Study On Key Techniques Of Digital Beamforming In Satellite Navigation Receivers

Interference and multipath are two difficult problems confronted by satellite navigation receivers. While digital beamforming (DBF) technology is implemented in satellite navigation receivers, according to different spatial features of signal, interference and multipath, the array weights are adjusted to form main beams towards satellites and nulls towards interference plus multipath. So the purpose of enhancing signals and suppressing interference plus multipath are reached. The engineering implementation of DBF technology in satellite navigation receivers relates to many aspects. This dissertation studied the following aspects, aiming at improving the quality of the received satellite signals.(1) Optimal weighting criterion is the key part of DBF technology. It constructs the theoretical foundation of computing the array weights, and decides the optimal performance of an array receiver."Optimal"refers to certain optimization target. While the optimization target differs, the designing result of the optimal array criterion is also different. Optimal criteria mostly used in satellite navigation systems include Power Inversion, Maximum Signal-to-Interference-plus-Noise Ratio, Minimum Mean Square Error, etc. Those criteria considered few aspects of suppressing multipath. According to the typical multipath environment of monitoring stations (reference stations) in satellite navigation systems, this dissertation proposed an optimal criterion to mitigate multipath, which is named Down-up-ratio Constrained Minimum Variance (DCMV) criterion. Simulation results showed that DCMV criterion could form a -20dB or even deeper null in the direction of multipath reflected from the ground. Since high precision civil receivers are vulnerable to continuous wave interference, this dissertation also proposed a criterion named Switchable Beam-steering/ Null-steering (SBN) criterion. It switches Beam Steering criterion to Null Steering criterion or inversely according to the impact of the continuous wave interference under different frequency. This criterion can extremely improve the quality of the received civil signals interfered by continuous wave. Experiment results showed that the array output carrier-to-noise-ratio under SBN criterion got 2~5 dB improvement compared to the one under Beam Steering criterion or Null Steering criterion separately.(2) The error of prior information is an important factor which affects the beam direction of the antenna array and the performance of suppressing interference and multipath. Many optimal criteria need the aid of prior information, such as Beam Steering, Maximum Signal-to-Interference-plus-Noise Ratio, DCMV, SBN, etc. In satellite navigation systems, the prior information of antenna array receivers includes the ephemeris data, the location of the receiver, and the platform attitude of the antenna array plus the relative positions of the array elements. Traditional literature usually focused on the calibration of one kind or the combined errors of the prior information, but neglected the error range of the prior information based on the application. Inevitably, the more the types of prior information are, the more complicated the calibrating procedure is, and the stricter the calibrating environment is needed, which may cause many difficulties in the implementation of DBF technology. This dissertation analyzed the error range of each type of the prior information in satellite navigation antenna array receivers for the first time. The estimation error of the signal arriving angle caused by errors from the ephemeris data, the location of the receiver and the platform attitude of the antenna array is less than 1 degree; Position errors of the array elements are usually less than 2% of the signal carrier wave length. The above errors have little impact on the beam direction and other array performance, and need no calibration. This conclusion provides the theoretical reference to the engineering implementation of DBF technology. Also, the platform attitude calibration with navigation signals was studied, and the suggestion about array configuration and baseline selection were given.(3) Array channel mismatch must be considered in the engineering implementation of DBF technology. This dissertation proposed a channel mismatch model according to the measured data of antennas and RF front ends, then defined three parameters: the amplitude mismatch, the phase mismatch, the difference between averaged group delays. And the affect of channel mismatch on array performance were analyzed in quantity. For antenna array satellite navigation receivers, traditional channel mismatch calibration has obvious disadvantages, for example, it is not able to solve the mismatch between antennas; the method of calibrating antenna array channel mismatch in anechoic chambers is able to calibrate mismatches between antennas, but the calibrating error may float when the hardware gets old, also the system error in the calibrating procedure should be controlled strictly. Nowadays, digital signal processing ability has been improved extremely, online calibrating with satellite signal is an ideal method. The key part of calibrating channel mismatch is to estimate the channel characteristic. Based on the space-time structure, the channel characteristic estimation has to change the space-time weights and capture the correlation peaks under each group of space-time weights; which costs large computation and time. This dissertation improves the online calibrating method, which only needs one group of space-time weights. The proposed method measures several values near the correlation peak, instead of measuring several correlation peaks, and then estimates the channel characteristic. The proposed method reduced the complexity of calibration, and also saved the computation time.(4) Along with the construction of the modern GPS, Galileo and Compass II, GNSS users will receive more satellite signals, but the amount of tracking channels of the receiver is limited. Therefore, satellite selection becomes an important step in receiver's processing. Satellite selection under DBF needs to consider not only the impact of satellite geometry, but also the feasibility of enhancing satellite signals with array beams. According to the above requirement, this dissertation proposed the optimal satellite selecting method for beamforming. Satellites are selected according to the geometry dilution of precision weighted by the array output signal to interference-plus-noise ratio. Based on the single-weight structure and the multi-weights structure of the antenna array receivers, the flow charts of the optimal satellite selecting procedure are suggested. Simulations are conducted to compare the performance of the conventional method and the proposed method. It shows that for typical seven-element antenna array receivers, the positioning success ratio is 98.8% and 100% respectively when there is one broadband interference; and the positioning success ratio is 91.5% and 99.9% when there are three broadband interference. Besides, the average positioning error of the proposed method in the 100% and 99.9% scenarios is almost the same with that of the conventional method in the 98.8% and 91.5% scenarios. It can be seen that the optimal satellite selecting method for beamforming improves the positioning success ratio, and at the mean time guarantees the positioning accuracy.The technique studied in this dissertation can be applied in the monitoring receivers of satellite navigation systems, the reference receivers of satellite navigation augmentation systems and the receivers for high-precision aerospace users.