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 60–90 degrees to the body's equator .
28-453: India's remote sensing program was developed with the idea of applying space technologies for the benefit of humankind and the development of the country. The program involved the development of three principal capabilities. The first was to design, build and launch satellites to a Sun-synchronous orbit . The second was to establish and operate ground stations for spacecraft control, data transfer along with data processing and archival. The third
56-404: A = 7200 km , i.e., for an altitude a − R E ≈ 800 km of the spacecraft over Earth's surface, this formula gives a Sun-synchronous inclination of 98.7°. Note that according to this approximation cos i equals −1 when the semi-major axis equals 12 352 km , which means that only lower orbits can be Sun-synchronous. The period can be in the range from 88 minutes for
84-595: A Sun-synchronous orbit. The angular precession per orbit for an Earth orbiting satellite is approximately given by where An orbit will be Sun-synchronous when the precession rate ρ = d Ω / d t equals the mean motion of the Earth about the Sun n E , which is 360° per sidereal year ( 1.990 968 71 × 10 rad /s ), so we must set n E = Δ Ω E / T E = ρ = Δ Ω / T , where T E
112-738: A larger launch vehicle to launch a given payload to a given altitude than for a near-equatorial orbit at the same altitude, because it cannot take advantage of the Earth's rotational velocity . Depending on the location of the launch site and the inclination of the polar orbit, the launch vehicle may lose up to 460 m/s of Delta-v , approximately 5% of the Delta-v required to attain Low Earth orbit . Polar orbits are used for Earth-mapping , reconnaissance satellites , as well as for some weather satellites . The Iridium satellite constellation uses
140-417: A polar orbit to provide telecommunications services. Near-polar orbiting satellites commonly choose a Sun-synchronous orbit , where each successive orbital pass occurs at the same local time of day. For some applications, such as remote sensing , it is important that changes over time are not aliased by changes in local time. Keeping the same local time on a given pass requires that the time period of
168-470: A satellite in Sun-synchronous orbit might ascend across the equator twelve times a day, each time at approximately 15:00 mean local time. 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
196-525: 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 ). Sun-synchronous orbits are mostly selected for Earth observation satellites , with an altitude typically between 600 and 1000 km over the Earth surface. Even if an orbit remains Sun-synchronous, however, other orbital parameters such as argument of periapsis and
224-412: 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, the satellite must complete
252-402: 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
280-407: 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 that need to limit the Sun's influence on the measurements, as it is possible to always point
308-411: Is the earth orbital period while T is the period of the spacecraft around the earth. As the orbital period of a spacecraft is where a is the semi-major axis of the orbit, and μ is the standard gravitational parameter of the planet ( 398 600 .440 km /s for Earth); as p ≈ a for a circular or almost circular orbit, it follows that or when ρ is 360° per year, As an example, with
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#1732791040170336-487: Is the largest constellation of remote sensing satellites for civilian use in operation today in the world, with 11 operational satellites. All these are placed in polar Sun-synchronous orbit and provide data in a variety of spatial, spectral and temporal resolutions. Indian Remote Sensing Programme completed its 25 years of successful operations on March 17, 2013. Data from Indian Remote Sensing satellites are used for various applications of resources survey and management under
364-798: The Indian Space Research Organization (ISRO) started off in 1988 with the IRS-1A , the first of the series of indigenous state-of-art operating remote sensing satellites, which was successfully launched into a polar Sun-synchronous orbit on March 17, 1988, from the Soviet Cosmodrome at Baikonur. It has sensors like LISS-I which had a spatial resolution of 72.5 metres (238 ft) with a swath of 148 kilometres (92 mi) on ground. LISS-II had two separate imaging sensors, LISS-II A and LISS-II B, with spatial resolution of 36.25 metres (118.9 ft) each and mounted on
392-689: The National Natural Resources Management System (NNRMS) for which the Department of Space (DOS) is the nodal agency, providing operational remote sensing data services. Data from the IRS satellites is received and disseminated by several countries all over the world. With the advent of high-resolution satellites, new applications in the areas of urban sprawl , infrastructure planning and other large scale applications for mapping have been initiated. The IRS system
420-418: 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
448-399: 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 . A satellite orbiting a planet such as Venus that is almost spherical will need an outside push to maintain
476-581: The Earth's movement around the Sun . This precession is achieved by tuning the inclination to the altitude of the orbit (see Technical details ) such that Earth's equatorial bulge , 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 600–800 km (370–500 mi) in altitude, with periods in
504-516: The National Natural Resources Management System (NNRMS). Following is the list of those applications: The initial versions are composed of the 1 (A,B,C,D). The later versions are named based on their area of application, including OceanSat, CartoSat, ResourceSat. Some of the satellites have alternate designations based on the launch number and vehicle (P series for PSLV). From 2020, the name was changed to
532-588: The Pacific (CSSTEAP) Center located at Dehradun of Uttrakhand State in India. Sun-synchronous orbit A Sun-synchronous orbit ( SSO ), also called a heliosynchronous orbit , is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time . More technically, it is an orbit arranged so that it precesses through one complete revolution each year, so it always maintains
560-581: The generic EOS, which stands for Earth Observation Satellite. Data from IRS is available to its users through NRSC Data Centre and also through Bhuvan Geoportal of ISRO. NRSC data center provides data through its purchase process, while Bhuvan Geoportal provides data in free and open domain. The capacity building programme of ISRO for IRS and other remote sensing applications is through Indian Institute of Remote Sensing (IIRS) Dehradun and UN affiliated Center of Space Science and Technology Education in Asia and
588-421: The instruments towards the night side of the Earth. The dawn/dusk orbit has been used for solar-observing scientific satellites such as TRACE , Hinode and PROBA-2 , affording them a nearly continuous view of the Sun. A Sun-synchronous orbit is achieved by having the osculating orbital plane precess (rotate) approximately one degree eastward each day with respect to the celestial sphere to keep pace with
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#1732791040170616-424: The orbit be kept as short, which requires a low orbit. However, very low orbits rapidly decay due to drag from the atmosphere. Commonly used altitudes are between 700 and 800 km, producing an orbital period of about 100 minutes. The half-orbit on the Sun side then takes only 50 minutes, during which local time of day does not vary greatly. To retain a Sun-synchronous orbit as the Earth revolves around
644-534: 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 spacecraft of EUMETSAT and RADARSAT-2 of the Canadian Space Agency , are all operated in such Sun-synchronous frozen orbits. Polar orbit Launching satellites into polar orbit requires
672-403: 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.) When one says that a Sun-synchronous orbit goes over
700-586: The same relationship with the Sun. A Sun-synchronous orbit is useful for imaging , reconnaissance , and weather satellites , because every time that the satellite is overhead, the surface illumination angle on the planet underneath it is nearly the same. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared 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,
728-401: The spacecraft in such a way to provide a composite swath of 146.98 kilometres (91.33 mi) on ground. These tools quickly enabled India to map, monitor and manage its natural resources at various spatial resolutions. The operational availability of data products to the user organisations further strengthened the relevance of remote sensing applications and management in the country. Following
756-418: The successful demonstration flights of Bhaskara-1 and Bhaskara-2 satellites launched in 1979 and 1981, respectively, India began to develop the indigenous Indian Remote Sensing (IRS) satellite program to support the national economy in the areas of agriculture , water resources , forestry and ecology, geology, water sheds, marine fisheries and coastal management . Towards this end, India had established
784-443: Was to use the data obtained for various applications on the ground. India demonstrated the ability of remote sensing for societal application by detecting coconut root-wilt disease from a helicopter mounted multispectral camera in 1970. This was followed by flying two experimental satellites, Bhaskara -1 in 1979 and Bhaskara-2 in 1981. These satellites carried optical and microwave payloads. India's remote sensing programme under
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