Development of radio seeing monitor using LEO satellite beacons
Advanced Global Communications Technologies for Astronomy II
Introduction In the astronomical observations using the radio interferometer at centimeter and millimeter waves, the phase fluctuation caused by the atmospheric turbulence is a dominant factor limiting the angular resolution and degrading the coherency of received signals. It mainly occurs when the radio waves passing through the irregular water vapor distributions in the troposphere. Thus, the phase fluctuations reflect the spatial and temporal variations of the density of atmospheric water
... tmospheric water vapor. Study of the characteristics of the atmospheric water vapor distributions is a key item to improve the quality of radio interferometric observations, especially at higher than 5GHz. It means that atmospheric water vapor distributions are conversely given by interferometric observations of well defined reference sources. Statistical characteristics of the atmospheric fluctuations at centimeter and millimeter waves are theoretically studied  , and observations are also made with short-baseline interferometers , long-baseline interferometers , and radiometers  . From these studies, it was found that the atmospheric turbulence largely varies with the weather condition. Observations with the various instruments revealed that the outer scale of the Kolmogorov turbulence is about 10 km and the strength of phase fluctuations strongly depend on the azimuth direction as well as the elevation angle. Recently, a method using the Global Positioning System (GPS) is developed, where the zenith wet delay (ZWD) is estimated from positional variations of a ground-base station . The total precipitable water vapor (PWV) is easily obtained from the ZWD. The measurement accuracy is a few centi-meters in ZWD and few milli-meters in PWV. With this method the measurement of the wet delay along the wave propagation path from a satellite to the ground-base station is tested, which gives us the estimate of spatial distribution of the water vapor. However, the shortest sampling interval of the PWV measurement with GPS is an order of minute and is insufficient to study fine structures of the atmospheric water vapor distributions, which gives large affection to the astronomical observations using the radio interferometer. To reveal the irregularity of the atmospheric water vapor distribution and its variations, we have developed two types of radio interferometer, one is a real-time VLBI (Very Long Baseline Interferometer) observing geostationary satellite beacons - and the other a short-baseline radio interferometer observing LEO satellite beacons . The former is suited to the measurements of long-term variations, and the latter to the mapping of the spatial distributions. The accuracy of estimated atmospheric delay is an order of sub-millimeter. In this paper, the results obtained with the short-baseline radio interferometer observing LEO satellite beacons are described, and their relations with the ground-base weather conditions are discussed.