Skipping Z-counts and accurate time in GPS receivers

摘要

A navigation-satellite receiver comprises high-sensitivity radio frequency front-end and navigation processor associated with a client CPU. The client CPU runs an operating system that serially communicates with the navigation processor. The client CPU is also able to obtain navigation data system transmissions from a network server and provides such when the direct satellite signals in the high-sensitivity environment are too weak to be demodulated directly. A low power, low frequency oscillator with a watch-type crystal and counter are used as a real time clock to keep time uncertainty under fifty milliseconds when the receiver is hibernating. If the time uncertainty and position uncertainty are below certain maximums when the receiver is re-awakened, then a minimum number of satellites will be needed and making a preliminary z-count to each can be avoided.