Research On MEMS IMU/GNSS Ultratight Integration Navigation Technology

The 21st century is the era of rapid development of information, and precision-guided technology which depends on information will further accelerate the development. In the new century, the late 20th century developed precision-guided technology will become a practical technology. At the present, low-cost navigation and guidance technology is the primary aspect to develop the practical precision guidance technology. The high points of MEMS IMU/GNSS, low-cost, low-size, low-weight and low power consumption, wholly meet the requirements for wide application fields, such as vehicle positioning, fine farm machinery navigation, UAV for forest fire, precision guided munitions and satellite detection. Preliminary research on rapid GNSS/INS alignment technology was funded by the Shanghai Academy of Spaceflight Technology in 2008. On the basis of the preliminary background research of navigation and guided system, this dissertation would give attention to new GNSS/INS technology with ultratight coupling or deep integration so as to enhance GNSS tracking, anti-jamming and positioning. Considering modern target's characteristic with high dynamic, the ability of GNSS receiver to acquire and track signal is challenged, and thus GNSS/INS integrated navigation capability is badly affected. The objective of this dissertation is to introduce modern IMU to the GNSS receiver's tracking loops, to analyze GNSS receiver's improvements in acquisition, tracking and anti-jamming, and to design realizable MEMS IMU/GNSS tight integration with IMU-aided receiver tracking loops and characterize its navigation performance in signal-attenuated applications. Furthermore, the dissertation would like to explore an ultratight coupling navigation system with supreme reliability and integrity to improve system performance and survivability. The primary research contributions and roadmap are summarized as follows:1. Collection and summary of all the related literatures and previous works. With this information, and taking into account the technique requirements and project limits, MEMS IMU/GNSS tight integration with IMU-aided receiver tracking loops and ultratight coupling are chosen to be researched. 2. To investigate MEMS IMU-aided GNSS receiver acquisition technology. The acquisition capability index including mean acquisition time and signal detection index is developed. Several main factors influencing the acquisition performance with IMU assistance are analyzed. Based on MEMS IMU-aided Doppler accuracy and almanac-based Doppler accuracy in different receiver startup modes, MEMS IMU-aided acquisition time is quantitatively determined and compared with standard GNSS capability, and acquisition experiments are conducted.3. To implement MEMS IMU-aided GNSS receiver tracking technology. Based on the analysis of DLL and PLL total tracking errors, the structure and implementation of both high dynamic GNSS receiver and MEMS IMU-aided GNSS receiver are provided, and the design of both optimal bandwidth and bandwidth threshold is given. According to the designed MEMS IMU-aided GNSS receiver tracking loops, the tracking and anti-jamming capability are assessed and compared with designed high dynamic GNSS capability, and tracking experiments are implemented.4. To develop MEMS IMU/GNSS integrated navigation technology with IMU-aided GNSS receiver tracking loops. The dissertation designed an EKF-based tightly integrated navigation algorithm trough observability analysis and proposes a reduced navigation algorithm by differencing across satellites. A high-credibility simulation platform of navigation system is constructed for verifying the autonomous navigation accuracy in various signal-attnuated scenarios. In addition, this dissertation draws attention to two time synchronization issues in the implementation of MEMS IMU-aided GNSS/IMU navigation system, and each synchronization scheme is given.A flexible low-cost time synchronizer with FPGA is developed, and vehicle experiments are conducted.5. To investigate MEMS IMU/GNSS ultratight integration navigation technology based on vector tracking structure in order to imporove the performance of dynamics and anti-jamming. Based on vector-based tracking structure of GNSS receiver, the reduced-dimension filter algorithm by differencing across satellites is designed for integrated navigation filter. Consequently, the schematic design of an ultratight MEMS IMU/GNSS receiver is tentatively proposed.The key technology and innovations in the research focus on the following points:1. The assessment method on IMU-aided acquisition, tracking and anti-jamming capability is built. The results of GNSS improvements on acquisition time, signal tracking threshold, bandwidth threshold and RF anti-jamming are displayed. The assessment results and method make contributions to both loop design and prototype development of high dynamic IMU-aided receiver.2. An IMU-aided third-order PLL structure and PLL-aided second-order DLL structure is proposed to satisfy the high dynamic requirement with 10g/s jitter. An iteration-based bandwidth threshold resolution is proposed to make the receiver stably track the GNSS signal in required carrier-noise-ratio environments.3. A novel algorithm based on the differencing pseudorange measurements and Doppler measurements across satellites is proposed so as to eliminate the clock components including receiver clock bias and clock drift. The computational efficiency is improved, and good positioning performance can be kept in various signal-attnuated applications.4. A synchronization software scheme based on recursion Kalman is proposed to synchronize the IMU-aided NCO information and GNSS baseband measurements, and make contributions to MEMS IMU/GNSS ultratight system development.The reseach conclusions are summarized as follows:1. With the aid of MEMS IMU, the acquisition capability in cold start and warm start are improved 30% and 96% respectively, and the reacquisition capability in hot start is enhanced 40%. The remarkable enhancenment in warm start shows wide prospect of IMU-aided acquisition technique in civil applications. According to the test of MAX2769-based GNSS SDR, the acquisition capability is examined.2. In the design of high dynamic GNSS receiver, third-order PLL and PLL-aided second-order DLL can meet the high dynamic requirement with 10g/s jitter.3. With the aid of MEMS IMU, the bandwidth threshold of GNSS third-order PLL is reduced from 18Hz to 1Hz, the signal tracking threshold is reduced from 28dB-Hz to 21dB-Hz, and the mean anti-jamming performance is improved by 7.4dB.4. The MRS errors of position and velocity, estimated by proposed navigation algorithm based on differencing across satellites, are 7m and 0.2m/s respectively. Compared with the traditional navigation algorithm, the computational efficiency is enhanced by 31.1%.5. The field van test result of the low-cost time synchronizer demonstrate that the synchronizer with FPGA developed in the research can achieve an accuracy of around 600 us between MEMS IMU and GNSS receiver.