Low Earth Orbits

A satellite can also be placed in Low Earth Orbits (about 1,000 kilometers above the Earth (between 400 miles and 1,600 miles)). However, satellites in LEO need a higher velocity than Geostationary orbits.  For example, a satellite which is placed in an orbit at altitude of 200 kilometers will need an orbital velocity of approximately 29000 kilometer per hour. Similarly, a satellite placed in an orbit at around 1730 kilometers will need a speed of 25,400 kilometers per hour.

Key Features of LEO

Unlike GEOs, the LEO satellites appear travelling across the sky from earth. A typical LEO satellite takes one and half hours to orbit the Earth, which means that a single satellite is “in view” of ground equipment for a only a few minutes. As a consequence, if a transmission takes more than the few minutes that any one satellite is in view, a LEO system must “hand off” between satellites in order to complete the transmission. In general, this can be accomplished by constantly relaying signals between the satellite and various ground stations, or by communicating between the satellites themselves using “inter-satellite links.”

International Space Station

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

Applications of Low Earth Orbit Satellites

LEO systems are designed to have more than one satellite in view from any spot on Earth at any given time, minimizing the possibility that the network will lose the transmission. Because of the fast-flying satellites, LEO systems must incorporate sophisticated tracking and switching equipment to maintain consistent service coverage. The need for complex tracking schemes is minimized, but not obviated, in LEO systems designed to handle only short-burst transmissions.

The advantage of the LEO system is that the satellites’ proximity to the ground enables them to transmit signals with no or very little delay, unlike GEO systems. LEO satellites rotate the earth and currently deliver significant voice quality over the Geosynchronous (GEO) satellite systems. Now days, LEO Satellites are used in constellations such as Globalstar and Iridium constellations. In addition, because the signals to and from the satellites need to travel a relatively short distance, LEOs can operate with much smaller user equipment (e.g., antennae) than can systems using a higher orbit. In addition, a system of LEO satellites is designed to maximize the ability of ground equipment to “see” a satellite at any time, which can overcome the difficulties caused by obstructions such as trees and buildings.

Orbital Decay

The satellites particularly in the LEO are subject to a drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The amount of this drag keeps increasing or decreasing depending upon several factors including the solar activity. The more activity heats of the upper atmosphere and can increase the drag. This drag in a long duration causes a reduction in the altitude of a satellite’s orbit, which is called orbital decay.

So, the major cause of the orbital decay is Earth’s atmosphere. The result of the drag is increased heat and possible reentry of satellite in atmosphere causing it to burn. Lower its altitude drops, and the lower the altitude, the faster the decay. Apart from Atmosphere, the Tides can also cause orbital decay, when the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. Mars’ moon Phobos is one of the best examples of this.

LEO systems Pros and Cons

• 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.
• However, since these LEO orbits are not geostationary, a network (or “constellation”) of satellites is required to provide continuous coverage.
• The transmission delay associated with LEO systems is the lowest of all of the systems.
• Because of the relatively small size of the satellites deployed and the smaller size of the ground equipment required, the LEO systems are expected to cost less to implement than the other satellite systems.
• The small coverage area of a LEO satellite means that a LEO system must coordinate the flight paths and communications hand-offs a large number of satellites at once, making the LEOs dependent on highly complex and sophisticated control and switching systems.

LEO satellites have a shorter life span than other systems. There are two reasons for this: first, the lower LEO orbit is more subject to the gravitational pull of the Earth and second, the frequent transmission rates necessary in LEO systems mean that LEO satellites generally have a shorter battery life than others.

Medium Earth Orbit

MEO systems operate at about 8,000-20,000 km above the Earth, which is lower than the GEO orbit and higher than most LEO orbits.  The MEO orbit is a compromise between the LEO and GEO orbits. Compared to LEOs, the more distant orbit requires fewer satellites to provide coverage than LEOs because each satellite may be in view of any particular location for several hours. Compared to GEOs, MEOs can operate effectively with smaller, mobile equipment and with less latency (signal delay).

These orbits are primarily reserved for communications satellites that cover the North and South Pole. Although MEO satellites are in view longer than LEOs, they may not always be at an optimal elevation. To combat this difficulty, MEO systems often feature significant coverage overlap from satellite to satellite, which in turn requires more sophisticated tracking and switching schemes than GEOs.Typically, MEO constellations have 10 to 17 satellites distributed over two or three orbital planes.Most planned MEO systems will offer phone services similar to the Big LEOs. In fact, before the MEO designation came into wide use, MEO systems were considered Big LEOs. Examples of MEO systems include ICO Global Communications and the proposed Orblink from Orbital Sciences. Unlike the circular orbit of the geostationary satellites, MEO’s are placed in an elliptical (oval-shaped) orbit