Experimental Evaluation of Fine-Time Aiding in Unsynchronized Networks

摘要

We demonstrate that the long-term stabilities of the signalsfrom an unsynchronized cellular network can be exploitedto provide fine-time aiding to a mobile GPS receiver. Thetime to first fix (TTFF) of a GPS receiver can be shortenedby providing accurate GPS time to the receiver, thus improvingthe user experience. We present the results of experimentsmade to measure the stability of an unsynchronizedcellular network. These show a large heterogeneity inthe overall stabilities of the time signals from the individualcellular base stations and the importance of the mechanismsused to control disciplined oscillators. We introducean ensemble filter with the goal of improving the long-termstability of the local oscillator in a mass-market GPS device,without the need for specific equipment in the cellularnetwork. The filter is evaluated for two types of oscillatormodels and we show that a simple random-walk model canprovide time aiding with an accuracy better than 50 μs over4 days in a GSM unsynchronized network.