Effects of atmosphere on radio imaging for target detection

Abstract

The predictions of atmospheric effects on millimeter wave radio imaging of launch vehicles can be obtained by theoretical extrapolation of measured data from propagation studies for millimeter wave satellite communication systems. Attenuation exceedance statistics versus percentage of time have been estimated for both relatively wet and dry temperate climates. Attenuation problems are minimal at 10 GHz, but increase with frequency. Excessive rain attenuation is expected at 30 GHz for a small fraction of time. Cloud attenuation becomes also significant at 100 GHz. Water vapor in clear air can often cause more than 10 dB attenuation at 300 GHz. By use of the reciprocity theorem the effects of angle-of-arrival fluctuations on beam spot resolution have been found to be negligible for a spacebased telescope detecting Earth-launched vehicles. Beam coverage area averaging can reduce cloudinduced signal scintillations to less than 1 dB. These statements are valid except at very low elevation angles approaching tangent to the Earth, where the dominant problem is excessive signal attenuation.Atmospheric effects on millimeter wave radio imaging are similar to millimeter wave propagation problems in radio astronomy, satellite communications, and radar systems. Radio imaging indeed will use the same radiometric technique of radio astronomy, i.e., to map an incoherent source distribution with scanning or multiple beams of a coherent radio receiving system. To minimize atmospheric effects, radio astronomers usually conduct millimeter waves observations in clear dry weather. Communication and radar systems must be usable in adverse weather conditions to achieve high reliability. Previous propagation data [Cox and Arnold, 1982] in the above related field will be used as realistic benchmarks for theoretical extrapolation to the required predictions for radio imaging of launch vehicles.In section 2 we estimate the attenuation statistics. servations the incoherent source distributions are far away, and the atmospheric layer is right in front of the receiving telescope. For a space-based telescope looking for Earth-launched vehicles, the atmosphere is very close to the incoherent source. This difference in relative positions of the atmospheric layer does not change signal attenuation by the atmosphere but 731 732 CHU: EFFECTS OF ATMOSPHERE ON TARGET DETECTION