Daily Earth rotation determinations from IRIS very long baseline interferometry
Classical optical determinations of Earth rotation (UT1) require several weeks of observations to achieve an accuracy of ∼1.5 ms of time, whereas modern techniques have demonstrated an accuracy of 0.7–0.9 ms on timescales of 1–5 days l,2. Here we have probed the limits of the accuracy and time resolution of very long baseline inteferometry (VLBI) determinations of UT1 by conducting a series of 1-h observing sessions using only one baseline. Comparison of the results from the 1-h sessions with corresponding determinations from 24-h multi-baseline sessions establishes that the accuracy of the 1-h determinations is close to 0.1 ms of time. These observations were scheduled as part of the project MERIT (monitor earth rotation and intercompare techniques of observation and analysis3) intensive observing campaign.
Comparison of Earth rotation as inferred from radio interferometric, laser ranging and astrometric observations
New techniques are being developed to improve the spatial and temporal resolutions achievable in measurements of the rotation vector of the Earth. Such improvements are expected to expose fine details of this rotation and to aid in understanding their physical causes. Although not yet fully developed, two of these techniques are now in routine use: radio interferometry and laser ranging. In the former technique, antennas up to thousands of kilometres apart are used to observe compact extragalactic radio sources. The differences in arrival time of the signals propagating from each source to the antennas, measured by the method of very-long-baseline interferometry (VLBI), are sensitive to variations in each of the components of the Earth's rotation vector. In the latter technique, lasers are used to measure the distance (range) to artificial Earth satellites (satellite laser ranging, SLR) and to optical retroreflectors on the Moon (lunar laser ranging, LLR). To assess the accuracy being achieved in the measurement of Earth rotation it is useful to compare results obtained with these different techniques. National programmes and international cooperation, exemplified by the International Astronomical Union/International Union of Geodesy and Geophysics project MERIT1,2, are encouraging the collection of data suitable for accurate intercomparisons. Here we compare results for a 400-day period from late September 1980 to December 1981, for which data from all three types of measurements are available. We also compare corresponding results from classical astrometric observations which demonstrate the degree of improvement in accuracy afforded by the new techniques.
Direct Overwrite by Light Power Modulation on Magneto-Optical Multi-Layered Media
The present authors have developed a new direct overwrite method on multi-layered Magneto-optical(MO) memory media by light power modulation [1]. (In this experiment double-layered MO memory media are used.) The key points were the media constituing of a memory layer and a reference layer, and an auxiliary magnet which initializes the reference layer. The materials of the two thin film layers we have tested were TbFe/TbFeCo and TbFe/GdTbFe. The both combinations worked well. The former conbination gave us the following result; media velocity = 13.4m/sec, readoput C/N (RBW = 30kHz) = 54dB, 47dB, 44dB for 1MHz, 6MHz, 8MHz respectively.