![]() Reference positions are also provided by satellite positioning services, such as SAPOS in Germany or CORS in the United States. This base station may be set up by the user, for example on a trigonometric point or on a user-defined reference point (see Fig. These achieve much higher accuracies in the centimeter range by measuring the position of the mobile unit-the UAV-simultaneously to that of a static base station with known coordinates. Many professional-grade UAS are now equipped with real-time kinematic (RTK) or post-processing kinematic (PPK) GNSS. Automatic flight control system plus#Combining signals from various GNSS greatly improves accuracy, and most recent UAS are able to receive signals from GPS plus further systems, depending on the region. The microwave signals from the satellites are subject to fluctuations and noise that limit the accuracy of GNSS measurements to the range of 1–10 m. Both absolute and relative accuracy of this position are highly dependent on the receiver design and GNSS antenna quality as well as local measurement conditions, such as atmospheric disturbance and satellite shadowing. This position is used for navigating the UAV and also for tagging aerial photographs with coordinates that may later be used for georeferencing and photogrammetric orientation. ![]() Global satellite navigation (GPS or its Russian, European, and Chinese equivalents, GLONASS, Galileo, and BeiDou) allows the aircraft to determine its position in real time. A global navigation satellite system (GNSS) receiver and an inertial measurement unit (IMU) are therefore central components of the flight-control system. However, autonomous and assisted flight modes require exact knowledge and control of the UAVs position and attitude. Remotely piloted platforms do not necessarily need navigation systems to fly-they may also be manually flown by a human pilot in visual line-of-sight without feedback about position and attitude from the aircraft to the remote control. Aber, in Small-Format Aerial Photography and UAS Imagery (Second Edition), 2019 8-3.2 Navigation and Flight-Control Systems A basic structure of an aircraft autopilot is shown in Fig. 4.2. Rate-based autopilots use information about the rate of the aircraft and move control surfaces to counter the speed of change that causes the error. The autopilot system moves the control surfaces so that the aircraft's motion reduces the error between the desired aircraft attitude and recorded actual aircraft attitude. A position-based autopilot controls the aircraft so that it always follows desired position profile. ![]() The first is position based and the second is rate based. ![]() Two basic autopilot structures can realise this. The autopilot system should be able to correct the error and restore the aircraft to the states desired by the flight task automatically. When the plane fails to follow the desired states, an error occurs. One critical ability of the autopilot system is error correction. Automatic flight control system software#The control software reads the aircraft's current speed, pose, height and location and then issues control signal to a flight control system, which is a lower-level actuator controller, to adjust the control surfaces of the aircraft in order to maintain the aircraft's attitude, height and speed while guaranteeing the lateral, vertical and longitudinal stability. Autopilot systems use computers to generate the control output. ![]()
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