Transmitting Time and Frequency Data by Using Broadcast TV Signals Observed in Common-View
Originally Aired - Sunday, April 14 | 12:10 PM - 12:30 PM PT
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We will use unmodified broadcast TV signals in common-view to transmit time and frequency information between sites. In this method, several sites receive the same signal and measure the difference between the times of arrival of the signal at each site and the local clock. The method attenuates the fluctuations in the path delay and the accuracy is insensitive to the statistics or characteristics of the transmitter clock, since these effects are common-mode and cancel in the differences between the data received at the sites. The initial measurements will be conducted between the NIST laboratory in Gaithersburg, Maryland and the Naval Research Laboratory in Washington, DC. Both of these sites have clock ensembles that are synchronized to Coordinated Universal Time (UTC) within a few nanoseconds. The synchronization is realized by methods independent of the current tests, so that the results of the test provide an estimate of the fluctuations in the path delay and other effects that are not completely common to both signals. The method does not require any modifications to the transitter, but if the transmitted signal is synchronized to UTC then these data would provide an additional check on the accuracy and stability of the method. The initial measurements would use a measurement of the time difference between the received preamble data with respect to the local clock. Since the two stations would not necessarily be observing the same packet, the method would provide an estimate of frequency stability but not time accuracy. The method would be used for time transmission if both sites measured the same packet, and this could be done by a cross correlation after the fact (since the time difference is slowly varying) or in closer to real time by exchanging packet information between the sites. This exhcnage would have to be dne only at the start of the measurements -- the two sites would then track the same packet with no further data exchanges required. This general method does not depend on the details of the transmitted signal, and requires only that the preamble bits be recognized. The details of the payload are not significant. This method has been used for many years to compare clocks at remote sites by using signals from GPS satellites in common-view. Once the sites lock onto the preable, it may be possible to measure the phase of the transmitted carrier with respect to the local clock. This tehcnique has also been widely used in the analysis of GPS data. It provides additional resolution but not increased accuracy, since a measurement of the phase of the received carrier is ambguous modulo the period of the transmission.
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