Unmanned Systems Technology 004 | Delair-Tech DT18 | Autopilots | Rotron RT600 | Unmanned surface vehicles | AMRC | Motion control | Batteries

27 Delair-Tech DT18 | Dossier it is integrated, and is intended to provide an ‘onboard’ piloting view. The payload camera is not gimbal-mounted since that approach would tip the overall weight beyond the 2 kg target. It is designed to point downwards, perpendicular to the airframe, and is free from vibration thanks to the use of an electric propulsion motor. There are three standard imaging payloads offered for the DT18. The first is the basic payload still camera (plus the ‘piloting’ video camera), while the second adds a third camera to the package, this being a near-infrared still payload camera – especially useful for agricultural work. Even with this addition however, the DT18’s take-off weight is still within the 2 kg limit. Option three is not to use a still camera at all but for the payload to take the form of an infrared video camera for thermal imaging in darkness; the standard ‘piloting’ video camera is retained though. This option is useful for surveillance work at night, for example tracking pipeline thieves. Control of the payload camera(s) is in accordance with the software developed by Delair, which specifies the parameters for image capture. Payload imaging data is stored uncompressed using an on board SD card, although it can also be sent back to the ground station in compressed form, which is useful for monitoring the progress of a job. The DT18 uses CCD-type still image sensors with a global shutter. Sensors used in still cameras are normally of either that or the CMOS type. The latter usually uses a rolling shutter, meaning rows of pixels are exposed sequentially, whereas the global type exposes all pixels at once so, given the motion of the craft, a rolling shutter is inappropriate. The sensor is 8.446 mm wide and 7.0656 mm tall, and is normally used with a 12 mm focal length lens. Each pixel has a diameter of 3.45 microns and it follows that at a height of 150 m the camera sensor will ‘see’ 43 mm on the ground as 1 pixel wide. The lens aperture is fixed, with the shutter speed adjusted to obtain the correct exposure given the ambient light conditions. That adjustment can be as per user request or an automatic function. De Lagarde remarks that the DT18’s sensor is larger than that used by rival systems. “This provides more light sensitivity, increasing shutter speed [for given light conditions]. Our shutter speed can be kept to less than 1.5 milliseconds and is normally less than 1 ms, avoiding the [risk of the] motion of the craft causing a blurred image,” he says. The usual operational altitude for photography is 100-120 m, with 60 m typically the lowest. Clearly, the lower the altitude the higher the resolution from the camera, whereas the higher then the greater the area included in each image. Photographs are normally overlapped to create a mosaic of a given area of land. A mosaic with about 80% overlap of individual images is required for digital surface modelling (DSM) to automatically generate a 3D model. This can be accompanied by digital terrain modelling (DTM), which provides a map of the ground regardless of the cover photographed from the UAV. For example, DTM will estimate the ground form underneath heavy tree cover. De Lagarde gives the example of power line monitoring on behalf of the French national grid. A DT18 is used to photograph power lines and their immediate surroundings, and from this data DSM creates a model from which power line proximity to trees and other objects can be automatically assessed. Where the tree canopy is identified as dangerously close to a line, field workers can be sent to cut it back. That model can also be stored for future comparison with more recent versions. De Lagarde notes that the national grid has been surveying power lines for more than 50 years, either by inspection from the ground or using light aircraft or helicopters, but now the DT18 offers a more cost-effective option. Operation While the DT18 can be launched by hand, Delair produces an optional launcher, which will be standard for its next model, the larger and heavier DT26. It is a hand-wound bungee cord launcher that is simple, robust and inexpensive. Hand-launching implies that the person doing it runs along with the craft, keeping it level; once the runner has reached take-off speed the propeller will automatically speed up and, on hearing that, the operator can release it. Unmanned Systems Technology | Autumn 2015 Attaching the DT18’s wing to its fuselage The image sensor in the DT18 is larger than that used by rival systems, giving more light sensitivity and allowing for faster shutter speeds