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) satellites are in an orbit about 400 to 800 miles above the Earth’s surface - far below geostationary (GEO) satellite orbit. Orbits lower than this are not stable, and will decay rapidly because of atmospheric drag.

A LEO satellite orbits a local horizon in approximately 20 minutes. The orbiting periods range anywhere from 90 minutes to two hours, at approximately 17,000 mph. LEOs are considered to have no delay. A LEO system must use a satellite-to-satellite hand-off to maintain communications and are best for short, narrowband communications. Once one satellite moves out of the area a new one will move in.

There are two types of LEOs - little LEOs and Big LEOs.
Big LEO

Big LEOs are used for technology devices such as high-speed, high-bandwidth data communications, and video conferencing. They carry voice and high-speed data services. They are aimed at data communications and real-time voice into hand-held devices. Big LEO can also offer global services, which are also subject to regulatory requirements. There have been five Big LEO already licensed. The five Big LEO are Iridium, Global Star, Odyssey, ICO Global Communications, Teledesic.
Little LEO

Little LEOs are required to offer non-voice services for example vehicle tracking, environmental monitoring and two-way data communication. A little LEO is a constellation of small, low-earth orbiting satellites, used for short, narrowband communications. Little LEOs are a small, low-cost, class of satellites. LEOs are to use a spectrum allocated between 137-138MHz for space-to-Earth downlinks, and 148-150.05 MHz for Earth-to-space uplinks. ORBCOMM is a Little LEO that Stevens uses for environemental monitoring application.
ORBCOMM

ORBCOMM, a leading LEO system used in data collection and environemtal monitorng, provides global coverage with 30 satellites, and is capable of sending and receiving tow-way alphanumeric packets of data anywhere in the world. Stevens is ORBCOMM’s value added reseller (VAR) or a.k.a. Message Server for environmental monitoring applications.

The Users communicators (SCs) pass data messages to and from Gateway Control Centers (GCC) over ORBCOMM satellites . Messaging traffic flows between the satellites and a GCC through tracking stations called Gateway Earth Stations (GESs) that connect with satellites as they pass overhead. The GCC than route messages to third party value added service providers such as Stevens Water Monitoring Systems, Inc. Stevens collects, processes, stores and delivers the data message to the user via Internet, e-mail or dedicated linesWhen a satellite is not connected to a GES, it can still support SC messaging in a store-and-forward mode.




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-vrequirement 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.



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.