Sky & Telescope April 1983 [antikvár]

Early Views from Landsat 4SATELLITE IMAGERY of the Earth is nothing new Explorer 6 started it all in 1959. Nor are the scenes here and on the following three pages the best that technology has to offer (reconnaissance satellites do orders of magnitude better).But Landsat 4. launched last July 16th. is making its mark nonetheless. From a vantage point 438 miles up. the spacecraft has been recording crisply detailed pictures of our planet through an experimental im-aging system called a thematic mapper (TM). This device rapidly scans the scene below with 100-foot resolution in six wavelength bands; a seventh channel in the thermal infrared records temperature data. (More details about the spacecraft appear on page 22 of the July, 1982, issue.)In fact. Landsat 4 actually outstrips the capability of computers to process its vast data. The first TM image was received last summer, yet only recently have theseand other .scenes become available; the overilow is being stored on tape for later processing. Also, until NASA's advanced communications satellites come on line later this year, coverage will be limited to regions surrounding the three ground stations (one each In the U. S., Canada, and Italy) now equipped to receive the TM's stunning views. Once fully operational, however, the spacecraft will scan nearly all of the Earth's land mass every 16 days.Here is a 28-by-25-mile section of a Landsat 4 image acquired over the eastern San Luis Valley in southern Colorado. To obtain such vibrant colors, an image taken through a 0.66-micron (red) filter is displayed in blue, one made in the near-infrared at 1.6 microns in green, and one at 2.2 microns in red. The Sangre de Cristo mountains at upper right appear deep blue because the snow that covers them reflects light poorly in the infrared bands, so the corresponding colors (green and red) make little contribution. Vegetation, highly reflective at 1.6 microns, appears green here; note the circular fields at lower left, which are watered by sprinkler systems pivoting around a central mast. Yellows and browns indicate generally barren terrain^ like that in the Great Sand Dunes National Monument at lower right. Landsat 4 took all these images during midmornin^ (local time); illumination is therefore from the lower right.September, 1983. Sky & Telescope 189