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To: David who wrote (2418)	4/14/1998 7:26:00 PM
From: Daniel Johnson	Respond to of 3506

April GPS World had an article on timing. Atomic clocks were said to often cost $55,000 per unit, with a disadvantage of additional costs in administering, operating, and maintaining the system. Atomic clocks were said to sometimes experience failures without giving any indication that they are off-frequency. A network must be setup to accept another network's timing until the problem clock is corrected. With these problems, the article said that "more telecommunications companies are turning to GPS because of its ability to self-calibrate, superior accuracy and traceability, and relatively low cost ($8000-40,000) per unit, depending on the features desired." The article described a source of top-level timing as a "system that includes a GPS receiver and precision time assembly, which includes a high-quality local oscillator that uses digital signal processing techniques to derive frequency from the 1-pulse-per-second output from the GPS sensor. As a consequence of the local oscillator's inherent stability, the GPS receiver surveys its position and then enters into a known position mode. Once in this mode, the receiver may derive time information with only a single satellite in view."