In 2003, Meteorological and Geophysical Bureau introduced an atomic clock and time server to provide complete time service in Macao; meanwhile, we participated in the project of JATC (Joint Atomic Time System of China) with the NTS (National Time Service, China). Due to the necessity for higher precision and accuracy in time service, we are planning to implement the usage of “common view observation” for time and frequency comparison with NTS and other international time service institutes. The techniques of time and frequency transfer can be classified as below:
l GPS/GLONASS Common View Observation
l Frequency synchronization using GPS Carrier Phase
l TWTFT (Two-Way Satellite Time and Frequency Transfer)
GPS
and GLONASS Common View Observation
GPS (Global Positioning System) of USA – USA first began the plannning and the study of global navigational satellite in 1970, follow by announcing the operation of GPS system in 1993. GPS was use in the military at the beginning; then they launched a partial of GPS in civil use. GPS is not only a precise navigational system. All time service institute in the world are using Common View to trace the time and frequecy to BIPM (Bureau Internationale de Poids et Measures), and its precision can be nano-second.
GLONASS (Global Navigation Satellite System) of Russia - It’s functions are the same as GPS, the precision of time transfer can be up to the level of nano-second.
GSPS (Galileo Satellite Positioning System) of European Union- This system is estimated will be launched in 2008. GSPS includes 30 satellites with a distance of around 24,000 kilometers from the Earth, the precision of GSPS will be higher than GPS and GLONASS.
At present, GPS and GLONASS are able to provide an economic, high precisely system of aeronautical and nautical navigation, space and geodesy observational technique and time-transfer system. In the technique of GPS/GLONASS common view observation, two GPS receivers simultaneously observes the same satellite, analyzes and compares the signals received, then the time signals of two different sites can be compared and adjusted. This observation can be used to synchronize clocks over a large geographical area with precision as good as nano-second.