| The measurement of geopotential and orthometric height are basic research issues of geodesy.With the rapid development of time and frequency technology,relativistic geodesy become popular these years.ESA and CNES made a joint plan of ACES mission,in which two atomic clocks of high precision will be launched to ISS in 2021.Using these clocks,it will be possible to transfer time and frequency signals between ISS and ground stations with planned precision of 2×10-16.And one of ACES's science objects is to test gravitational redshift with high precision and do researches on measuring geopotential.Based on ACES mission's three microwave frequency links,this paper established a pioneering model named tri-frequency combination to extract relativistic redshift values and then obtain geopotential and orthometric height of ground stations.In this research,Doppler frequency shift,refractive frequency shift,ionospheric and tropospheric frequency shift,transverse Doppler frequency shift and Shapiro frequency shift are taken into account,to the precision level of c-3.Furthermore,we analyzed high order ionospheric part,tidal effects,etc.,and results show these residual errors will not influence much with the precision target of meter level.To further test our model,we designed a simulation experiment of ACES mission,in which real gravity field,ISS orbit,parameters of ionosphere and troposphere and various clock noises are taken into account.The orthometric height obtained from our experiment is only 0.4 m away from that obtained by gravity model,satisfying well with ACES's precision demand.In the meantime,we analyzed the demanded precision of input parameters,including the position,velocity and time information of ISS and ground station.Tri-frequency combination model will be useful for ACES data processing,to obtain geopotential and orthometric height of at least meter level.It is also possible to be applied to Chinese Space Station under construction.If more precise error corrections are taken into account,it is possible to obtain results with higher precision level. |
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