# Wind Speed Measurement via Visual Recognition of Wind-Induced Waving Light Stick Target
## 1 Introduction
The paper describes the design and construction of a telltale wind indicator for the Mars Phoenix lander, which was sent to Mars by NASA to study the Martian arctic environment. The wind measurement system involves a video camera focused on the movement of a suspended lightweight Kapton tube, based on which wind measurement, direction, and turbulence can be estimated. This paper proposes a computer-vision-based wind speed and direction measurement methodology for horizontal wind speed measurement in small, enclosed areas such as a stationary train equipment cabin. The proposed system consists of a single camera focused on a lightweight stick target that swings according to wind strength and direction. The pinhole camera model is implemented to model the relationship between the real-world angular displacement of the target and its corresponding displacement in the image plane when the camera is positioned relative to the lightweight stick target in front, side, and bottom orientations. The method combines the advantages of being lightweight, and having less intrusion on the flow field, small size, low cost, simple set up, and acceptable accuracy. These qualities make the proposed method particularly promising for two-directional wind speed measurement in enclosed areas. The comparative advantages of the proposed method compared to traditional methods such as the plate anemometer immediately stand out. Compared to the plate anemometer, for instance, the proposed method is less intrusive, less prone to human error, and occurs in real time. Furthermore, the proposed method is not as sensitive to flow direction as the pitot tube and cup anemometer which are, in addition, more flow-intrusive. At the core of our proposed measurement methodology is the real-time detection of the lightweight stick, based on which wind speed and direction can be estimated. Therefore, our proposed method and the works of [PERSON] et al. [21] and [PERSON] et al. [22] also share similarities in that all involve vision-based detection of the fluttering, waving, or movement of some target. However, the former's method is based on laboratory training data with an ultrasonic sensor as the reference device. This creates significant room for bias against the real working environment of the boat (sea and/or ocean). Although the authors attempted to address the problem, it remained an important limitation, as they admitted. The proposed method addresses this by establishing a well-defined relationship between the real-world movement of the target and its corresponding displacement in camera coordinates to allow for real-time measurement that directly depends on the active scene condition. Furthermore, in the application of the proposed method, several targets were lined across the breadth of the enclosed space in different positions, and focused on the same camera, to attain high accuracy and robustness, as suggested by [PERSON] et al. [21]. The latter's method relies on the kinetic energy of the target and several empirical factors. Moreover, the determination of wind speed wholly depends on the relative tilt of the target, which necessitates dependence on one orientation where the camera plane is parallel to the movement of the target. If the camera were to be placed at an angle to the telltale, the output tilt angle would be incorrect, leading to incorrect wind speed measurement. In contrast, the proposed method relies on the geometric attitude of the target and attempts to address the heavy dependence on the relative camera-target orientation by formulating four different camera orientation scenarios, including one that couples multiple orientations. Thus, we propose a wind speed and direction measurement method based on the movement of a lightweight stick. Detailed formulations are given for different camera orientations to correctly capture the target's movement. Based on this displacement and together with the wind tunnel calibration coefficients, the wind speed and direction are determined.