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A challenge in the development of multi-channel millimetre wave imaging radiometers is overcoming effects associated with the temperature dependence of receiver responsivity. In this paper, the stability of absolute radiation temperature measurements, made with direct and heterodyne detection radiometers, is investigated theoretically and experimentally. The agreement between theory and experiment is found to be good. Changes in measured radiation temperatures were found to be between 6°K at 35 GHz and 1 45 °K at 220 GHz, for a one degree change in instrumental temperature. Suggestions are made, as to how the temperature stability ofradiometers may be improved.

The status of passive millimetre wave radiometry as a thermal imaging technique will be discussed. Methods will be identified for overcoming the usual difficulties of poor spatial resolution and slow response time. High quality images will be presented to demonstrate the potential of this technology. INTRODUCTIONThe advantages of detecting millimetre waves rather than infra-red radiation in thermal imaging systems arise from the superior penetrating properties of millimetre waves through cloud, fog and rain. The contrast is high at millimetre waves since an image in this waveband is dominated by large cold sky reflections1. For this reason a millimetre wave image can look similar to a visible picture given the same spatial resolution.The early imagers2'3 developed in the late 1950's by the Defence Research Agency (DRA) at Malvern (then the Royal Radar Establishment) were bulky, had poor spatial resolution and low thermal sensitivity. One imager known as "Green Minnow" consisted of 16 "Dicke" switched 35GHz radiometers feeding a O.5m diameter stepped polythene lens. This imager was flown in a Hastings aircraft and used a pushbroom scan pauern. It contained 374 thernuonic valves and weighed 500kg. The noise temperature of each channel was approximately 4500K, including the input waveguide losses, and an intermediate frequency (IF) bandwidth of 10MHz was used. O-8194-1515-4/94/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/06/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

We present a regularized pseudoinverse method which has successfully superresolved passive millimetre-wave images and that can be performed in the frame time of the imager.

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