CHAPTER 27

Low−Earth−Orbit Satellites (LEOs)

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 160–2,000 km (100–1,240 miles) above the Earth's surface. The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.

Satellite systems are employed for telephone and data communications. There are geostationary satellites flying in high orbit (22,000 miles) where they can maintain the same position above the earth's surface at all times. The only problem, with such high-flying satellites is that there is a noticeable delay in real-time communications, and the power requirement to communicate with the satellites is too high for portable devices.

LEOs are more practical for mobile communication devices like mobile phones, PDAs, and automobile communication systems. An LEO satellite orbits in a relatively low earth orbit of a few hundred miles. In this orbit, the round-trip time for transmission is minimal, as are the power requirements for earth-bound communication devices. The downside of LEO satellites is that a fleet of them is required. Because of their low orbit, they move faster relative to a point on the surface, so a fleet of LEO satellites is required to maintain communications over a single point. As one LEO moves out of position, the other moves in. Each satellite covers an area that could be compared to a cell in a cellular system, except that the cell moves as the satellite orbits.

Space debris

The LEO environment is becoming congested with space debris. This has caused growing concern in recent years, since collisions at orbital velocities can be damaging or dangerous, and can produce more space debris in the process (Kessler Syndrome). The Joint Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), currently tracks more than 8,500 objects larger than 10 cm in LEO,[4] however a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters, which are too small to be visible from Earth.

ORBITAL DISTANCES

Any satellite can achieve orbit at any distance from the earth if its velocity is sufficient to keep it from falling to earth. The farter the satellite is from the earth, the longer it takes for a transmission to reach the satellite. The altitudes at which satellites can orbit are split into two categories:

Ø  Low Earth Orbit (LEO)

Ø  Medium Earth Orbit (MEO)

LOW EARTH (LEO)

Satellites in low earth orbit (LEO) satellites complete one orbit roughly every 90 minutes at a height of between 100 and 500 miles above the earth's surface. This means that they are fast moving ( >17,000mph) and sophisticated ground equipment must be used to track the satellite. This makes for expensive antennas that must track the satellite and lock to the signal while moving.

MIDDLE EARTH (MEO)

Most of the satellites in middle earth orbit circle the earth at approximately 6,000 to 12,000 miles above the earth in an elliptical orbit around the poles of the earth. Any orbit that circles around the poles is refered to as a 'polar orbit'. Polar orbits have the advantage of covering a different section of the earth's surface as they circle the earth. As the earth rotates, satellites in polar orbits can cover the entire surface of the earth. Fewer satellites are required to create coverage for the entire earth, as these satellites are higher and have a larger footprint. Spy satellites typically use middle earth, polar orbits to cover as much of the earth's surface as possible from one satellite.

GEOSTATIONARY/GEOSYNCHRONOUS (GEO)

 At 22,240 miles above the earth, craft inserted into orbit over the equator and traveling at approximately 6,880 miles per hour around the equator following the earths rotation. This allows these satellites to maintain their relative position over the earth's surface. Since the satellite follows the earth, and takes 24 hours to complete it's orbit around the earth, geostationary orbits are also called geosynchronous.

http://www.answers.com/topic/low-earth-orbit

http://www.linktionary.com/l/leo.html

http://en.wikipedia.org/wiki/Low_Earth_orbit