ABSTRACT
The fundamental reason for using microwaves for remote sensing is that they are different. Active sensors, such as radar systems, generate their own illumination by transmitting pulses of microwave radiation and then using a specialised receiver system to measure the reflected signal from the area of interest. Those radiometers that are used to measure emission from the atmosphere are termed microwave sounders. A unique feature of microwave spectroscopy for radiometry is the ability to design instrumentation which simultaneously gives the best possible spectral resolution and sensitivity of thermal emission measurements at long wavelengths. There are countless applications for which microwave remote sensing has proved capable of providing information comparable with, and sometimes beyond, that obtained with optical/IR sensors. Passive sensors, or radiometers, measure the microwave energy that is radiated or reflected by the Earth's surface or atmosphere. For most practical applications of passive imagers the most significant factors effecting the measured intensities are temperature, salinity and liquid water content.
