Passive and active imaging at 94 GHz for environmental remote sensing

We report on the use of the All-weather Volcano Topography Imaging Sensor (AVTIS) 94 GHz dual mode radar/radiometric imager for environmental monitoring. The FMCW radar yields 3D maps of the terrain whilst the passive radiometer records brightness temperature maps of the scene. AVTIS is a low power portable instrument and has been used operationally to survey terrain at ranges up to 6 km. AVTIS was originally developed for the ground-based measurement of active volcanoes and has been used successfully to measure the Arenal Volcano in Costa Rica and the Soufriere Hills Volcano on Montserrat. However, additional environmental remote sensing applications are emerging for this technology and we will present details of how the instrument is used to perform terrain mapping and thermal surveys of outdoor scenes.

The extraction of digital elevation maps is the primary function of the AVTIS radar mode. We review this process covering range drift compensation, radar cross section (RCS) histogram analysis and thresholding, and georeferencing to GPS. Additionally, we will present how careful calibration enables RCS imaging of terrain and the extraction of the intrinsic reflectivity of the terrain material (normalized RCS, or sigma-nought) which can potentially be used to classify terrain types. We have validated the passive mode imagery against infrared thermal imagery and they show good agreement once the differences in spatial resolution are accounted for. This comparison also reveals differences in propagation due to obscurants (steam, gas, ash) in the two wavebands.