Active Microwave Remote Sensing

Active remote sensors create their own electromagnetic energy that is transmitted from the sensor towards the terrain (and is largely unaffected by the atmosphere), interacts with the terrain producing a backscatter of energy, and is recorded by the remote sensor's receiver. The strength of the backscattered signal is measured to discriminate between different targets, and the time delay between the transmitted and reflected signals determines the distance to the target. Active microwave sensors are generally divided into two distinct categories: imaging and non - imaging. The most common form of imaging active microwave sensors is imaging RADAR. RADAR is an acronym for Radio Detection And  Ranging, which essentially characterizes the function and operation of a radar sensor. Non - imaging microwave sensors include altimeters and scatterometers. In most cases, these are profiling devices which take measurements in one linear dinension, as oppoosed to the two- dimensional representation of imaging sensors. Radar altimeters transmit short microwave pulses and measure the round trip time delay to targets to determine their distance from the sensor. Generally altimeters 'look' straight down at nadir below the platform and thus measure height or elevation. Radar altimetry is used on aircraft for altitude determination, on aircraft and satellites for topographic mapping, for the estimation of the height of the sea surface, etc. (Bhatta B, 2009, page 165.)

Reference: Bhatta B. 2009. Remote Sensing and GIS, Oxford, page 165.