These sources, plus the transportation sector, are the major originators of increased nitrogen oxides. The problem of acid rain not only has increased with population and industrial growth, it has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. The same remote glaciers that provide evidence of natural variability in acidic deposition show, in their more recently formed layers, the increased deposition caused by human activity during the past half century. A remote-sensing system that transmits its own radiation to detect an object or area for observation and receives the reflected or transmitted radiation.
Radar is an example of an active system.
Compare with passive system. Used to refer to the conversion of analog data to its digital equivalent. A five-channel scanning instrument that quantitatively measures electromagnetic radiation, flown on NOAA environmental satellites. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice. The act or process of establishing a forest, especially on land not previously forested.