Research on Signal Design Method of Pseudolite 'Near-Far Effect' Based on TDMA Technique

摘要

The Global Navigation Satellite System (GNSS) provides comprehensive and instantaneous positioning information for human life and production, causing huge changes in various industries and fields. However, in practical application, it is obvious that the accuracy of GNSS cannot meet all industry requirements. The integrity, reliability and continuity of GNSS are seriously affected by the satellite distribution. In some special locales, such as basement, indoors, open mines and tunnels, GNSS often cannot offer correct positioning information; furthermore, it doesn't function at all. The requirements of high accuracy and good reliability promote the proposition and development of pseudolite. There are many problems to be solved in pseudolite application, such as multipath effect, time synchronization and near-far effect. In this paper, we study how to solve the near-far effect. In short, the near-far effect is the large difference in the received signal power caused by the difference in the distance between the orbiting satellites, pseudolites and receivers. We choose TDMA technique to design pseudolite pulse signal, after analysing the existing three methods TDMA, FDMA and CDMA techniques' advantages and disadvantages. A 10% duty cycle hopping sequence pulse method is proposed by using RTCA and RTCM pulse mode for reference. In MATLAB simulation environment, the pulsed signal acquisition method based on the segmented FFT is used to achieve pulsed signal acquisition simulation in the case of different SNR (signal to noise ratio) of pseudolites. And then, the closed-loop delay based on the lead-lag half symbol and the three-order phase-locked loop (PLL) assisted by second-order frequency-locked loop (FLL) are used to achieve pulsed signal tracking simulation in the case of different SNR. The simulation results show that the design of pseudolite pulsed signal can overcome the near-far effect.