History of Earth observation   During World War I reconnaissance aircraft flew over enemy forces in order to observe troop movements. Multispectral scanners     Satellite with multispectral scanner. The radiation from the scan area on the surface of the Earth is recorded by detectors for each of the sensor's different channels. Radar sensors     Radar image of the Hubbard glacier in southeastern Alaska, acquired from the south. In this image, the southward slopes strongly reflect the microwave pulses emitted by the radar sensor back to the satellite, and appear bright. Dark shadows are seen because of the shallow incidence hiding the northern slopes from the sensor. However, where data is recorded, it is very detailed, as the crevasses on the surface of the glacier clearly show. (Source: Government Raster Image Processing Software and data, Meridian Data, 1989).