Study On Key Technologies Of Two-way Satellite Time Transfer Based On Mobile Station

The Two-way Satellite Time Transfer(TWSTT) is currently one of the most accurate remote time transfer technologies, the precision of time transfer can reach 0.3ns in the world, and can reach 0.3-0.5ns at home. As the time synchronization has a very high degree of accuracy, and now it has been widely used in time-frequency, navigation and communication etc.In military applications, strategic needs are considered fully by the time synchronization technology, so it must have a high precision, dynamicity, flexibility, anti-interference ability and the ability of independent running. In all kinds of time synchronization technologies, the technology of TWSTT need less time and it have a high precision. It is the principal method of international time transfer used by BIPM. At the same time, this topic is put forward based on the cooperation project by National Time Service Center(NTSC)and naval. It is a research on key technologies of TWSTT based on mobile station.(1) This paper introduces the principle of the TWSTT and has an analysis of the impact on the precision caused by all kinds of error sources. There are three factors which caused the impact on the precision of TWSTT: First, it is the error of equipment time delay; Second, it is the error of time delay of the transmission path; Finally, it is the error caused by the satellite movement. The most important error source is the error of equipment time delay, which is composed by the errors of time delay of counter, MODEM, satellite transponder and transmitting and receiving equipment of the ground station. The error of transmission path time delay is composed of that of ionosphere and troposphere; the error caused by the satellite movement is composed of that of asymmetrical geometric path between the two static stations and the satellite, Sagnac effect.(2) This paper analyses the principle of TWSTT between a static station and a mobile station based on the principle between two fixed stations, In the time interval of the measurement, it put forward that ?prop_ delay is the delay variation of the relative transmission caused by the relative motion of the comparison station platform, the variation of ?Sagnac depends on the absolute position of the two stations. Meanwhile, there are some key technical problems based on mobile station or stations in TWSTT, that are stable antenna system, rapid acquisition of mobile station’s coordinate, data processing and error compensation, effect of signal quality on the accuracy of time comparison.(3) The stable antenna system is one of the key issues based on mobile station or stations in TWSTT. Relative position of the satellite and Antenna pointing have changed because of external factors or man-made interferences, and the antenna of earth station doesn’t receive signals or receives poor signal quality and the uplink and downlink’s geometry paths of the signal are asymmetrical between the station and the satellite and which causes the transmission time delay is changed. By the experiment of testing the impact of the error of the antenna pointing on the performance of TWSTT, some preliminary data are obtained. It is concluded that the antenna azimuth is more sensitive than the pitch angle. In theory, whether azimuth angle or pitch angle, the maximum angles deviated from the center position on both sides are symmetrical. But in the actual experiment, it is asymmetric due to the impact of the surrounding environment(such as weather, electromagnetic wave and other factors),so when the antenna’s position of earth station is adjusted and the signal in the antenna’s main lobe zone is ensured, the azimuth angle or pitch angle’s angle range of deviations from the steady state are different, these can offer a powerful reference for the antenna rotation in earth station and provide theoretical foundation for the effect of earth station’s movement on precision in TWSTT.(4) The rapid acquisition of coordinate of mobile station is one of the key issues based on mobile station or stations in TWSTT. The effect of the position changing of mobile station on TWSTT using the data gathered from the real-time changing of vehicle position test, the effect includes two factors: the change of geometric path time delay and the change of the sagnac effect from the satellite to mobile station. By the analysis: when the position changing of mobile station is 1 meter, the change of transmission path time delay is aboat 3.1ns; the sagnac effect caused by the error of the station’s longitude, latitude and the satellite’s longitude is more than that caused by the error of the station’s height and the satellite’s latitude. So, in the TWSTT based on the mobile station, when sagnac effect is calculated and the time comparison accuracy is maintained in the nanometer level, the longitude and latitude of the station and the longitude of satellite must be ensured and kept in a certain area.(5) For the limiting of test condition, emulation and simulation are carried out on TWSTT using the ship shore. The impact of the TWSTT precision are analyzed which are caused by movement of the low-speed ship using the method of the vector algebra and calculus, and the mathematical model of TWSTT based on ship shore is concluded. By this model, the time delay errors are simulated, calculated and summarized, they are caused by the speed, the heading of the ship and the wave fluctuation. At the same time, the paper introduces: the height change of the ship is related to the ratio of the wavelength of sea wave to the length of ship and the height of wave, and one method of initial error compensation during the moving of ship based on the ship and shore in TWSTT.(6) The effect of space signal quality on the accuracy of time transfer is one of the key issues based on mobile station or stations in TWSTT. An important indicator to measure the signal quality is the carrier to noise ratio. In order to analyze the effect on the quality of signal receiving and time transfer accuracy in TWSTT caused by the motion of the satellite, with reference to the GPS precise single point positioning, this paper analyses the effects of the AM2 satellite deviated from the normal track on the link of NTSC-PTB in the Eurasian link. After the satellite started to deviated, the efficient observation data is reduced. The carrier to noise ratio of received signal decreased, and signal quality and time transfer accuracy are decreased, which has certain reference value for the research of effects of satellite or earth station motion on the measurement precision in the TWSTT.In short, based on the TWSTT between static stations, this paper presents the key technical issues of the TWSTT between the static station and the mobile station, each problem is tested analyzed, simulation and obtained using the experiment data or analog.