对载波跟踪混合DS跳频扩频测控系统的研究-外文翻译.docx

Research on Carrier Tracking in Hybrid DS/FHSpread Spectrum TT&C SystemAbstractBecause of the effect of carrier frequency hopping, the input IF signal of carrier tracking loop in DS/FHSS (Direct Sequence/Frequency Hopping Spread Spectrum) TT&C (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 TT&C system.Keywords: carrier tracking； DS/FHSS； frequency agility； aided； TT&CI・ INTRODUCTIONThe main function of TT&C (Telemetry, Tracking and Command) system is ranging and velocity measurement. Presently, the most common used TT&C systems are unit carrier system and unit spread spectrum system. For the unit carrier TT&C system, ranging is realized by measuring the phase difference between transmitted and received tones, and for the unit spread spectrum TT&C 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 TT&C systems depends on extracting the frequency difference resulting from the Doppler phenomena between the transmitted and received carrie匚 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 TT&C system・ As the space electromagnetism environment become more and more complicated, the capability of anti-jamming is required by the future TT&C system [1]. So we consider using the hybrid DS/FHSS (Direct Sequence/Frequency Hopping Spread Spectrum) technology to build a more robust TT&C system.For many ordinary hybrid DS/FHSS communication systems, the most important function is demodulating data but not measuring, so it is not necessary to measure the carrier frequency precisely. However, in hybrid DS/FHSS TT&C system, measuring and tracking the carrier precisely is the foundation of system, so some special problem needs to be solved・ In the hybrid DS/FHSS TT&C system, even the received signal has been dehopped by the pattern synchronization module, due to the Doppler Effect and carrier frequency hopping, the input frequency of tracking loop contains frequency agility severely・ As a result, the loop is likely to shift to the frequency step responses state again and again, and it seems to be impossible for frequency measurement and carrier tracking・ The paper is organized as follows. In section I, the frequency hopping pattern synchronization module in the DS/FHSS TT&C system is introduced. In section II, we analyze how the carrier frequency hopping influences the performance of the carrier tracking loop・ In section III, a carrier tracking loop aided by frequency hopping pattern and current spacecraft velocity is proposed・ In section IV, a simulation mode on the ground of actual requirement of TT&C system is built and the results of simulation show that this method is very simple and effective for DS/FHSS TT&C system. Finally, some conclusions are drawn in section V.II. INPUT SIGNAL OF CARRIER TRACKING LOOPAs the traditional TT&C and communication system, the input signal of carrier tracking loop must be a monotonous intermediate frequency signal, so the received RF signal should be dehopped by the frequency hopping pattemsynchronization module・ In FH communication system, the signal during a hop dwell time is a narrowband signal and the general power detector is commonly used to detect the frequency hopping signal [2]. But in the hybrid DS/FHSS TT&C system, the signal is submerged in the noise, it is impossible to acquire signal directly by power detector such as FH communication system. However, the signal during a hop dwell time in the system just is a direct sequence spread spectrum signal, so we can acquire it based on the acquisition of direct sequence spread spectrum signal. The acquisition methods, such as serial-search acquisition, parallel acquisition and rapid acquisition based on FFT have been discussed in a lot of papers [3-5], so we wont discuss the problem detailedly in this paper. In our system, since one hop dwell time is very short, the rapid acquisition based on FFT which can extract the phase delay and carrier frequency at one time will be the best way for acquisition. The scheme of the frequency。