Radar antennas alternately transmit and receive pulses of microwave energy. Radar was developed as an air defense system during World War II and is now the primary remote sensing system air traffic controllers use to track the 40,000 daily aircraft takeoffs and landings in the United States. One example of active remote sensing that everyone has heard of is radar, which stands for Radio Detection And Ranging. Both image data and elevation data can be produced by microwave sensing, as you will discover in the sections on imaging radar and radar altimetry that follow. Microwave sensing is unaffected by cloud cover, and can operate day or night. Active sensors like those aboard the European Space Agency's ERS satellites, the Japanese JERS satellites, and the Canadian Radarsat, among others, transmit pulses of long wave radiation, then measure the intensity and travel time of those pulses after they are reflected back to space from the Earth's surface. Active remote sensing systems solve this problem. Microwaves can penetrate clouds, but the sun and Earth emit so little longwave radiation that it can't be measured easily from space. Longwave radiation, or microwaves, are made up of wavelengths between about one millimeter and one meter.
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