Research And Implementation Of GNSS Signal Tracking Technology For High-orbit Spacecraft

The orbital altitude of high-orbit satellites is so high that one satellite can cover almost the entire hemisphere.Therefore,it plays an extremely important role in communications and meteorology.The issue of spacecraft orbit determination is the focus of research in the high-rail field.The traditional satellite orbit location technology uses ground stations for positioning.When the number of high-orbit satellites is too large,it will inevitably increase the processing difficulty of ground stations.At present,China has not completed the construction of globally covered ground stations.The Global Navigation Satellite System(GNSS)is a comprehensive navigation and positioning system that provides users with round-the-clock positioning and timing services.With the development of science and technology,the application scope of GNSS is becoming more and more extensive.It is of great practical significance to apply the existing GNSS system to the autonomous positioning of high-orbit spacecraft.Compared with the relatively well-developed low-orbit satellite orbit location technology,there are still many problems to be solved in the GNSS positioning of high-orbit spacecraft.At the beginning of design,GNSS navigation satellites mainly serve the ground users.Therefore,the radiation directions of the navigation satellite signals all point to the center of the earth.However,the orbit of the high orbit spacecraft is higher than that of GNSS satellite,so the GNSS receiver of the high orbit spacecraft can only receive signals from the opposite side of the earth.Due to the distance between the navigation satellite and the high orbit spacecraft,the path loss and atmospheric loss will increase much more than the terrestrial receivers,resulting in too low signal to noise ratio of the received signal.And due to the harsh conditions of the received signal,the number of visible stars in the receiver is also greatly reduced.In order to solve the problems of high orbit spacecraft autonomous orbit positioning,this paper analyzes the space environment of the high orbit spacecraft,determines the minimum sensitivity required for the receiver to complete accurate positioning,and the Doppler shift range of the received signal.Study the weak signal tracking strategy to improve the traditional GPS signal tracking loop.Considering that the improved tracking loop will be implemented and verified on the hardware platform,it is not appropriate to use an algorithm with too high computational complexity.This paper improves the signal coherent accumulation time and loop bandwidth to improve the signal-to-noise ratio of the output signal of the tracking loop to reduce the frequency tracking error.According to the proposed method,simulations were performed on the MATLAB platform.It was found that when the loop bandwidth is 5Hz and the coherent accumulation time is 20 ms,the tracking loop can perform reliable signal tracking when the SNR is as low as-39 dB.Then based on the space environment and application scenario of the high-orbit spacecraft,the gain of the beamforming technology for the RF front-end of the receiver is analyzed.Finally,the improved tracking loop was simulated and verified on the DSP hardware platform.The results were consistent with the software simulation and satisfied the minimum sensitivity required for autonomous orbit determination of the high-orbit spacecraft.