Analysis Of The Geometric Conditions In The Site Selection Of VLBI Observation Station

The selection of a new astrometric and geodetic VLBI observation site requires comprehensive investigations on the radio interference background,climate characteristics,historical disastrous weather records,geological conditions,geographical location,as well as transportation,water and electricity supply,and many other factors.With the progressively wide application of active emission in S band in national defense,economy and daily life,the radio interference to astrometric and geodetic VLBI observations is growing seriously.To setup new observation site in remote areas away from centers of industrial and similar activities and to make use of the surrounding terrain of a site as natural barrier of possible interference,which are feasible and effective ways to reduce active emission radio interference.The adverse effects of too high topographic barrier could also block some VLBI observations,and so could result in unexpected effects on the determination of important parameters of astrometric and geodetic VLBI such as the station coordinates,baseline length,coordinates of extragalactic radio sources and Earth Orientation Parameters.In addition to considering the shielding of observations by topographical features surrounding the astrometric and geodetic VLBI station,the mutual shielding effect between two telescopes located within limited space is also discussed in this paper.The main work and conclusions are summarized as followings.1.The effect of cut-off elevation on the sky coverage of observations is analyzed.It is shown that the shielding by topographical features surrounding the station should not be more than 10 degrees.Otherwise,the observable radio sources will be reduced by about 20%.And with every degree increase in the altitude angle,the influence on the sky coverage of observations is very significant.2.The correlation between the vertical component coordinate of observation station and the zenith excess delay correction is analyzed.It is shown that the preferable cut-off elevation angle is from 5 to 7 degrees.Otherwise,the determination of the vertical component coordinate of station will be detrimentally affected.3.The influence of cut-off elevation angle on the determination of baseline length repeatability and Earth Orientation Parameters is analyzed.Based on real data analysis of astrometric and geodetic VLBI observations,it is shown that a large cut-off elevation is detrimentally affect the determination of baseline length repeatability,and so the Earth Orientation Parameters.It is preferable that the cut-off elevation angle is no more than 7 degrees.4.In accordance with practical needs of a project,the mutual shielding effect between two telescopes located within limited space is analyzed.The concept of the externally connected sphere of an antenna in Azimuth-Elevation mount is proposed.An approximate calculation method via numerical integral of the area of a spherical crown(part)is given.5.The mutual shielding effect between the 25-m and 13-m antenna at Sheshan and between the 65-m and 13-m antenna at Tianma is analyzed.The result is of theoretical instructive meanings in the design phase of the project concerning the position selection of the 13-m antennas and the height design of the supporting towers.The analytical strategy and method could also be referred by similar project practice.