大氣折射與天線指向PPT課件

1、大氣折射與天線指向 the pointing accuracy9.5 for 25M telescope at 30 GHz. 2.0 for 110M telescope at 30 GHz. Thermosphere (熱電離層)Mesosphere (中間層)Stratosphere (同溫層)Troposphere (對流層)Within the neutral atmosphere, the propagation of radiowaves is most affected by the troposhere.物理機制物理機制 Molecular resonances in a n

2、eutral dielectric gas produce both absorption lines (nonzero imaginary) and changes in refractivity (changes in the real part). Each absorption line at frequency fabs contributes a fixed amount to the refractivity at all frequencies below fabs. The strongest water-vapor effects occur at infrared wav

3、elengths, making the radio refractivity of water vapor much 22 times higher than its to optical refractivity. Thus optical refraction data are useful predictors for the dry component of radio refraction but not the wet component. On the other hand, O2 absorption line at about 60 GHz that its effect

4、on the real part of radio refractivity cannot also be negligible, a observation below it. In summaries, the radio wavelength region the tropospheric refractivity is determined mainly by oxygen O2 and water vapour H2O.三段折射改正區(qū)域三段折射改正區(qū)域(1). Under the assumption that the atmosphere can be described by a

5、 set of plane parallel layers of slowly decreasing refractivity, zenith angles of 065 degrees, leads to the simple relation for the angle of refraction, where the angle z is the zenith angle and n0 is the index of refraction at the level of measurement.(2). If we describe the atmosphere by a number

6、of parallel spherical layers, zenith angles of 6580 degrees, the resulting angle of refraction contains an additional term with a coefficient about three orders of magnitude smaller than above, (3). GBT proved to be quite accurate down to zenith angles of 8085 degrees. A simple “fix” for this proble

7、m is to multiply the value of above by the factor, (經驗的)計算計算n0 where Pd is the partial pressure of the dry atmosphere, e the partial pressure of water vapour, both in hPa. And T is the temperature in K (開爾文).(1). With typical sea level values Pd=1000, e=10, T=288.(2). On a high and dry radio telesco

8、pe site we might have Pd=700, e=2, T=270. (3). The difference illustrates the importance of applying real-time refraction corrections determined from local temperature and atmospheric pressure measurements.折射對天線效率的影響折射對天線效率的影響The GBT or QTT primary diameter 110m is a finite fraction of the atmospher

9、ic scale, then the refraction angle difference between the lower and upper edges of the dish at low elevations,about 5 degree, is + 3.7 - - 3.7最后一部分Horizontal temperature gradients in the surroundings of the observing station probably make one of the most important sources of error in the determinat

10、ion of refraction. They are particularly dangerous if strong enough to be accompanied by the characteristic wind, the sea breeze, the lake breeze, the land forest breeze. it is evident that the temperature gradients will cause the astronomical refraction to be somewhat different in different azimuth

11、s.For z1=80, Tgrad=150 Another important effect is caused by large blobs of wet air passing through the beam of the antenna at relatively small distances from the aperture. We consider a wedge of wet air passing with the wind over the aperture in the Fresnel region (near-field) of the antenna. Assum

12、e that a gradient in the wet part of the refractivity 5 over an assumed thickness of the wedge of 100m. The pathlength variation will be 0.5mm. This pathlength difference between up-to-centre edges of the antenna of radius D will cause a change in a angle as, (度) Thus, over the width of an aperture of 110m this amounts to an angle of about 2.5 arcsecs. These total effects are as, 2.5 + 3.7 6.2 This effect has been seen with the