A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body . The terminator is defined as the locus of points on a planet or moon where the line through the center of its parent star is tangent . An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.
33-514: IRS-1B , Indian Remote Sensing satellite-1B, the second of the series of indigenous state-of-art remote sensing satellites, was successfully launched into a polar Sun-synchronous orbit on 29 August 1991 from the Soviet Cosmodrome at Baikonur . IRS-1B carries two sensors, LISS-1 and LISS-2, with resolutions of 72 m (236 ft) and 36 m (118 ft) respectively with a swath width of about 140 km (87 mi) during each pass over
66-404: 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
99-518: 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 the same relationship with the Sun. A Sun-synchronous orbit is useful for imaging , reconnaissance , and weather satellites , because every time that
132-743: A 3 km (1.9 mi) overlap. Data from the LISS-1 were downlinked on S-band at 5.2 Mbps and from the LISS-2 at 10.4 Mbps to the ground station at Shadnagar , India . The satellite was controlled from Bangalore , India. IRS-1B was operated in a Sun-synchronous orbit . On 29 August 1991, it had a perigee of 859 km (534 mi), an apogee of 915 km (569 mi), an inclination of 99.2°, and an orbital period of 102.7 minutes. IRS-1B successfully completed its mission on 1 July 2001, after operating for 10 years. Sun-synchronous orbit A Sun-synchronous orbit ( SSO ), also called
165-542: 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
198-425: A circle with a diameter that is approximately that of Earth. The terminator passes through any point on Earth's surface twice a day, at sunrise and at sunset , apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night . The circle separates the portion of Earth experiencing daylight from that experiencing darkness (night). While a little over one half of Earth
231-404: A focal length of 162.2 cm (63.9 in) generating a resolution of 72 m (236 ft) and a 148 km (92 mi) swath width. The LISS-2 sensor had eight 2048-element CCD imagers with a focal length of 324.4 mm (12.77 in) generating a ground resolution of 36 m (118 ft) and a 74 km (46 mi) swath width. The LISS-2 imager bracketed the LISS-1 imager providing
264-463: 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
297-525: 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
330-412: 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
363-402: 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
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#1732884100696396-508: Is a type of skywave propagation . Under good conditions, radio waves can travel along the terminator to antipodal points. The lunar terminator is the division between the illuminated and dark hemispheres of the Moon . It is the lunar equivalent of the division between night and day on the Earth spheroid , although the Moon's much lower rate of rotation means it takes longer for it to pass across
429-407: 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
462-420: Is illuminated at any point in time (with exceptions during eclipses ), the terminator path varies by time of day due to Earth's rotation on its axis. The terminator path also varies by time of year due to Earth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes , and its maximum angle is approximately 23.5° to
495-405: Is measured when the Sun still or already illuminates it while the base of the mountain remains in shadow. Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse , therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit . This prolongs
528-411: 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
561-418: 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
594-405: The pole during the solstices . At the equator , under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 metres per second (1,040 mph). This speed can appear to increase when near obstructions, such as the height of a mountain, as the shadow of the obstruction will be cast over the ground in advance of
627-399: 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
660-578: 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
693-417: The Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows provide accurate descriptions of the lunar terrain . The lunar terminator (or tilt) illusion is an optical illusion arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line perpendicular to
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#1732884100696726-746: The country. It was a part-operational, part-experimental mission to develop Indian expertise in satellite imagery . It was a successor to the remote sensing mission IRS-1A , both undertaken by the Indian Space Research Organisation (ISRO). IRS-1B was the second remote sensing mission to provide imagery for various land-based applications, such as agriculture, forestry, geology, and hydrology. Improved features compared to its predecessor (IRS-1A): gyroscope referencing for better orientation sensing, time tagged commanding facility for more flexibility in camera operation and line count information for better data product generation. The satellite
759-413: The dark side of the terminator, whereas the E and F layers above the D layer take longer to form. This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the "grey line". Amateur radio operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line" propagation , this signal path
792-421: 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
825-509: 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. Terminator (solar) On Earth , the terminator is
858-408: The particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the sun has set. Images showing a planetary terminator can be used to map topography: the position of the tip of a mountain behind the terminator line
891-403: 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
924-430: 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,
957-403: The surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth. Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when
990-405: The terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness). Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic transports are the only aircraft able to overtake the maximum speed of the terminator at the equator. However, slower vehicles can overtake
1023-406: The terminator at higher latitudes , and it is possible to walk faster than the terminator at the poles, near to the equinoxes . The visual effect is that of seeing the sun rise in the west, or set in the east. Strength of radio propagation changes between day- and night-side of the ionosphere . This is primarily because the D layer , which absorbs high frequency signals , disappears rapidly on
IRS-1B - Misplaced Pages Continue
1056-400: The terminator) should correspond with the position of the Sun , but does not appear to do so. The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based on planar geometry . Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere can create a fuzzier terminator. As
1089-759: Was a box-shaped 1.56 m x 1.66 m x 1.10 metres bus with two Sun-tracking solar panels of 8.5 square metres each. Two nickel-cadmium batteries provided power during eclipses. The three-axis stabilised Sun-synchronous satellite had a 0.4° pitch/roll and 0.5° yaw pointing accuracy provided by a zero-momentum reaction wheel system utilising Earth/Sun/star sensors and gyroscopes. IRS-1B carried two solid state push broom scanner Linear Imaging Self-Scanning Sensor (LISS): The satellite carried two LISS push broom CCD sensors operating in four spectral bands compatible with Landsat Thematic Mapper and Spot HRV data. The bands were 0.45-0.52, 0.52-0.59, 0.62-0.68, and 0.77-0.86 microns. The LISS-1 sensor had four 2048-element CCD imagers with
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