188.60.33.147: /* References */

2013-05-21T16:43:39Z

References

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	~~Golda~~ is ~~what~~'s ~~written~~ on ~~my beginning certificate although it~~ is not the name ~~on my birth certificate~~. ~~For years he~~'s been ~~living~~ in ~~Alaska~~ and ~~he doesn~~'~~t plan~~ on ~~changing~~ it. He is ~~an info officer~~. ~~To climb~~ is ~~some thing I truly enjoy doing~~.<br><br>~~Here~~ is ~~my webpage~~; ~~telephone psychic~~; [http://~~ehobbybox~~.~~com~~/~~profile~~-~~16378~~/info/ ~~ehobbybox~~.~~com~~],		{{Technical\|date=December 2013}}

			[[File:Heliosynchronous Orbit.png\|thumb\|right\|220px\|Diagram showing the orientation of a Sun-synchronous orbit (green) in four points of the year. A non-sun-synchronous orbit (magenta) is also shown for reference]]

			A '''Sun-synchronous orbit''' (sometimes called a '''heliosynchronous orbit'''<ref name="shcherbakova">{{Cite journal\|author=Shcherbakova, N. N.; Beletskij, V. V.; Sazonov, V. V. \|journal= Koheskie Issledovaniia, Tom 37\|issue= 4\|pages= 417–427 \|url=http://adsabs.harvard.edu/abs/1999KosIs..37..417S\|title=
			Stabilization of heliosynchronous orbits of an Earth's artificial satellite by solar pressure.}}</ref>) is a [[geocentric orbit]] which combines [[altitude]] and [[inclination]] in such a way that an object on that orbit ascends or descends over any given Earth latitude{{clarify \| reason = Whatever does that mean?\|date=September 2013}} at the same local [[mean solar time]]. The surface [[illumination angle]] will be nearly the same every time. This consistent lighting is a useful characteristic for [[satellite]]s 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 [[Osculating orbit\|osculating]] orbital plane [[precess]] (rotate) approximately one degree each day with respect to the [[celestial sphere]], eastward, to keep pace with the Earth's movement around the [[Sun]].<ref name="me">{{Cite journal\|first=M. \|last=Rosengren\|title= ERS-1 - An Earth Observer that exactly follows its Chosen Path\|journal= ESA Bulletin\|issue= 72\|date=November 1992\|url=http://adsabs.harvard.edu/abs/1992ESABu..72...76R}}</ref>

			The uniformity of Sun angle is achieved by tuning the inclination to the altitude of the orbit (details in section "Technical details") such that the [[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 km in altitude, with periods in the 96–100 [[minute]] range, and inclinations of around 98[[degree (angle)\|°]] (i.e. slightly [[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"/>

			Special cases of the sun-synchronous orbit are the '''noon/midnight orbit''', where the local mean solar time of passage for equatorial longitudes is around noon or midnight, and the '''dawn/dusk orbit''', where the local mean 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]], [[Hinode]] and [[Proba-2]], affording them a nearly continuous view of the Sun.

			Sun-synchronous orbits are possible around other [[Oblate spheroid\|oblate]] planets, such as [[Mars]]. But [[Venus]], for example, is too spherical to have a satellite in sun-synchronous orbit. See the article [[Venus]] where a flattening coefficient of zero for this planet is cited.

			== Technical details ==

			Equation (24) of the article [[Orbital perturbation analysis (spacecraft)]] gives the angular precession per orbit for an orbit around an oblate planet as
			:<math>\Delta \Omega = -2\pi\ \frac{J_2}{\mu\ p^2}\ \frac{3}{2}\ \cos i\,</math>
			where
			:<math>J_2\,</math> is the coefficient for the second zonal term (1.7555 · 10<sup>10</sup> km<sup>5</sup> / s<sup>2</sup>) related to the [[oblateness]] of the earth (see [[Geopotential model]]),
			:<math>\mu\,</math> is the gravitational constant of the Earth (398600.440 km<sup>3</sup> / s<sup>2</sup>)
			:<math>p</math> is the [[Conic section#Features\|semi-latus rectum]] of the orbit,
			:<math>i</math> is the inclination of the orbit to the equator.

			An orbit will be sun-synchronous when the precession rate, <math>\rho</math>, equals the mean motion of the Earth about the Sun which is 360° per [[tropical year]] (1.99106 · 10<sup>−7</sup> radians / s) so we must set <math>\Delta\Omega/P=\rho</math> where ''P'' is the orbital period.

			As the orbital period of a spacecraft is <math>2\pi\ a\sqrt{\frac{a}{\mu}}\,</math> (where ''a'' is the [[semi-major axis]] of the orbit) and as <math>p \approx a\,</math> for a circular or almost circular orbit it follows that
			:<math>\rho\approx -\frac{3J_2\cos i}{2a^{7/2}\mu^{1/2}}=-(360\text{° per year})\times(a/12352\text{ km})^{-7/2}\cos i=-(360\text{° per year})\times(P/3.795\text{ hrs})^{-7/3}\cos i</math>
			or when <math>\rho</math> is 360° per year,
			:<math>\cos i\ \approx\ -\frac{\rho\ \sqrt{\mu}}{\frac{3}{2}\ J_2}\ a^{\frac{7}{2}}=-(a/12352\text{ km})^{7/2}=-(P/3.795\text{ hrs})^{7/3},</math>

			As an example, for a=7200 km (the spacecraft about 800 km over the Earth surface) one gets with this formula a sun-synchronous inclination of 98.696 deg.

			Note that according to this approximation cos ''i'' equals −1 when the semi-major axis equals 12 352 km, which means that only smaller orbits can be sun-synchronous. The period can be in the range from 88 minutes for a very low orbit (''a''=6554 km, ''i''=96°) to 3.8 hours (''a''=12 352 km, but this orbit would be equatorial with ''i''=180°). (A period longer than 3.8 hours may be possible by using an eccentric orbit with ''p''<12 352 km but ''a''>12 352 km.)

			If one wants a satellite to fly over some given spot on Earth every day at the same hour, it can do between 7 and 16 orbits per day, as shown in the following table. (The table has been calculated assuming the periods given. The orbital period that should be used is actually slightly longer. For instance, a retrograde equatorial orbit that passes over the same spot after 24 hours has a true period about 365/364 ≈ 1.0027 times longer than the time between overpasses. For non-equatorial orbits the factor is closer to 1.)

			{\| class="wikitable"
			\|-
			! Orbits per day!!Period (hrs)\|\| Height above<br>Earth's surface<br>(km) !! Maximum latitude
			\|-
			\|16\|\| <math>1\tfrac{1}{2}</math> = 1 hr 30 min \|\| 282\|\|83.4°
			\|-
			\|15\|\| <math>1\tfrac{3}{5}</math> = 1 hr 36 min\|\| 574\|\|82.3°
			\|-
			\|14\|\| <math>1\tfrac{5}{7}</math> ≈ 1 hr 43 min \|\| 901\|\|81.0°
			\|-
			\|13\|\| <math>1\tfrac{11}{13}</math> ≈ 1 hr 51 min \|\| 1269\|\|79.3°
			\|-
			\|12\|\| <math>2</math> \|\| 1688\|\|77.0°
			\|-
			\|11\|\| <math>2\tfrac{2}{11}</math> ≈ 2 hrs 11 min \|\| 2169\|\|74.0°
			\|-
			\|10\|\| <math>2\tfrac{2}{5}</math> = 2 hrs 24 min\|\| 2730\|\|69.9°
			\|-
			\|9 \|\| <math>2\tfrac{2}{3}</math> = 2 hrs 40 min \|\| 3392\|\|64.0°
			\|-
			\|8 \|\| <math>3</math> \|\| 4189\|\|54.7°
			\|-
			\| 7\|\| <math>3\tfrac{3}{7}</math> ≈ 3 hrs 26 min\|\| 5172\|\|37.9°
			\|}
			When one says that a sun-synchronous orbit goes over a spot on the earth at the same '''local time''' each time, this refers to [[mean solar time]], not to [[apparent solar time]]. The sun will not be in exactly the same position in the sky during the course of the year. (See [[Equation of time]] and [[Analemma]].)

			The Sun-synchronous orbit is mostly selected for [[Earth observation satellite]]s that should be operated at a relatively constant altitude suitable for its Earth observation instruments, this altitude typically being between 600 km and 1000 km over the Earth surface. Because of the deviations of the gravitational field of the Earth from that of a homogeneous sphere that are quite significant at such relatively low altitudes a strictly circular orbit is not possible for these satellites. Very often a [[frozen orbit]] is therefore selected that is slightly higher over the Southern hemisphere than over the Northern hemisphere. [[European Remote-Sensing Satellite\|ERS-1, ERS-2]] and [[Envisat]] of [[European Space Agency]] as well as the [[MetOp]] spacecraft of the [[European Organisation for the Exploitation of Meteorological Satellites]] are all operated in Sun-synchronous, [[Frozen orbit\|"frozen"]] orbits.{{citation needed\|date=November 2011}}

			== See also ==
			* [[Orbital perturbation analysis (spacecraft)]]
			* [[Analemma]]
			* [[Geosynchronous orbit]]
			* [[Geostationary orbit]]
			* [[List of orbits]]
			* [[Polar orbit]]
			* [[World Geodetic System]]

			== References ==
			<references/>
			* [http://topex.ucsd.edu/geodynamics/14gravity1_1.pdf Sandwell, David T., The Gravity Field of the Earth - Part 1 (2002)] (p. 8)
			* [http://www.centennialofflight.gov/essay/Dictionary/SUN_SYNCH_ORBIT/DI155.htm Sun-Synchronous Orbit dictionary entry], from U.S. Centennial of Flight Commission
			* [http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970613a.html NASA Q&A]
			* {{cite web\|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37901/1/04-0327.pdf\|title=The A-B-Cs of Sun Synchronous Orbit Design \|publisher=Space Flight Mechanics Conference\|first=Ronald J.\|last=Boain\|date=February 2004}}

			==External links==
			*[http://www.zarya.info/Diaries/Launches/ss-LTDN.php List of satellites in Sun-synchronous orbit]

			{{orbits}}

			{{DEFAULTSORT:Sun-Synchronous Orbit}}
			[[Category:Earth orbits]]

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Reverted edits by 85.127.3.42 (talk) to last version by 131.114.10.32

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Golda is what's written on my beginning certificate although it is not the name on my birth certificate. For years he's been living in Alaska and he doesn't plan on changing it. He is an info officer. To climb is some thing I truly enjoy doing.<br><br>Here is my webpage; telephone psychic; [http://ehobbybox.com/profile-16378/info/ ehobbybox.com],

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188.60.33.147: /* References */

en>Ciphers: Reverted edits by 85.127.3.42 (talk) to last version by 131.114.10.32