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Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TTC System Abstract Because of the effect of carrier frequency hopping, the input IF signal of carrier tracking loop in DS/FHSS (Direct Sequence/Frequency Hopping Spread Spectrum) TTC (Telemetry, Tracking Command) System is characterized by the Doppler frequency agile. The tracking loop will shift to the frequency step response state ceaselessly and the measurement resolution severely decline, even the loop is likely to be unlocked. This paper presents a carrier tracking loop aided by frequency hopping pattern. In order to keep the stability of the tracking loop, the Doppler frequency agility in the next frequency hopping dwell is estimated and timely compensated to the frequency adjustment of carrier NCO according to the preset frequency hopping pattern and current spacecraft velocity. Simulation results show that this method effectively eliminates the instability due to carrier frequency hopping, and the resolution of loop meets the requirement of TTC system. Keywords：carrier tracking；DS/FHSS；frequency agility；aided；TTC INTRODUCTION The main function of TTC (Telemetry, Tracking and Command) system is ranging and velocity measurement. Presently, the most common used TTC systems are unit carrier system and unit spread spectrum system. For the unit carrier TTC system, ranging is realized by measuring the phase difference between transmitted and received tones, and for the unit spread spectrum TTC system, according to the autocorrelation properties of PN code, ranging is realized by measuring the phase delay between the received and local pseudonoise (PN) code. Velocity measurement in both of TTC systems depends on extracting the frequency difference resulting from the Doppler phenomena between the transmitted and received carrier. While all the processes mentioned above are finished on the ground of high resolution carrier tracking, and the phase lock loop is the common used method to implement it in TTC