| As an extremely important basic physical quantity,time plays a huge role in the field of space technology,especially high precision time frequency.It is more and more widely used in satellite navigation and positioning system,deep space exploration,verification of relativistic basic physical testing and geophysical measurement.These applications and contributions of high-precision time and frequency to physical science and space science require the time-frequency transfer technology with matching accuracy as the application support.For example,the manned space station in China uses the microgravity environment of the space station and the combination of atomic clocks with different characteristics to achieve the spatial time and frequency reference with uncertainty of 1E-18 to 1E-17 magnitude.With the precision of space time frequency reference getting higher and higher,there will be a series of significant research results based on high-precision time-frequency reference in the next 20 years.However,the current space time frequency transfer technology in China can only realize the frequency stability of 1E-15 and the time synchronization accuracy of 100 picoseconds,which is far from meeting the demand for the application of precision time frequency reference.High precision time frequency transfer has become the bottleneck of the application of precision time frequency reference in space science and technology.Aiming at the demand of high-precision time-frequency transfer between satellite and ground,this paper studies the precise time synchronization technology based on two-way time difference measurement system,focusing on the carrier phase cycle slip detection and repair method under the three-frequency measurement system and the integer ambiguity processing technology.The main contents of this paper include:1.The technical characteristics of the link in this paper are introduced.The multifrequency correction of link error and high-precision satellite-ground two-way time synchronization are realized by double downlink and single uplink link system.The two-way time synchronization principle based on pseudorange and the two-way time synchronization principle based on carrier phase are introduced.2.In view of the characteristics of high-dynamic spacecraft such as low orbit and fast flight speed,the visibility law of high-dynamic spacecraft and ground station is analyzed,and the error term in the satellite-ground measurement link is analyzed.Among them,the space distance error,clock error reduction error,atmospheric error,relativistic effect,Sagnac effect error and gravitational delay difference caused by the relative motion of the satellite and the ground are corrected by the corresponding algorithms and models.The multipath effect reduces its influence by external means,and the channel delay difference is corrected by continuous calibration.3.Considering the uplink cycle slip detection,a triple-frequency cycle slip detection and repair algorithm is proposed based on the uplink and downlink.Combined with the downlink double-frequency cycle slip detection and repair algorithm,all the preset cycle slips can be detected in the simulation experiment.It is found that the method of calculating the cycle slip by the test quantity is affected by the pseudorange observation noise.The high precision calculation of the cycle slip is realized by the least squares Legendre polynomial fitting,and the mm-level cycle slip repair accuracy on each link is realized.4.In this paper,the link system data has only one set of satellite and station.Because the channel delay makes the ambiguity lose the integer characteristic,the traditional method eliminates it through the double difference combination,so that the ambiguity restores the integer characteristic.Limited to the data characteristics in this paper,restoration of integer ambiguity by external UPD product calibration,and the ambiguity is solved by the combination of pseudorange and carrier phase.However,the pseudorange observation noise will affect the calculation accuracy.The influence of pseudorange observation noise is reduced by the method of bidirectional phase smoothing pseudorange,and the floating solution of ambiguity is solved.The ambiguity is fixed according to the Mohalanobis distance and acceptance test.5.Based on the simulation model of satellite-ground time synchronization with two-way time difference measurement system,the performance of satellite-ground time synchronization is evaluated by internal coincidence method,and the influence of different cycle slip types on satellite-ground two-way time synchronization at different stages is analyzed.The results show that the performance of satellite-ground time synchronization is effectively guaranteed by using the cycle slip detection and repair method proposed in this paper,and the time synchronization accuracy of subps level is realized.The performance of satellite-ground two-way time synchronization before and after the smoothing ambiguity floating point solution and the ambiguity fixed solution is analyzed.The results show that the two-way time synchronization performance of the smoothing floating solution is improved by one order of magnitude,and the optimal fixed solution can achieve high precision satellite-ground time synchronization accuracy of sub-ps level. |
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