Research On Decoding Of Low Earth Orbit Microsatellite Telemetry Data Based On Digital Joint Carrier Tracking Loop

Carrier tracking technology is one of the key technologies for digital demodulation in satellite telemetry receiver.The deep space high dynamic environment will affect the ground station decoding small satellite telemetry data.Although the ground monitoring station has conventional decoding equipment,it is expensive and is limited by satellite pattern.In addition,low-cost solutions can also cause Doppler frequency drift problems,which will further lead to Doppler frequency compensation errors.In view of the above problems,this thesis studies the original carrier tracking loop structure based on the decoding of the original telemetry data of low-orbit small satellites in the deep space measurement and control system,and does not have the Doppler compensation equipment,and improve the decoding accuracy of small satellite telemetry data.This thesis first introduces the characteristics of the low-orbit small satellite measurement and control communication system,the telemetry data frame structure type and the relevant communication protocol for data transmission.The structure and working principle of the phase locked loop and the frequency locked loop,which are widely used in the carrier tracking technology,are analyzed.The tracking precision of the phase locked loop and the frequency locked loop are simulated and analyzed.Then,the digital phase locked loop and two different types of frequency locked loop structures are studied.Aiming at the disadvantage of low tracking accuracy of traditional phaselocked loops under high dynamics with low SNR,the digital Costas loop which is insensitive to demodulation of small satellite modulation data is studied,and the loop filtering of digital Costas loop is given.The design of the device and the numerically controlled oscillator and the tracking characteristics are simulated.Aiming at the problem of dramatic change of Doppler frequency in high dynamic environment of deep space measurement and control system,a frequency locked loop structure model which can track the Doppler frequency change rate is studied,and the performance of the frequency locked loop tracking is simulated.The results show that the frequency locked loop can indeed track the Doppler frequency change rate.In addition,for the case that the frequency discrimination characteristics of the classical frequency discrimination algorithm are not ideal,the frequency locked loop structure composed of the band edge filter based on the maximum likelihood frequency estimator is studied,and its tracking performance is simulated under the GNU Radio platform.Based on the above research,this thesis proposes a new joint carrier tracking scheme combining a second order digital Costas loop with a second order frequency locked loop structure based on band edge filters.The simulation shows that the new joint carrier tracking loop has better tracking performance for both frequency step and frequency ramp signals.Finally,the GNU Radio platform compiles the code to realize the modularization of the tracking scheme,and applies it to the existing small satellite telemetry data decoder.The experimental results show that the carrier tracking accuracy of this scheme is high,and the number of decoded packets is about ten more than the number of decoded decoders before the improvement.