| Onboard time and frequency generation and maintenance systems are used toprovide accurate, reliable, stable and continuous time and frequency referencestandard for navigation satellites. Its implement is based on the output of space atomicclocks as reference and integrating the resources of hardware and software. Theperformance of reference standard is related to the precision of positioning, timingand velocity measurement of satellite navigation system. Accurate signal depends onthe performance of atomic clock and signal generated chain when reliable and stablesignal is maintained depending on high-precision measurement and steering capacity.Because the environment of onboard satellite is complicated and can’t intolerancefailure, signal hot-standby redundant is used widely to enhance the reliability. In orderto ensure a smooth transition to the standby signal when the master signal is failure,there is high-precision measurement device to keep the master and slave signalsynchronizing. So studying the methods that high-precision measure and autonomousintegrity monitor time and frequency reference signal of onboard satellites haveimportant scientific significance and practical value.The key issues of monitoring and measuring time and frequency reference signal ofonboard satellites includes: high-resolution measurement method of special frequency,autonomous integrity monitoring method of reference signals under limited resources.This paper analyzed the research results of onboard time-frequency generation andmaintenance at home and abroad, and then researched the particular runningenvironment of satellite. The high-resolution phase difference measurement andautonomous integrity monitoring method are proposed based on previously mentionedresearch. Finally, a prototype have been developed and tested to validate theperformance of proposed method.The main research contents and innovation as follows:(1)Propose a new scheme of onboard time-frequency generation andmaintenance system based on analyzing the advantages and disadvantages of existingscheme. And then the key issues are analyzed in that scheme. (2)According to the characteristics and requirement of space atomic clock outputfrequency and running environment, a high-resolution phase difference digitalmeasurement method is proposed. It can measure synchronously two signals toremove the system error. A prototype has been developed based on DSP, with highmeasurement accuracy, small size, low power consumption, and it is suitable forapplying in satellite.(3) Analysis and simulation are implemented to determine the reasons of thesystem errors and effect on measurement accuracy in theory. And then theimprovement measures are proposed correspondingly.(4)Analyzing typical abnormal states of space atomic clock and its influences,proposed a solving scheme that realize the autonomous integrity monitoring based oncommon oscillator of measurement system without additional hardware. The schemecan determine which clock is abnormal and what happen. Abnormal states that can bedetected include space clock failure, frequency of phase hopping.(5) A test platform has been built to test the performance of developed prototype.It achieves including: the phase difference measurement resolution better than1ps,frequency difference measurement resolution is better than10μHz.When the detectionthreshold of is set as0.3028mHz，the frequency jump bigger than0.8mHz and phasejump bigger than0.078ns can be easily detected and distinguished towards thesimulated reference frequency of5E-12/s.This paper follows three stages such as theory analysis, experiment simulation, andprototype testing, which argue the feasibility of proposed method. Prototype has beentested and achieved that typical noise floor is superior to1E-12/s, and the best noisefloor reach6.76E-13/s. The performance is better than the monitoring requirements ofonboard time and frequency reference standard. |
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