Landsat was first deployed in the early 1970s, and is now an old technology. I think that Landsat 7 is still operating with updated sensors
Landsat images are usually captured with a Multi-Spectral Scanner (MSS), which reads five to seven different infrared channels. The digital images are then processed into the false color images that we are accustomed to seeing by highlighting reflection from vegetation. Normally the vegetation is rendered in red, as in the images below, which makes the overall pictures easier to interpret. Sometimes they will attempt a natural-looking green rendering to show approximately what the view might look like from space (if there were no atmospheric haze). The processed images are still "false color" -- that is, the colors are only nominal, representative of some calculation that did not actually involve colored light -- but they look sort of realistic. Personally, I prefer the red-vegetation false color versions; surface details of civilization are much easier to spot.
On Landsat 4 and later, the imaging is produced by an onboard computer called the Thematic Mapper (TM -- just an abbreviation, not a trademark designation) and now Enhanced Thematic Mapper (ETM), which gives greater attention to some details, and overall results in more resolution from fewer bits. The San Francisco picture here is from a newer, but still not current, imaging system, and the quality is much better, I think you'll agree.
Another interesting tidbit: Landsat travels in a special near-polar orbit that brings it over any point of land at the same time of day every fifteen-ish days ("Sun-synchronous"). This makes comparison pictures easy, if the cloud cover cooperates, because the sun angles are identical. On the other hand, it means that Landsat may not capture news events in a timely fashion, just because sometimes the event and the orbiting schedules don't mesh well. Sometimes we get lucky and it does capture volcanoes and floods, and the pictures are quite spectacular.
Lastly, NASA published a spectacular book, Mission to Earth: Landsat Views the World, NASA SP-360, by Short, Nicholas, et al., 1976. It is now, regrettably, out of print. The NJ image here was scanned from that book.
False Color Image
|Boston, Massachusetts (100KB)
|San Francisco Bay (1984) (1MB)
|New Jersey (1972) (800KB)
SIR-C is a Side-Imaging Radar system carried aloft by the Space Shuttle for testing on some ground areas. The radar partially penetrates the ground and is reflected with changes in timing, amplitude, phase, and polarization. The false color rendering usually reflects differences in near-surface geology.
False Color Image
|New York City
|Houston & Galveston, TX (and Johnson Space Center)
NASA's web for all Earth Sciences: www.earth.nasa.gov
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