| Sun-synchronous Orbit |
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Information AboutSun-synchronous Orbit |
| CATEGORIES ABOUT SUN-SYNCHRONOUS ORBIT | |
| astrodynamics | |
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A sun-synchronous orbit (sometimes incorrectly called a heliosynchronous orbit) is a geocentric (rotate) approximately one degree each day, eastward, to keep pace with the Earth's revolution around the Sun . The uniformity of sun angle is achieved by tuning natural range, and inclinations of around 98 ° (ie slightly Retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit). Variations on this type of orbit are possible; a satellite could have a highly eccentric sun-synchronous orbit, in which case the "fixed solar time of passage" only holds for a chosen point of the orbit (typically the Perigee ). The orbital period chosen depends on the desired revisit rate; the satellite crosses the equator at the same solar time on every passage, but it will usually be at a different longitude since the Earth rotates underneath it. For example, an orbital period of 96 min, which divides evenly into the Earth solar day (15 times) means the satellite will cross at fifteen different longitudes on consecutive orbits, at the same Local Solar Time for each location, and begin again at the first longitude every fifteenth passage, once per day. Special cases of the sun-synchronous orbit are the noon/midnight orbit, where the local solar time of passage for equatorial longitudes is around noon or midnight, and the '''dawn/dusk orbit''', where the local solar time of passage for equatorial longitudes is around sunrise or sunset, so that the satellite rides the terminator between day and night. Riding the terminator is useful for active radar satellites as the satellites' solar panels can always see the Sun, without being shadowed by the Earth. It is also useful for some satellites with passive instruments which need to limit the Sun's influence on the measurements, as it is possible to always point the instruments towards the night side of the Earth. The dawn/dusk orbit has been used for solar observing scientific satellites such as Yohkoh , TRACE and Hinode , affording them a nearly continuous view of the Sun. As the satellite's altitude increases, so does the required inclination, so that the usefulness of the orbit decreases doubly: first because (for an Earth-observing satellite) the satellite's photographs are taken from ever farther away, and second because the increasing inclination means the satellite won't fly over higher latitudes. A sun-synchronous satellite designed to fly over the continental United States , for example, would need its inclination to be 132° or less, which means an altitude of ~4600 km or less. Sun-synchronous orbits are possible around other planets, such as Mars . TECHNICAL DETAILS For a sun-synchronous orbit, the precession is Retrograde (that is, opposite to the Earth's spin direction) in order to slow the apparent movement of the satellite around the Earth to match the Earth's rotation. A good approximation of the precession rate is: where : is the precession rate ( Rad /s) : is the Earth's equatorial radius (6.378 137 Mm) : is the satellite's orbital radius : is its angular frequency ( radians divided by its period) : its inclination : is the Earth's ''second dynamic form factor'' (1.08×10-3). This last quantity is related to the Oblateness as follows: where : is the Earth's oblateness : is the Earth's rotation rate (7.292115×10-5 rad/s) : is the product of the universal Constant Of Gravitation and the Earth's mass (3.986004418×1014 m³/s&2) SEE ALSO REFERENCES
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