Effects of atmosphere on radio imaging for target detection


  • T. S. Chu


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 space-based telescope detecting Earth-launched vehicles. Beam coverage area averaging can reduce cloud-induced 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.