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Small UAVs are often operated in a relatively low speed range, where the flight speed is not much greater than the wind speed. As a result, their performance is greatly affected by the wind condition. Estimating wind can improve the control performance and increase the capability of UAV to perform various missions, and itself can become an important application.
A steady wind component in the atmosphere is estimated under a novel extended Kalman filter setup. The estimation utilizes the vector relation between the air, ground, and wind velocity and the phenomenon that the ground speed increases with a tail-wind and it decreases with a head-wind.
Here is a set of fight test data. The first figure shows the flight trajectory and the time history of flight altitude during the experiment. The aircraft took off from around (10 m, 30 m). Then, the aircraft climbed to the command altitude of 100 m and maintained the altitude until t=340 sec, while it circled around (-100 m, -75 m) with a radius of 100m. Then it performed a landing procedure while lowering the altitude until it finally touched down at t=510 sec. In the figure, the aircraft snapshots are also displayed along the flight trajectory at every 3 seconds.
< Flight Trajectory and Altitude >
The first plot in the next figure shows the airspeed measurement (thin solid line), and the airspeed estimate in which the estimated bias of -1.2 m/s is compensated. It also shows the ground velocity. The ground velocity oscillates in the range between 6 m/s and 23 m/s depending on the relative wind configurations. The ground velocity is minimum at a head-wind configuration, and it is maximum at a tail-wind configuration.
The estimated wind speed and direction are shown in the second and thirt plot as solid lines . They generally agree well with the Weather Station wind data (dotted lines).
< Flight data and Wind Estimation >
Autonomous Crabbing
An efficient way of flying a very low-speed aircraft in a confied area under a strong and steady wind condition, which is often found in the community of model glider pilots, is to have the aircraft crab in zigzags. In this flying skill, the glider is never placed into a tail-wind configuration. Instead, the aircraft is manipulated such that it either flies very slowly in an exact head-wind or it is slightly deviated from the exact head-wind in order to move sideways left or right. By cleverly combining such maneuvers the plane can stay in a confined area.
Here is a set of fight test data. The figure below shows the flight trajectory. The aircraft took off from around (25m, 5m). Initially, it flew a bit randomly while gaining altitude to about 150m. Then it turned counter-clockwise about three and a half times along a circle, during which the wind estimation was stabilized indicating that a strong steady wind component exists from the south-east, as shown in the next figure. Then autonomous crabbing guidance was engaged, in which the aircraft flew mostly sideways around the center position (-30m, -160m) for more than a hour during the rest of the flight while leaving a number of figure-8 like patterns which are perpendicular to the wind direction, as indicated by the trajectory plot .
< Flight Trajectory >
The figure below shows the estimation variables from the flight test. It indicates that the estimation was stabilized by about t=1000 seconds which corresponds to the point where the aircraft had turned about three and a half times along the circular path. Once after the filter was stabilized the average wind speed estimate was about 6m/s while the airspeed was about 10m/s. The estimated wind direction is displayed in the third plot. In the period after the filter was stabilized the wind direction estimate was 130 degrees in average, which agreed pretty well with the wind data from a nearby Automatic Weather Station of Korea Meteorological Administration.
< Flight Data: Wind & Airspeed Estimation >
The next figure shows the trajectory corresponding to one cycle of the autonomous crabbing from the flight experiment. It also indicates the aircraft attitude along the trajectory by the snapshots displayed every two seconds using the onboard flight data. The wind blowing from south-east corresponds to the direction from bottom to top in the diagram. The crabbing is well indicated in the wide mid-portion in the diagram by the series of snapshots compared to the trajectory line. Also the slow ground speed at head-wind is well indicated at each end by the relatively shorter distances between the aircraft snapshots in this area.
< Flight Data: Autonomous Crabbing >
Last Update - November 16, 2018
