International Symposium on Antennas and Propagation


Session Number:4A09




Masanori NISHIO,  Yuki KAWAGUCHI,  Chikara MINAMITAKE,  Tomoyuki MIYAZAKI,  


Publication Date:2008/10/27

Online ISSN:2188-5079


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Water vapor is one of essential components of the earth’s atmosphere. It condenses into clouds and falls to the ground as rains or snows, carrying a large amount of energy as the latent heat to control the weather condition. In addition, the water vapor is a green house gas with an absorption line in the infrared region also to control the global weather condition. Its behavior, however, is still unclear because observations of the atmospheric water vapor, especially those in the troposphere, are insufficient. Further development of methods and instruments of observations of the tropospheric water vapor distributions has been required [1][2]. While most of atmospheric components such as nitrogen and oxygen molecules distribute uniformly, the atmospheric water vapor tends to localize with sizes from millimeter to hundred kilometers. Such a spatial ununiformity and its time variations are major factors of delay fluctuations of microwaves propagating in the atmosphere. In precise positioning with GPS (Global Positioning System), these cause positioning errors of the order of centimeters. In astronomical observations using radio interferometers, these are major factors limiting minimum detectable levels of radio sources and quality of observed images. Understanding of statistical property of the delay fluctuations is one of fundamental approaches to improve the observation accuracy. The propagation delay and its time variations due to the atmospheric water vapor are measured by precise positioning observations using GPS networks, and are given as residuals in positions relatively determined to a reference frame [3]. A minimum detectable level of the delay is of the order of 0.01 ns. Those physical quantities obtained up to date are ones averaged over a celestial hemisphere at an observation site, but measurements of the delay along the wave propagation path from a GPS satellite to the ground-base station are also reported [4][5]. For further improvement of the sensitivity and the spatial resolution of the atmospheric propagation delay observations, we have reported a new method, in which a radio interferometer designed for receiving low earth orbit (LEO) satellite beacons are used [6]. In this system, the spatial ununiformity and its time variations are given by phase fluctuations of received signals. Thus absolute values of the propagation delay cannot be obtained, but an improvement of the minimum detectable level is expected. Performance tests of the new method have been made using beacons of a satellite cellular phone system [7][8]. In this paper, a result of statistical analysis is described for the atmospheric phase fluctuation data obtained with the radio interferometer. Seasonal variations of the fluctuations will be also discussed in the presentation.