This chapter considers the atmospheric window in the microwave region with the intention of measuring surface properties but will utilise quite different measurement techniques to effectively bypass the whole issue of angular resolving power. It also considers rain radar, where frequencies are chosen so that weather does influence the signal. Since the basic operation of any radar is to estimate target distance from a measured echo time delay, the accuracy with which this distance can be measured in a fundamental quality indicator of a radar system. Active microwave systems suffer from the same limitations of spatial resolution as passive sensors, the long wavelengths meaning that large antennas or high frequencies must be used even for moderate resolutions. Altimeters and scatterometers can normally be expected to achieve ground footprint sizes in the order of tens of kilometres. The alternative approach to the fan-beam geometry is to use conical scanning, such as that used by the Quickscat scatterometer on the Sea-Winds platform.