Millisecond Pulsar Rivals Best Atomic Clock Stability

摘要

The measurement time residuals between the millisecond pulsar PSR 1937+21 and the reference atomic time scale UTC(NBS) have been significantly reduced. Analysis of data for the most recent 768 day period indicates a fractional frequency stability, (modified Allan variance) of 3 x 10(exp-l6) for an integration time of 240 days. The improved stability relative to the earlier analysis is a result of three significant improvements. First, an upgraded data acquisition system was installed at Arecibo Observatory, substantially reducing several kinds of systematic measurement errors, Second, the Loran-C link to the Arecibo Observatory clock was replaced by a GPS common-view link, effectively removing the link noise from consideration. Third, the measurements were made in two widely separated frequency bands. And fourth, the reference atomic clock was improved. Using the information from these measurements allowed us to partially account for dispersion caused by free electrons along the 12,000 to 15,000 light year path from the pulsar to the earth. With data taken every two weeks, the final residuals are nominally characterized by a white phase noise at a level of 243 ns. The total interstellar electron content was found to follow a random walk by up to 12ppm over the 768 days.