A hierarchical braking control strategy based on driver braking acceleration analysis is proposed in this paper. Firstly the vehicle state data of the driver during braking collected by driving simulator. Then by analyzing the acceleration of the vehicle during braking, the driver's desired acceleration during collision avoidance is determined, and the TTC threshold is divided according to this desired acceleration value. The two-level warning and two-level braking collision avoidance strategies are designed based on the second-order collision time model. Finally, the overall simulation model of the collision warning system is constructed in Simulink/ Carsim. The co-simulation test results demonstrate that the hierarchical braking and warning strategy of this system can effectively avoid crash.

BeiDou signals are modulated with a secondary code of NH (Neumann–Hoffman) code to obtain a better positioning performance. The data bit rate increases to 1 kbps and bit transitions can occur in every 1 millisecond (ms) random sampling signal. As a result, the frequent bit transitions lead to an acquisition-sensitivity attenuation of classic integration algorithms. In order to improve acquisition sensitivity for BeiDou weak signals and resolve the problem that frequent bit transitions limit integration time, a novel BeiDou weak signal acquisition scheme based on modified differential correlation (M-DC) is proposed. First, conventional differential combination method is modified. The differential operation is moved from the post-correlator stage to the IF (intermediate frequency) samples to weaken the influence of data bit reversions. Second, an acquisition scheme implemented via fast Fourier transform is provided. Code phase and carrier frequency are estimated by one-dimensional search using fast Fourier transform. Third, power consumption and computation complexity of the proposed method are analyzed. Finally, Monte Carlo simulations and real data tests are conducted to analyze the performance of the proposed acquisition scheme. The results show that the proposed acquisition scheme can weaken the influence of data bit transitions to extend the integration time and enhance signal-to-noise ratio in weak signal environment. With the increase of accumulation time, the detection probability and acquisition sensitivity increase as well. The detection probability is 0.97 at CN0 (carrier-to-noise ratio) of 25 dB-Hz with an accumulation of 20 ms. This method is applicable to all kinds of BeiDou satellites and is also applicable to GPS, Galileo and other satellites navigation systems with or without secondary codes.

In traditional satellite navigation receivers, the parameters of tracking loop such as loop bandwidth and integration time are usually set in the design of the receivers according to different scenarios. The signal tracking performance is limited in traditional receivers. In addition, when the tracking ability of weak signals is improved by extending the integration time, negative effect of residual frequency error becomes more and more serious with extension of the integration time. To solve these problems, this paper presents out research on receiver tracking algorithms and proposes an optimised tracking algorithm with inertial information. The receiver loop filter is designed based on Kalman filter, reducing the phase jitter caused by thermal noise in the weak signal environment and improving the signal tracking sensitivity. To confirm the feasibility of the proposed algorithm, simulation tests are conducted.

BeiDou system navigation messages are modulated with a secondary NH (Neumann-Hoffman) code of 1 kbps, where frequent bit transitions limit the coherent integration time to 1 millisecond. Therefore, a bit synchronization algorithm is necessary to obtain bit edges and NH code phases. In order to realize bit synchronization for BeiDou weak signals with large frequency deviation, a bit synchronization algorithm based on differential coherent and maximum likelihood is proposed. Firstly, a differential coherent approach is used to remove the effect of frequency deviation, and the differential delay time is set to be a multiple of bit cycle to remove the influence of NH code. Secondly, the maximum likelihood function detection is used to improve the detection probability of weak signals. Finally, Monte Carlo simulations are conducted to analyze the detection performance of the proposed algorithm compared with a traditional algorithm under the CN0s of 20~40 dB-Hz and different frequency deviations. The results show that the proposed algorithm outperforms the traditional method with a frequency deviation of 50 Hz. This algorithm can remove the effect of BeiDou NH code effectively and weaken the influence of frequency deviation. To confirm the feasibility of the proposed algorithm, real data tests are conducted. The proposed algorithm is suitable for BeiDou weak signal bit synchronization with large frequency deviation.