of ‘Reset Compass’, so even though we have back-up systems we’re moving away from relying on magnetic fields and more towards new RTK innovations for ensuring our heading is always correct.” Gimbal Capturing sufficient data about a particularly complex cell tower requires around 1000 photos, or just under an hour of total flight time. That therefore entails a quick battery swap partway through to enable the M6 or M4 to take off again to gather the remaining imagery. For stabilising the camera during a flight, Skyfish works with Gremsy, which builds a customised version of its T3 gimbal for Skyfish. As standard, the gimbal weighs 1.2 kg, measures 288 mm long and 237 mm at its greatest width, and features a heated and temperature-controlled IMU as well as eight connectors in its I/O module. These comprise ports for CAN, COM, SBus, JR, USB, AUX and HDMI interfacing, as well as a 15-52 V power plug. June/July 2023 | Uncrewed Systems Technology The gimbal is a customised version of Gremsy’s T3, which Skyfish further modifies in-house before installing an extra computing model underneath
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