A 94GHz dual-mode imaging "radarometer" for remote sensing

We report on the continued development of our 94GHz dual-mode radar/radiometric imager, AVTIS. To date we have concentrated on refining the radar mode and can now acquire state-of-the-art long range, high resolution radar images. More recently we have worked to improve and integrate the radiometric mode, to complete the dual-mode functionality. One notable problem of the monostatic architecture is that of radar transmitter leakage via the circulator/antenna assembly, even with the transmitter signal heavily attenuated. A related issue is the high level of AM noise in the local oscillator signal due to the IMPATT multipliers and power amplifiers used. This far-off-the-carrier noise, in combination with the leakage, desensitises the receiver and raises the noise floor to give a receiver noise temperature of approximately 6000K. This yields a thermal resolution of 2K in 4ms that is considered adequate for the intended volcanological application. In comparison, tests with a high power Schottky diode multiplier chain as the LO source has yielded a noise temperature of 750K in a separate radiometer. Thermal calibration is also of concern and we have implemented an IF noise adding circuit in which the radiometer alternates between the scene and hot and cold IF noise references. This accounts for dominant receiver fluctuations in the IF amplifiers but sky tipping curves are used to calibrate the overall response. Whilst the radar is already capable of producing high resolution topographic maps with surface reflectivity overlaid, it is hoped that co-aligned radiometric brightness temperature data will lead to a better understanding of the emissivity and surface roughness of the terrain being surveyed.