Low Earth Orbit Satellite (LEOs)

CHAPTER 27 - LEOs (Low Earth Orbit Satellite)

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 requirements 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

• Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles.
• Low Earth Orbit satellites must travel very quickly to resist the pull of gravity — approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.
• Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
• Low Earth Orbit satellites typically operate in polar orbits.
• Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS).
• Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

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.[1][2] The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.

The delta-v needed to achieve low earth orbit starts around 9.4km/s. With the exception of the lunar flights of the Apollo program, all human spaceflights have either been orbital in LEO or sub-orbital. The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 km.

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. LEO is an orbit around Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km because that would be impractical due to the larger atmospheric drag.

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement of any orbit. Orbits with a high inclination angle are usually called polar orbits.

Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation.

The International Space Station is in a LEO that varies from 320 km (199 mi) to 400 km (249 mi) above the Earth's surface.

While a majority of artificial satellites are placed in LEO, where they travel at about 7.8 km/s (28,080 km/h), making one complete revolution around the Earth in about 90 minutes, many communication satellites require geostationary orbits, and move at the same angular velocity as the Earth. Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. ENVISAT is one example of an Earth observation satellite that makes use of this particular type of LEO.

Although the Earth's pull due to gravity in LEO is not much less than on the surface of the Earth, people and objects in orbit experience weightlessness due to the effects of freefall.

Atmospheric and gravity drag associated with launch typically adds 1.5–2.0 km/s to the delta-v launch vehicle required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h).

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

              http://www.ssec.wisc.edu/sose/pirs/pirs_m1_leo.html