# Large-Scale Aircraft Pose Estimation System Based on Depth Cameras
## 1 Introduction  
The real-time pose information of objects is of great significance in various fields, such as defense, aerospace, and industrial automation. For example, wind tunnel experiments must analyze the aerodynamic performance of the mechanical structure of aircraft models, and the pose of the models via real-time measurements [1; 2; 3]. In addition, real-time pose information is important for independent navigation, robotic arm tracking, and the automatic sorting of robots in the industrial and medical fields [4; 5]. It is also important in the field of aerospace, particularly for connecting and docking, fault spacecraft capture, and the clearance of space fragments [6]. Since the 1960s, estimation and measurement methods for the wind tunnel model have received widespread attention in industrial and academic circles. These methods can be divided into the inertial measurement method and the optical measurement method, depending on the type of sensor. The inertial measurement method uses inertial sensors inside the model, including electrolytic tilt sensors, accelerometers, and gyroscopes [7]. Electrolytic tilt sensors measure the attitude angle of the model by solving the output voltage. The principle of this method is the imbalance between the two sensing electrodes. In 1994, at the National Transonic Facility at the NASA Langley Research Center (LaRC), [PERSON] achieved a model angle measurement accuracy of 0.1\({}^{\circ}\) using a shell temperature of 71 \({}^{\circ}\)C in electrolytic tilt sensors [8]. Accelerometers are widely used in many applications because of their high precision and repeatability. The latest-generation accelerometer, Q-Flex, consists of pendulums flexures composed of fused quartz. In 2007, [PERSON] improved the angle measurement system at LaRC by using three quadratically mounted Q-Flex accelerometers to measure the pitch and roll angles of a craft simultaneously, and the model attitude error was reducedto less than 0.01\({}^{\circ}\)[9]. Microelectronic mechanical sensors (MEMS) can be used, but are limited in their measurement capabilities with regard to small models by size restrictions. In NASA LaRC.18, [PERSON] et al. performed laboratory testing of MEMS-based capacitive and servo accelerometers. Their calibration error range was higher than 0.04\({}^{\circ}\), and their confidence interval was 95% [10]. However, accelerometers suffer from vibration in a wind tunnel, which causes angle measurement errors. Gyroscopes can be used to measure both vibration and acceleration, and are widely used on aircraft and ships. The principle of gyroscopes is similar to that of accelerometers; the difference is that gyroscopes have a diagonal speed response rather than a linear acceleration response. Therefore, the gyroscope relies on the accelerometer to zero during angle measurement, resulting in random walk and measurement drift. In 2008, Boeing developed a measurement method that used a pair of gyroscopes, an electrolytic sensor, and a servo accelerometer. The gyroscopes drifted 0.08\({}^{\circ}\) in 50 s, and the overall uncertainty range of the system was between 0.05\({}^{\circ}\) and 0.1\({}^{\circ}\)[11; 12]. Inertial sensors are less accurate than this system because they can change the load of the model, thereby increasing the uncertainty of the pose measurement. In addition, in a hypervelocity wind tunnel, the size of the model is small, and there is no space for an inertial sensor. Optical pose measurement typically uses non-contact methods to measure the model's pose and can be divided into photogrammetry and laser-based methods. Photogrammetry is the most commonly used optical pose measurement technology. Multiple cameras are positioned to have different perspectives, tracking a marked point on the model, and judging the displacement of the marked point by the triangular measurement method. In 2014, [PERSON] et al. proposed a high-speed pose visual measurement method for high-speed rolling targets in supersonic wind tunnels.