Radar remote-sensing instruments

Any of a range of active REMOTE-SENSING instruments that propagate radio or microwave radiation and measure the BACKSCATTERING (echoes) that returns sometime later. Such systems can provide two types of information. By measuring the time taken for each echo to return to the sensor, distance (range) from the sensor can be determined.

At the same time, the intensity and POLARISATION of backscattered radiation may also be detected. There are three generic types of spaceborne RADAR instruments: altimeters, scatterometers and imaging radars. Altimeters (or nadir [downward] looking radars) are used to derive elevation profiles under the orbit track. These profiles are constructed by accurately measuring the time delay for a radar echo (pulse) to propagate to the surface and return back to the sensor. This has applications in mapping surface TOPOGRAPHY and in OCEAN and GLACIER monitoring. 

Scatterometers measure the radar cross section of a target (surface reflectivity), which is a function of how the target interacts with microwave radiation, and are typically used to measure wind speed and direction over water or detect rainfall. Imaging radars, such as synthetic aperture radar (SAR), are used to acquire high spatial resolution (a few metres to a few tens of metres) images, measuring range, intensity and sometimes polarisation of backscattered radiation over large areas. SAR instruments emit radar pulses as the platform moves and, by correcting the pulses for the transmission and reception times, an aperture can be synthesised that has a much greater size than the physical size of the antenna, which results in a finer spatial resolution. Radar instruments are generally composed of several parts: A transmitting source; an antenna, which shapes the transmitted energy into a beam pointing in a certain direction and collects energy from this direction; and equipment for processing and storing the data.