39th Annual Precise Time and Time Interval (PTTI) Meeting COMBINATION OF GPS PPP AND TWO-WAY TIME TRANSFER FOR TAI COMPUTATION

To follow the CCTF 2006 recommendation of using high-precision GPS carrier-phase data (CP) for TAI generation, BIPM computes monthly the GPS PPP (Precise Point Positioning) time links and publishes the result on the TAI ftp site. The GPS PPP is a combined solution of the codes and CP information. The latter assigns the high precision to the PPP result. It is shown that the PPP links have a better time stability than the TW ones in short and middle terms (up to a week). On the other hand, TW could be more stable on the long term, due to its symmetric measurement procedure and its more accurate absolute calibration. The combination of PPP and TW may benefit the advantages of both links. Because the GPS and TW are completely independent techniques, combination of them makes the results less noisy, less biased, and more robust. The combination gives also a possibility to analyze the errors presented in each technique. One of them is the diurnal deviation in some TW KU band links. In this paper, we first briefly review the recent study on the uncertainties of PPP and TW, then quickly recall the model used for the combination, and finally evaluate of the method proposed using measuring data. The TAI time network is highly redundant. This approach gives a new idea about fully using the potential of the two fundamental time transfer techniques: GPS and TW