Recently, small unmanned aerial vehicles (UAVs) have been widely investigated for a variety of applications, including remote sensing and aerial surveying. For such applications, current small UAV platforms use a camera to generate a 3D map from aerial images obtained by the UAV. To generate a 3D map, accurate position and attitude data of the UAV is necessary. However, the typical positioning accuracy of a single frequency global navigation satellite system (GNSS) receiver is 1-3 m, and the attitude accuracy obtained from a low-cost micro electro mechanical system (MEMS) sensor is limited to approximately 1-3 degrees. This accuracy is not sufficiently high for accurate 3D mapping.The goal of this study is establish an accurate position and attitude determination technique by using low-cost GNSS receivers for small UAVs. The key idea behind the proposed method is using multiple low-cost and single-frequency GNSS antennas/receivers to accurately estimate the position and attitude of a UAV. Using the "redundancy" of multiple GNSS receivers, we improve the performance of Real-time kinematic (RTK)-GNSS by using the single-frequency GNSS receivers. This method consists of two approaches: hybrid GNSS fix solutions and consistency check of the GNSS signal strength. In multipath environments, the carrier-phase multipath affects the ambiguity resolution of RTK-GNSS. Different GNSS signal propagation paths are caused at each GNSS antenna. As a result, different multipath errors are caused in each GNSS receiver. It can be used to detect the multipath signals. With this method, we can enhance the availability of carrier-phase ambiguity solutions by using a single-frequency GNSS receiver. Furthermore, we propose a direct attitude estimation technique for a small UAV by using the multiple GNSS receivers. To estimate the absolute attitude of the UAV, we used relative GNSS antenna positions determined by GNSS carrier-phase measurements. It is difficult to resolve the ambiguity for a low-cost single-frequency receiver. In this paper, baseline length constraints can be applied between the GNSS antennas to estimate a reliable ambiguity.To evaluate the proposed method, we conducted a static test in a narrow-sky environment. First, we determined the attitude by using the proposed technique. Using the proposed method, we could almost perfectly solve the GNSS carrier-phase ambiguity by using low-cost single-frequency GNSS receivers in multipath environments. Next, we evaluated the position determination by using the proposed technique. The fix rates are improved in every GNSS antenna by using the proposed multipath elimination technique. Finally, the fix rate reaches 99.9 %, and it can be concluded that the proposed technique offers increased positioning accuracy in urban environments.
Precise UAV Position and Attitude Estimation by Multiple GNSS Receivers for 3D Mapping
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