Analysis and Evaluation of Time Tag Bias of USB Measurements

The Unified S-band system(USB) is a main TTC(Telemetry,Tracking and Command) network used in manned space missions and lunar exploration programs of china.Because of the influence of propagation environment of radio signal,measure environment change in USB station,instrument error and other interference,it's inevitable to contain a great variety of error in USB measurements.Therefore,it's necessary to perform error correction before orbit determination.Usually,most error correction of USB measurements principally includes the refraction error of troposphere,ionospheric delay and channel delays.Other errors such as time tag bias of observations is omitted.The influence of time tag bias of the observations of USB range and range-rate on observation accuracy is studied in detail,the characteristics of the results are analyzed,and error correction model is formulated in this paper.Compared with the influence of spacecraft ephemeris bias on observation accuracy,it shows that the influence of time tag bias is equivalent to that of orbit error along-track,thus making it difficult to separate them apart.A method to separate the time tag bias based on real time positioning results of onboard GPS receiver is proposed in this paper.A sample is given by using the real measurements obtained in a mission to separate the time tag bias.In view of the fact that the influence of time tag bias is of the same order of magnitude as observation error,it is proposed that the recording accuracy of time tag should improve to 0.1ms to decrease the influence of time tag bias.