Remote Synchronization of Onboard Crystal Oscillator for QZSS Using L1/L2/L5 Signals for Error Adjustment Toshiaki Iwata, Michito Imae, Tomonari Suzuyama National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba, Japan Akira Iwasaki, Kenji Kokubu Department of Engineering University of Tokyo Tokyo, Japan Yuji Hashibe, Satoshi Fukushima Space Engineering Development Co. Ltd. Tokyo, Japan Fabrizio Tappero, Andrew Dempster School of Surveying and Spatial Information Systems University of New South Wales Sydney, Australia Abstract— A new error adjustment method for remote synchronization of the onboard crystal oscillator for the quasi- zenith satellite system (QZSS) using three different frequency positioning signals (L1/L2/L5) is proposed. The error adjustment method that uses L1/L2 positioning signals was demonstrated in the past. In both methods, the frequency- dependent part and the frequency-independent part were considered separately, and the total time information delay was estimated. By adopting L1/L2/L5, synchronization was improved by approximately 15% compared with that using L1/L2 and approximately 10% compared with that using L1/L5. I. INTRODUCTION The Japanese Quasi-Zenith Satellite (QZS) System (QZSS) is a three-satellite navigation/positioning system conceived to improve the positioning performance (satellite availability and position accuracy) of the presently available global positioning system (GPS) in urban areas where high- rise buildings reduce the number of visible GPS satellites [1]. A new timekeeping method of the QZSS, named the remote synchronization system for an onboard crystal oscillator (RESSOX), has been planned by the National Institute of Advanced Industrial Science and Technology (AIST) [2]. RESSOX is a remote control method that permits synchronization between a ground station clock and QZS clocks. In its original concept, various delay models are used for the estimation of the delay of the RESSOX control signal that includes time information of QZSS-Time and is advanced with respect to QZSS-Time to compensate the delay during the transmission. Furthermore, pseudo-ranges of positioning signals obtained at the ground station, named the time management station (TMS), are used for error adjustment, where QZSS-Time is a standard time of QZSS, like GPS Time for GPS, and refers to UTC (NICT). The proposed Japanese QZSS has the following properties regarding its timekeeping system (TKS): (1) it is possible to control the system over a 24-hour period as long as a good choice of the TMS site is made; (2) a high-stability crystal oscillator is superior to an atomic clock in terms of short-term frequency stability [3]; and (3) the QZSS employs a maximum of three satellites, which is not too many to monitor from the ground. RESSOX reduces overall costs, satellite power consumption, onboard weight and volume, and has a longer lifetime compared with a system with onboard atomic clocks. RESSOX ground experiments and computer simulations have been conducted since 2003. QZS will broadcast four positioning signals as availability enhancement signals; L1C/A, L1C, L2C, and L5 [4]. The tentative target of our research is synchronization within 10 ns between the ground site and the QZS site and frequency stability better than 1 × 10 -13 for 100,000 s. Primary experimental results using only the L1 and L1/L2 positioning signals and experimental apparatus have been introduced in our previous papers [5-9]. We have developed a new feedback method using L1, L2 and L5 positioning signals of the QZS and proved that we could improve the performance by 15% compared with the former L1/L2 method. . Evaluations of the effects of the range error magnitude and least-squares filter used at the ground site will also be discussed. II. SIMULATION MODEL To investigate this new RESSOX technique, a specific software simulator has been developed. The actual onboard crystal oscillator will be MINI-OCXO manufactured by CMAC, and it is modeled as follows: 0.3324755V 1.7795196 - 10 023 . 1 7 + × = f 1-4244-0647-1/07/$20.00 ©2007 IEEE 1312