大气折射与天线指向

the pointing accuracy ~9.5'' for 25M telescope at 30 GHz. ● ~9.5'' for 25M telescope at 30 GHz. ~2.0'' for 110M telescope at 30 GHz.

Thermosphere (热电离层) Mesosphere (中间层) Stratosphere (同温层) Troposphere (对流层) Within the neutral atmosphere, the propagation of radio waves is most affected by the troposhere.

物理机制 Molecular resonances in a neutral 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 wavelengths, 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 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.

三段折射改正区域 (1). Under the assumption that the atmosphere can be described by a set of plane parallel layers of slowly decreasing refractivity, zenith angles of 0—65 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 of parallel spherical layers, zenith angles of 65—80 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 80—85 degrees. A simple “fix” for this problem is to multiply the value of above by the factor, (经验的)

计算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 (开尔文). (1). With typical sea level values Pd=1000, e=10, T=288. (2). On a high and dry radio telescope 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.

折射对天线效率的影响 The GBT or QTT primary diameter 110m is a finite fraction of the atmospheric 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 determination 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 azimuths. For z1=80, Tgrad=1'50''

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. Assume 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 newer and accurate millimeter telescopes (30-100GHz) at a number of different sites and appears to be a general feature of a not fully stable atmosphere. 总结一下 对指向有影响 → 调指向 对效率有影响 → 调主动面 wet

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