Study On Space Station And Loran Common-view Time Comparison Method

The common-view time comparison method has come out since thousand years ago,and it has been widely used in the time and frequency field.In ancient times,the moon eclipse,the eclipse of secondary planet of the Jupiter,the meteors,and so on,were used as common-view references to realise the time comparison with an accuracy of several mimutes.And now the navigation satellites as the common-view references can realise a time comparison accuracy of the magnitude of nanosecond.The development history of the common-view technology reveals that the upgrade of the common-view reference results in the improvement of the common-view time comparison accuracy.On the premise of considering the developing actuality of time and frequency technology and the national improtant actions,two kinds of common-view references,the space stations and the ground-based Loran transmit stations,are studied in the paper.During the study course,the characteristics of the two references are analyzed and the traditional common-view method is optimized.The more accurate atomic clocks will be loaded in the space station,and microwave time comparison link with high performance is plan to be constructed.Therefore,the time comparison accuracy of several ten picoseconds will be realized based on the space station common-view time comparison,which is two orders of magnitude better than that of navigation satellite common-view technology in theory.On the other hand,the accurate ground-based time service system that is included in the Chinese thirteenth “five-year plan” started up in 2018.When the construction of this system is completed,alomost all the Chinese territory will be covered with the Loran timing signals.So the Loran common-view method will be applicated widely.In the aspect of the space station common-view time comparison,the main researching contents are explained as bellow.(1)The limitations are researched when traditional common-view method is applicated to the space station.Due to the orbit differences between the space station and the navigation satellites,there are two limitations if the traditional common-view time comparison method is applied to the space station.First,low orbit height results in the small earth overlay aera of the space station.So the receivers in some areas cannot receive the signal send by the space station simultaneously,and the traditional common-view time comparison method is invalid when the ground stations are in these areas called common-view blind areas.Second,the space station position error can not be cancelled effectively by the traditional method,which will restrict the time comparison accuracy to the magnitude of several hundred picoseconds or nanoseconds.(2)Combining with the high accurate space atomic clock and microwave time comparison link,asynchronous common-view method is put forward to solve the space station position error and common-view blind area problems.The space station position errors are cancelled when the geometrical position relationship between the space station and two ground stations accords with the judging condition proposed in the asynchronous common-view method.The space-ground clock bias is modeled and extrapolated in order to obtain the space-ground clock bias of the two gound stations of the same time.Then,the common-view method can be used to compare the time difference of the two ground stations.(3)The space station common-view time comparison experiments are done.The simulated platform is built,where the space station one-way time comparison,traditional and asynchronous common-view time comparison experiments are done.The experiment results show that the asynchronous common-view time comparison method is valid to realize the time comparison with the accuracy of several ten picoseconds.And it also shows that the asynchronous common-view method is helpful in solving the problem of blind area that exists in traditional space station common-view time comparison method.In the aspect of Loran common-view time comparison,the feasibility and advanteges of Loran common-view are analysed,and the basic principle of Loran common-view time comparison is researched.And then several Loran common-view time and frequency comparison experiments are done based on Loran signal transmiting landforms which are nearly the same or different.The experiment result shows that Loran ground wave signal transmiting delay need not to be calibrated in the condition that the landforms between the Loran transmit station to the two common-view stations are nearly the same and the common-view baseline is short.In this condition,several ten nanoseconds time comparison accuracy will be reached.If the landforms between the Loran transmit station to the two common-view stations are far different,although the baseline is short,the signal transmiting delay need to be calibrated.Otherwise,the common-view time comparison accuracy is limited to the level of microsecond.Loran signal transmiting delay need not to be calibrated in the common-view frequency comparison,and the difference operation between time bias data is valid to cancel most of the effect of the transmiting delay.Otherwise,the integrated positioning method of Loran and GNSS is researched.The integrated positioning algorithm based on calibration-fusion is presented to correct the systematic error of Loran system.Some integrated positioning experiments are done.The research result shows that the calibration-fusion method is good at improving the performance of integrated positioning,and the accuracy of integrated positioning is nearly at the same level as that realized by single GNSS system.