Satellite Orbits (Part 1)

A. Low Earth Orbit (LEO)

A Low Earth Orbit (LEO) is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 160 – 2000 km above the Earth’s surface.

Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80-500 km up) or exosphere (approximately 500 km and up), depending on orbit height. The altitude is usually not less than 300 km because that would be impractical due to the larger atmospheric drag.

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B. Polar Orbit

           

A polar orbit is a particular type of Low Earth Orbit in which a satellite passes above or nearly above both poles of the planet orbiting (north-south direction) on each revolution. It therefore has an inclination of (or very close to) 90 degrees to the equator. Except in the special case of a polar geosynchronous orbit, a satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.

Polar orbits are often used for earth-mapping, earth observation and reconnaissance satellites, as well as some weather satellites. During a 12-hour day, a satellite in such an orbit can observe all points on the Earth. The disadvantage to this orbit is that no one spot on the Earth’s surface can be sensed continuously from a satellite in a polar orbit.

To achieve a polar orbit requires more energy, thus more propellant, than does an orbit of low inclination. A polar orbit cannot take advantage of the “free ride” provided by the Earth’s rotation, and thus the launch vehicle must provide all of the energy for attaining orbital speed.

 

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C. Sun-Synchronous Orbit

A sun-synchronous orbit describes the orbit of a satellite that provides consistent lighting of the Earth-scan view. Like a polar orbit, the satellite travels from the north to the south poles as the Earth turns below it. In a sun-synchronous orbit, though, the satellite passes over the same part of the Earth at roughly the same local time each day. For example, a satellite’s sun-synchronous orbit might cross the equator twelve times a day each time at 3:00 p.m. local time. The orbital plane of a sun-synchronous orbit must also precess (rotate) approximately one degree each day, eastward, to keep pace with the Earth’s revolution around the sun.

Typical Sun-synchronous orbits are about 600-800 km in altitude, with periods in the 96-100 minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the direction of Earth’s rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).

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 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.

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D. Near Equatorial Orbit (NEqO)

A near equatorial orbit is an orbit that lies close to the equatorial plane of the object orbited. This orbit allows for rapid revisit times (for a single orbiting spacecraft) of near equatorial ground sites.

 

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Sources:
1. http://en.wikipedia.org
2. http://www.centennialofflight.gov
3. http://www.thetech.org

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