Astronaut Photography
Astronaut
photography of the Earth from the International Space Station, although generally of low-resolution, has
achieved resolutions close to those available from commercial remote sensing satellites — with many photographs
having spatial resolutions of less than 6 m. Some of these images are particularly impressive and show a wealth of
detail, for example see the high-res versions of the images of Kiritimati
taken on the Stations second mission. These images are all the more remarkable when it is borne in mind that astronauts
take the photographs by hand and physically compensate for the motion of the spacecraft relative to the Earth while the
images are being acquired.
Images taken from early Space Shuttle and Space Station missions made use of traditional mechanical cameras with 35 mm and 70 mm film. Astronauts onboard the Space Station currently image the earth with sophisticated digital cameras such as the Kodak DCS760 with lenses of up to 800 mm. These image are taken from a low orbit of around 360 km — an advantage over the higher orbiting satellites. Because they are taken literally 'out of the window' the look-angle can vary. Thus, images can be taken from directly overhead or at highly oblique angles.
The resolution of these images varies and depends on numerous factors such as spacecraft altitude, lens used, film type, lighting conditions, atmospheric interference and look angle. In general spatial resolutions of 30 m to 100 m are common. This is similar to the resolution of the Landsat Thematic Mapper (approximately 30 m). As noted above, spatial resolutions of around 6 m have also been achieved, which equals all but the most sophisticated of commercial earth-observing satellites.
MODIS
The MODIS instrument
provides 12-bit sensitivity in 36 spectral bands ranging in wavelength from 0.4 µm
to 14.4 µm. Two bands are imaged at a nominal resolution of 250 m at nadir, with five bands at 500 m, and
the remaining 29 bands at 1 km. A ±55° scanning pattern at an orbit of 705 km in height achieves a 2,330-km
swath and provides global coverage every one to two days.
The optical system consists of a two-mirror off-axis afocal telescope, which directs energy to four refractive objective assemblies; one for each of the VIS, NIR, SWIR/MWIR and LWIR spectral regions to cover a total spectral range of 0.4 to 14.4 µm. The Scan Mirror Assembly uses a continuously rotating double-sided scan mirror to scan ±55°.