Research On Systematic Errors Calibration Method Based On Double Mobile Stations In Two-way Satellite Time Transfer

Two-way satellite time and frequency transfer(TWSTFT) is currently one of the most accurate remote time transfer technology. With an outstanding feature of nearly symmetry on the signal transmission path, TWSTFT eliminate majority signal path delay. However, equipment transmission delay on a TWSTFT ground station is variable, which leads TWSTFT to need a regular calibration.The traditional calibration methods mainly include the satellite simulator method and the single mobile station method. The satellite simulator method is an absolute calibration. Its calibration process is complicate, and the calibration period is long. The single mobile station calibration method is a relative calibration, which measures equipment time delay difference between the mobile station and each fixed station respectively, and then obtains the equipment time delay difference between pairs of fixed station. At present, there are mainly four kinds of modems of TWSTFT in China. Moreover there are mainly two kinds of signals, the C band and Ku band, in TWSTFT. According to the traditional single mobile station calibration method, eight kinds of mobile stations need to be developed for meeting our requirement of TWSTFT calibration. The cost is too high. In addition it is also difficult to calibrate the systematic errors among mobile stations. Therefore a new systematic errors calibration method is proposed based on double mobile stations.The main research and originality of this paper are as follows:(1) Studying on the link errors of the TWSTFTThe effect of tropospheric is studied for TWSTFT using Atmospheric Millimeter Wave Propagation Model and Marini Model. The results show that the influence of tropospheric is only a few picoseconds for TWSTFT, so it is negligible. The dual frequency correction model can be applied in correcting the influence of ionospheric delay in TWSTFT. And Sagnac effect which has a significant influence on the TWSTFT, can be calculated with the corresponding formula.The satellite motion is also a significant influence factor on the TWSTFT results, so it needs to be considered. Equipment time delay is the main error in TWSTFT. This paper uses double mobile stations to calibrate equipment time delay difference between pairs of TWSTFT stations.(2) Proposing a new TWSTFT calibration methodBased on the requirements of TWSTFT calibration in China, a method to calibrate the systematic errors between pairs of TWSTFT ground stations is proposed by using double mobile stations. The specific calibration process is designed according to features of system: Firstly two mobile stations are placed in A site to do zero baseline calibration in order to obtain systematic errors T1 of the two mobile stations at A site environment(mainly including equipment time delay difference); Secondly the two mobile stations are respectively placed in A and B sites to test and get the measured results T2, where two clocks placed in A and B sites provide the reference signals; Thirdly the two mobile stations are both placed in B site to recheck systematic errors of the two mobile stations by zero baseline calibration for obtaining systematic errors T3 of the two mobile stations at B site environment. The mean of T1 and T3 is considered as the systematic errors of the second step. After the T2 deduct the systematic errors, the actual clock difference is measured between A and B stations by double mobile stations method. Calibrating the clock difference measured by two fixed ground stations using the clock difference measured by two mobile stations, the systematic errors of two fixed ground stations can be calibrated by double mobile stations method.(3) Studying on the effect factors on double mobile stations calibrationThe influence factors of double mobile stations calibration errors are studied. The factors contain code rate, signal power, temperature, operating frequency, rain attenuation, location and so on. The results show that equipment time delay of the prototype is associated with signal power of the transmitting and receiving. Fixed transmit and receiving power are required in TWSTFT, so that the effect of signal power for equipment time delay in repeated testing is minimum. The results on sunny days are better than the ones on rainy days nearly one orders of magnitude. So the test on rain is not recommended.(4) Developing principle prototype of double mobile stationsDouble mobile stations consists of two identical equipments. Each equipment is composed of pulse signal distribution amplifier, modem, Ku band RF transceiver channel, network time server, data transmission module and time synchronous precision evaluation software between pairs of stations.(5) Developing high precision time synchronous precision evaluation software between pairs of stationsThe main function of the software is to receive measurement data from two stations and transmit the local measurement data. It also has other functions such as the parameter setting, data acquisition, data processing, time difference calculation, condition monitoring, data storage, display, output results and so on. So it can realize the evaluation of time synchronous precision between pairs of stations.(6) Designing and fulfilling various experiments to verify the performance of double mobile stationsThe disassembly consistency and switch consistency tests of equipment delay are designed. The consistency of before and after disassembling equipments and the consistency of before and after switching equipments are validated. Zero baseline test is designed to test accurately systematic errors of double mobile stations based on the zero baseline. Lintong-Luonan short baseline test and Lintong-Changchun long baseline test are designed to validate test performance of the system; Satellite 1 and 2 were respectively used to research the effect of different satellites for TWSTFT calibration.All in all, in this paper the error distribution law and calibration method of TWSTFT in the subnanosecond are analyzed. The systematic errors calibration method based on double mobile stations is proposed. Accroding to the theoretical analysis and tests, systematic errors of TWSTFT can be calibrated within 0.5ns by using double mobile stations method. The advantage of the proposed method is its signal system and work way independed. The systematic errors in other time comparison system can be calibrated by means of double mobile stations method.Therefore this method is very promising.