Design And Realization Of Low Cost UAV Oblique Photography System Based On Beidou

Low-cost, small-scale unmanned aerial vehicles (UAV) have a very wide range of applications in the municipal planning, agricultural land survey and other works,the applications need to immediately get map or other geographical information. UAV can achieve a high degree of customization, it can be designed to be disassemble or folding for the convenience of transport, and be easy to achieve ejection-take-off and parachute drop or vertical takeoff and landing, without the demand of airport runway or limited by the situation of site.Traditional aerial photography remote sensing using the camera vertically installed on the fuselage, and take photography. During the mission, the camera and the plane were controlled and kept vertical with the ground. After geometric correction, images from traditional aerial photography can obtain a higher geometric orientation, which can produce high-precision orthophoto maps. At the same time,during the three-dimensional reconstruction of the objects’ feature, the orthographic image map can only show the top of the features of the texture information, the side of the model can’t be completely reconstructed due to the lack of side texture information, especially when there is partial occlusion, the generated three-mode model will have hollow.UAV based oblique photography is a kind of technology which is using UAV to carry serval cameras on the same aerial photography platform, can obtain the texture of the object from top and side. Through the strict multi-view geometry and three-dimensional matching algorithm, we can automatically reconstruct the 3D surface model of the feature, reduce the workload of the three-dimensional reconstruction in the field, improve the operation speed, and can be used in a wide range of areas. However, low-cost UAV has disadvantages and need to make a choice when designing and using, due to their limited carrying capacity, battery capacity,structural strength and other conditions. The problems of the UAV can be included as follws:First is the GNSS positioning accuracy, the unmanned aerial vehicles equipped with GNSS recivers and generated POS data contains the UAV’s three-dimensional coordinates and attitude data at a certain time interval, the data has an important impact on the accuracy of the object which generated from three-dimensional reconstruction, but in complex environments such as mountainous areas, the signal will be blocked on the aerial drone operations altitude, and the accuracy of the single GNSS positioning is bad. Therefore, a single GNSS signal source is difficult to meet the applicability of UAV oblique photography in complex environments. By combining the different GNSS positioning signals such as Beidou and GPS, it can enhance the navigation and positioning accuracy of UAV oblique photography and have a better route planning.Secondly, the oblique photography produces a large number of images, in order to carry out fast and accurate three-dimensional reconstruction, we need to build a POS-based dense matching method for the UAV oblique photography on the condition of without ground control.Last but not the least, is the UAV’s endurance capacity, due to the low-cost UAV’s load is limited, it is difficult to carry a large battery for the long endurance in the conditions which need to carry a precise imaging payload, so low-cost UAV’s endurance is often difficult to meet the long flight needs of UAV oblique photography.Aiming at the above problems, this paper designs and implements a UAV oblique photography system based on low cost materials, strong endurance and have a high precision three-dimensional reconstruction. The main work is as follows:1) Study on the The problem of attitude stability of UAV, and the fusion of inertial data obtained from two sets of IMU units and two sets of geomagnetic sensing are proposed by means of extended Kalman filter (EKF) method, To enhance the stability of the UAV, and have a access to high-precision attitude data, which provides a guarantee for the follow-up positioning accuracy.2) Study on the UAV’s onboard Beidou / GPS integrated navigation technology,which achieves combined positioning by optimizing the visual satellite of Beidou and GPS. This method can achieve high positioning accuracy under the condition of poor signal.3) Study on the three-dimensional reconstruction technique, firstly, the SIFT feature extraction method is used to generate a large number of feature points, on this basis, the KD-TREE matching method is used to match the stereo points automatically to establish the relative connection points, and the connection points of the oblique images are automatically obtained, and the reconstruction of azimuth and foreign parameters of the camera by multi-view geometry. Then, we use CMVS (cluster multi-view stereo) to cluster the sequence image, and decrease the data to improve the operation speed. Finally, using the algorithm of PMVS (patch-based multi-view stereo), dense matching is accomplished through three steps : match, expand and filter.4) We designed and developed a low-cost fixed-wing UAV. The fuselage is made by foamed plastics, and carbon fiber rods is supplemented to reinforce the fuselage,the thrust is offered by a propeller driven by the motor. UAV loads a bi-camera oblique photography, and a microcontroller is used to synchronize the bi-camera’s exposure.5) Through two test flights, we adjusted the parameters of the flight control system and the fuselage’s center of gravity, the stability of the UAV and endurance power were improved. The cost of UAV is controlled within ten thousand RMB, and the cost of equipment of ground station is less than five thousand. According to the test flight time and the estimating the remaining capacity of the battery after the test, the operation time of this UAV is about 1 hour and 30 minutes, and the operating range is about 70 km. It shows that the desired design goals are achieved.The UAV oblique photography system developed in this study can carry out the oblique photography on the large area, and use the photographic results to carry out three-dimensional reconstruction. This system can be applied to the civil construction supervision, municipal planning, agricultural land survey and other related works about geographic information, which needs instant access to maps and geographic information and accept a certain degree of error when mapping or modeling.