Low – Earth – Orbit Satellites (LEOs): 2012

Low-Earth-orbiting satellites

A Low Earth Orbit (LEO) typically is a circular orbit about 400 kilometres (250 mi) above the earth’s surface and, correspondingly, a period (time to revolve around the earth) of about 90 minutes. Because of their low altitude, these satellites are only visible from within a radius of roughly 1000 kilometers from the sub-satellite point. In addition, satellites in low earth orbit change their position relative to the ground position quickly.

Low earth orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high signal strength (Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic). Thus there is a trade off between the number of satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions.

LEOs are either elliptical or (more usual) circular orbits at a height of less than 2,000 km above the surface of the earth. The orbit period at these altitudes varies between ninety minutes and two hours. The radius of the footprint of a communications satellite in LEO varies from 3000 to 4000 km. The maximum time during which a satellite in LEO orbit is above the local horizon for an observer on the earth is up to 20 minutes. (A satellite with an orbiting altutude less than geostationary travels at a speed faster than the earth's orbit.) Although there are long periods during which the satellite is out of view of a particular ground station. This may be acceptable for a store-and-forward type of communication system as in an ecological/earth monitoring application. Most small LEO systems employ polar, or near-polar, orbits. Accessibility can of course be improved by deploying more than one satellite and using multiple orbital planes. A complete global coverage system using LEO orbits requires a large number of satellites, in multiple orbital planes, in varied inclined orbits.

The topology of a full service LEO-based communication network is dynamic; the network must continually adapt to changing conditions to achieve the optimal (least delay) connections between terminals. When a satellite serving a particular user moves below the local horizon, it needs to be able to hand over the service to a proximal or succeeding one in the same or adjacent orbit. Depending on the system design, individual satellites may cross-link with one another to relay a signal typically via a rapid packet switching technique (as in Iridium) or may return the signal to an earth terminal for rerouting.

Their main advantage is how close they are, providing shorter delays for faster communications. However, for consistent communications they require a constellation of satellites so that communications can be maintained as one satellite moves out of range and another moves within range of the ground station. LEO satellites are less expensive to build, typically less powerful, and have a shorter average life span.

Space Junk

The LEO environment is getting very crowded. The United States Space Command keeps track of the number of satellites in orbit. This is a graphic display of the objects in low earth orbit. According to the USSC, there are more than 8,000 objects larger than a softball now circling the globe.

Some people worry about the number of items now in low earth orbit. Not all of these things are working satellites. There are pieces of metal from old rockets, broken satellites, even frozen sewage. At 17,000 mph, even a small bolt can hit a space shuttle with the impact of a hand grenade. Which is exactly why the US Space Command keeps track of these things!

Advantages and Disadvantages of LEO

Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time.

There are two disadvantages to having things so close, however. The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth.

The second disadvantage has to do with how quickly a satellite in LEO goes around the Earth. As you can imagine, a satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. So what happens if we want a satellite to spend all of its time over just one part of the Earth? For instance, a weather satellite wouldn't be very effective for us in North America if it didn't have a long dwell time over us. (Dwell time = the time a satellite sits over one part of the globe.) Also, a communications satellite wouldn't work very well for us in North American if it spent most of its time over Africa or Asia.

There are two ways to accomplish this. One solution is to put a satellite in a highly elliptical orbit and the other is to place the satellite in a geosynchronous orbit.

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Wednesday, March 7, 2012

Low – Earth – Orbit Satellites (LEOs)