![Heliosynchronous orbit](https://upload.wikimedia.org/wikipedia/commons/7/79/Heliosynchronous_orbit.svg)
A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit,
is a nearly
polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of about ...
around a planet, in which the satellite passes over any given point of the planet's surface at the same local
mean solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time ( sundial ...
. More technically, it is an orbit arranged so that it
precesses through one complete revolution each year, so it always maintains the same relationship with the Sun.
Applications
A Sun-synchronous orbit is useful for
imaging
Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of an image).
Imaging technology is the application of materials and methods to create, preserve, or duplicate images.
...
,
reconnaissance
In military operations, reconnaissance or scouting is the exploration of an area by military forces to obtain information about enemy forces, terrain, and other activities.
Examples of reconnaissance include patrolling by troops (skirmisher ...
, and
weather satellite
A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or ...
s,
because every time that the satellite is overhead, the surface
illumination angle The angle of incidence, in geometric optics, is the angle between a ray incident on a surface and the line perpendicular (at 90 degree angle) to the surface at the point of incidence, called the normal. The ray can be formed by any waves, such as ...
on the planet underneath it is nearly the same. This consistent lighting is a useful characteristic for
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...
s that image the Earth's surface in visible or
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths, such as weather and spy satellites, and for other remote-sensing satellites, such as 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.
![Sun-Synchronous Orbit with LST Zones](https://upload.wikimedia.org/wikipedia/commons/2/25/Sun-Synchronous_Orbit_with_LST_Zones.svg)
Special cases of the Sun-synchronous orbit are the noon/midnight orbit, where the local mean solar time of passage for equatorial latitudes is around noon or midnight, and the dawn/dusk orbit, where the local mean solar time of passage for equatorial latitudes is around sunrise or sunset, so that the satellite rides the
terminator
Terminator may refer to:
Science and technology
Genetics
* Terminator (genetics), the end of a gene for transcription
* Terminator technology, proposed methods for restricting the use of genetically modified plants by causing second generation s ...
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 that 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
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisoto ...
such as
TRACE
Trace may refer to:
Arts and entertainment Music
* ''Trace'' (Son Volt album), 1995
* ''Trace'' (Died Pretty album), 1993
* Trace (band), a Dutch progressive rock band
* ''The Trace'' (album)
Other uses in arts and entertainment
* ''Trace'' ...
,
Hinode and
PROBA-2, affording them a nearly continuous view of the Sun.
Orbital precession
A Sun-synchronous orbit is achieved by having the
osculating orbital plane
precess
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...
(rotate) approximately one degree eastward each day with respect to the
celestial sphere to keep pace with the Earth's movement around the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
.
This precession is achieved by tuning the inclination to the altitude of the orbit (see
Technical details) such that Earth's
equatorial bulge
An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis. A rotating body tends to form an oblate spheroid rather than a sphere.
On E ...
, which perturbs inclined orbits, 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 around Earth are about in altitude, with periods in the 96–100-
minute range, and inclinations of around 98°. This is slightly
retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit, and 90° represents a polar orbit.
Sun-synchronous orbits are possible around other
oblate planets, such as
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
. A satellite orbiting a planet such as
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
that is almost spherical will need an outside push to maintain a Sun-synchronous orbit.
Technical details
The angular
precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...
per orbit for an Earth orbiting satellite is given by
:
where
: is the
coefficient for the second zonal term related to the
oblateness of the Earth,
: is the mean radius of the Earth,
: is the
semi-latus rectum of the orbit,
: is the inclination of the orbit to the equator.
An orbit will be Sun-synchronous when the precession rate equals the mean motion of the Earth about the Sun, which is 360° per
sidereal year (), so we must set , where is the orbital period.
As the orbital period of a spacecraft is
:
where is the
semi-major axis of the orbit, and is the
standard gravitational parameter of the planet ( for Earth); as for a circular or almost circular orbit, it follows that
:
or when is 360° per year,
:
As an example, with = , i.e., for an altitude ≈ of the spacecraft over Earth's surface, this formula gives a Sun-synchronous inclination of 98.7°.
Note that according to this approximation equals −1 when the semi-major axis equals , which means that only lower orbits can be Sun-synchronous. The period can be in the range from 88 minutes for a very low orbit ( = , = 96°) to 3.8 hours ( = , but this orbit would be equatorial, with = 180°). A period longer than 3.8 hours may be possible by using an eccentric orbit with < but > .
If one wants a satellite to fly over some given spot on Earth every day at the same hour, the satellite must complete a whole number of orbits per day. Assuming a circular orbit, this comes down to between 7 and 16 orbits per day, as doing less than 7 orbits would require an altitude above the maximum for a Sun-synchronous orbit, and doing more than 16 would require an orbit inside the Earth's atmosphere or surface. The resulting valid orbits are 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 ≈ 1.0027 times longer than the time between overpasses. For non-equatorial orbits the factor is closer to 1.)
:
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
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time ( sundial ...
, not to
apparent solar time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...
. The Sun will not be in exactly the same position in the sky during the course of the year (see
Equation of time
In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...
and
Analemma
In astronomy, an analemma (; ) is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure ...
).
Sun-synchronous orbits are mostly selected for
Earth observation satellites, with an altitude typically between 600 and over the Earth surface. Even if an orbit remains Sun-synchronous, however, other orbital parameters such as
argument of periapsis and the
orbital eccentricity evolve, due to higher-order perturbations in the Earth's gravitational field, the pressure of sunlight, and other causes. Earth observation satellites, in particular, prefer orbits with constant altitude when passing over the same spot. Careful selection of eccentricity and location of perigee reveals specific combinations where the rate of change of perturbations are minimized, and hence the orbit is relatively stable a
frozen orbit, where the motion of position of the periapsis is stable. The
ERS-1, ERS-2 and
Envisat of
European Space Agency, as well as the
MetOp
Metop (Meteorological Operational satellite) is a series of three polar-orbiting meteorological satellites developed by the European Space Agency (ESA) and operated by the European Organisation for the Exploitation of Meteorological Satellites ( ...
spacecraft of
EUMETSAT
The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) is an intergovernmental organisation created through an international convention agreed by a current total of 30 European Member States.
EUMETSAT's primary ...
and
RADARSAT-2 of the
Canadian Space Agency, are all operated in such Sun-synchronous frozen orbits.
See also
*
Orbital perturbation analysis (spacecraft)
*
Analemma
In astronomy, an analemma (; ) is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure ...
*
Geosynchronous orbit
*
Geostationary orbit
*
List of orbits
Summary
A simple list of just the common orbit abbreviations.
List of abbreviations of common Earth orbits
List of abbreviations of other orbits
Classifications
The following is a list of types of orbits:
Centric classifications
* Gal ...
*
Polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of about ...
*
World Geodetic System
The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descr ...
References
Further reading
Sandwell, David T., The Gravity Field of the Earth - Part 1 (2002)(p. 8)
from U.S. Centennial of Flight Commission
*
External links
List of satellites in Sun-synchronous orbit
{{Portal bar, Physics, Mathematics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science
Earth orbits
Satellites in low Earth orbit