In this paper we investigate methods to achieve highly accurate time synchronization among the satellites of the COMPASS global navigation satellite system(GNSS).Owing to the special design of COMPASS which implements several geo-stationary satellites(GEO),time synchronization can be highly accurate via microwave links between ground stations to the GEO satellites.Serving as space-borne relay stations,the GEO satellites can further disseminate time and frequency signals to other satellites such as the inclined geo-synchronous(IGSO)and mid-earth orbit(MEO)satellites within the system.It is shown that,because of the accuracy in clock synchronization,the theoretical accuracy of COMPASS positioning and navigation will surpass that of the GPS.In addition,the COMPASS system can function with its entire positioning,navigation,and time-dissemination services even without the ground link,thus making it much more robust and secure.We further show that time dissemination using the COMPASS-GEO satellites to earth-fixed stations can achieve very high accuracy,to reach 100 ps in time dissemination and 3 cm in positioning accuracy,respectively.In this paper,we also analyze two feasible synchronization plans.All special and general relativistic effects related to COMPASS clocks frequency and time shifts are given.We conclude that COMPASS can reach centimeter-level positioning accuracy and discuss potential applications.
WANG ZhengBoZHAO LuWANG ShiGuangZHANG JianWeiWANG BoWANG LiJun
An improved method for stabilizing a frequency-quadrupled 214.5-nm tunable diode laser system is re- ported. Improvements to the method include a homemade logic circuit and the use of a Fabry-Perot optical spectrum analyzer as a transfer cavity. Lasers locked with this method exhibit megahertz-level frequency stability measured with an optical frequency comb referenced to a cesium atomic standard. The laser can be locked for hours to days, depending on experiment requirements. Being relatively inexpensive, stable, and robust, the control method can be applied to stabilizing essentially all lasers of deep ultraviolet wavelengths.
