Difference between revisions 1813701 and 1813703 on enwikiversity

[[Image:Skylab-73-HC-440HR.jpg|thumb|right|200px|The Saturn V SA-513 lifts off to boost the Skylab Orbital Workshop into Earth orbit on March 14, 1973. Credit: NASA.]]
[[Astronomy]] is performed by location and is subject to local conditions. The shapes and sizes of observatories have changed over time, as have their altitude. The motivations for putting an observatory manned or unmanned at different altitudes has led to a great variety in '''lofting technology'''.
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[[Image:ERS 2.jpg|thumb|left|200px|The photograph shows a full-size model of ERS-2. Credit:Poppy.]]
[[Image:Ariane42P rocket.gif|thumb|right|200px|The ERS-2 is carried into a sun-synchronous polar orbit by an Ariane 4 similar to the one imaged. Credit: NASA.]]

"A '''Sun-synchronous orbit''' (sometimes called a heliosynchronous orbit<ref name="shcherbakova">Shcherbakova, N. N.; Beletskij, V. V.; Sazonov, V. V. - Kosmicheskie Issledovaniia, Tom 37, No. 4, p. 417 - 427,
|url=http://adsabs.harvard.edu/abs/1999KosIs..37..417S</ref>) is a [[w:geocentric orbit|geocentric orbit]] which combines [[w:altitude|altitude]] and [[w:inclination|inclination]] in such a way that an object on that orbit ascends or descends over any given Earth latitude at the same local [[w:mean solar time|mean solar time]]. The surface [[w:illumination angle|illumination angle]] will be nearly the same every time. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths (e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean and atmospheric remote sensing instruments that require sunlight).  For example, a satellite in sun-synchronous orbit might ascend across the equator twelve times a day each time at approximately 15:00 mean local time. This is achieved by having the [[w:Osculating orbit|osculating]] orbital plane [[w:precess|precess]] (rotate) approximately one degree each day with respect to the [[w:celestial sphere|celestial sphere]], eastward, to keep pace with the Earth's movement around the [[Sun (star)|Sun]].<ref name="me">M. Rosengren: ERS-1 - An Earth Observer that exactly follows its Chosen Path, ESA Bulletin number 72, November 1992</ref>

"The uniformity of Sun angle is achieved by tuning the inclination to the altitude of the orbit ... such that the [[w:Equatorial bulge|extra mass near the equator]] causes the orbital plane of the spacecraft to precess with the desired rate: the plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis. Typical sun-synchronous orbits are about 600–800&nbsp;km in altitude, with periods in the 96–100 minute range, and inclinations of around 98[[w:degree (angle)|°]] (i.e. slightly [[w:retrograde motion|retrograde]] compared to the direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).<ref name="me"/>"<ref name=SunSynchronousOrbit/>

"'''European [[w:remote sensing|remote sensing]] satellite''' ('''ERS''') was the [[w:European Space Agency|European Space Agency]]'s first [[w:Earth observation satellite|Earth-observing satellite]]. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782&ndash;785&nbsp;km."<ref name=EuropeanRemoteSensingSatellite/>

ERS-1 carried an array of earth-observation instruments that gathered information about the Earth (land, water, ice and atmosphere) using a variety of measurement principles. These included:
* RA (Radar Altimeter) is a single frequency [[w:nadir|nadir]]-pointing radar altimeter operating in the [[w:Ku band|K<sub>u</sub> band]].
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