传播效应对干涉仪的影响

以不同基线长度的干涉仪为准,从大气折射、二次波的影响和天线遮挡等三方面讨论了旋转综合孔径中的传播效应。文中给出了大气折射引起的超额路径差的估算式,地面反射波存在的条件和它引起的相位差计算式,以及考虑空隙不足有绕射损耗时天线遮挡的临界角的计算式。用一些典型数据计算了传播效应的影响,得出了一些主要结论。 

PROPAGATION EFFECT ON INTERFEROMETER

From three aspects——atmospheric refraction,effect of secondary waves andscreen effect between antennas located at each terminal of the base line,we discussed the propagation effect on interferometers with various lengthes of base line.According to the result calculated with some typical parameters,we obtained the following conclusions:1.Horizontal inhomogenity of the troposphere and that of the ionosphere are the main causes to produce the maximum relative excess path difference for shorter and longer (especially for VLBI) base line respectively,but it is mainly due to the maximum initial elevation angle difference between two terminals of the base line,for longer base line at low elevation angle.Prom the point of view that phase errores were not larger than 20°,the characteristics of the synthesized beam would not be influenced,the atmospheric refraction effect on interferometer with base line shorter than 1 km can be neglected.2.According to the accuracy of atmosphere measurement and the statistical distribution of maximum horizontal gradient of atmospheric inhomogenity,we know that the minimum spatial coherent scales are about 10 km and 400 km for troposphere and ionosphere respectively,the minimum temporal coherent scale is about 1/2 Hr.for both.So that,it is unreasonable to correct the atmospheric refraction by utilizing atmospheric model with poor accuracy,for interferometer with base line shorter than 10 km.3.In the presence of ground reflected wave,there will be introduced an phase difference error,Δφ.For VLBI the maximum of which is determined by the reflection coefficients,A_r at each terminal of the base line,that is,(Δφ)_max=sin-1 A_r1+sin-1A_r2 For shorter base line,because of unequal heights of the antennas relative to each reflecting plane,Δφmay be large at low elevation angle,but it is reduced at high elevation angle by the reduction of the equivalent reflection coefficient from the secondary beam effect of the antenna.4.The larger the ratio of the diameter of the parabolic antenna,d_a to the base line's length,D,and that of the wave length,λ,to D are,the larger the diffraction loss due to defect of the Fresnel's clearance is.But from the example calculated with (d_a:D)_max=(9:12),and λ=1.293m,onr result is that the diffraction loss is less than 1 db.