Reaearch On Two-way Satellite Time And Frqquency Transfer Errors

Two-way satellite time and frequency transfer（TWSTFT）is one of the mostaccurate mean for remote clock comparison with an uncertainty in time of less then1ns and with relative uncertainty for frequency of about1part in1014at averagingtimes of one day. It has been used to calculate the Coordinated Universal Time（UTC）through ITU’s recommendation by1999. TWSTFT is widely used in time andfrequency community，telecommunication administration and satellite navigationsystem.The signal is almost symmetrical in TWSTFT. It is mostly canceled out forionosphere delay, troposphere delay and geometrical delay and so on in principal. It isthe advantage of TWSTFT and the reason for high accuracy. Along with thedevelopment of TWSTFT and the denseness of observation，it is showed that diurnalvariation systematic errors about1～3ns in TWSTFT. This problem became theresearch hotspot on the world and ITU tries to solve it. It is studied for each error indetailed through the data of Transfer Satellite Orbit Determination Net andinternational TWSTFT links:(1) The atmosphere effect: For avoiding the disturbing, the frequencies aredifferent for up link and down link signals. It results to the atmosphere effect inTWSTFT. This includes ionosphere and troposphere effects. The troposphere effect isvery small. If the time comparison accuracy is not high，it can be ignore. Theionosphere effect is obvious. It can used the IGS ionosphere product to modify theerror for after-processing technology. The ionosphere variation is about0～0.05nsand0～0.7ns in KU band and C band respectively and has the diurnal variationcharacter.(2) The equipment time delay: The equipments delay is closely relation withtemperature variation. It is showed that the equipment delay variety with temperatureis linear in the normal temperature. The outdoor part is diurnal variation with theenvironment temperature. The modem is the major equipment in TWSTFT. It isstudied for each error effect related with modem. The resolution is proposed.(3) The satellite transponder effect: It is studied in detailed for the satellitetransponder by the data of international TWSTFT links. It is analysis that differentsatellite transponder can highly increase the amplitude of diurnal variation in one TWSTFT link. This is the major reason for the diurnal variation systematic errors inTWSTFT. The function fitting is used to cancel out this problem.(4) The satellite motion effect: The geostationary satellite is not absolutelystationary，but it dose the diurnal motion that the amplitude is small. It is regarded asstationary for geostationary satellite and the Sagnac effect is constant in conventionalmethod. It is studied the effect of satellite motion in detailed through the data ofTransfer Satellite Orbit Determination Net for TWSTFT. Satellite motion is a kind ofthe incomplete reciprocity and can result that the uplink geometry paths aren’t equalto the downlink geometry paths. Satellite motion’s correction depends on the sumsatellite velocity at sight direction and time difference between the signal from twostations arrive at the satellite. It is corrected the effect by the pseudorange differencebetween the epochs method. It is about0.1ns and0.05ns respectively for the satellitemotion result to the Sagnac effect’s and geometry paths delay’s diurnal variation.The GNSS has been the major time transfer technology form1980’s. Theshort-term stability of GNSS is better than TWSTFT，but the long-term stability ofTWSTFT is better than the GNSS. It can decrease the amplitude of diurnal variationthrough the method of combination of GNSS time transfer and TWSTFT usingVondrak-Cepek filter. It has the important significant to many fields for studying theTWSTFT.