A virtual device driver (VxD) is a software device driver that emulates hardware and other devices so that multiple applications running in protected mode can access hardware interrupt channels, hardware resources and memory without causing conflicts. The Vxd was superseded by the Windows Driver Model (WDM) and is now obsolete.
Low earth orbits (LEO) are satellite systems used in telecommunication, which orbit between 400 and 1,000 miles above the earth's surface. They are used mainly for data communication such as email, video conferencing and paging. They move at extremely high speeds and are not fixed in space in relation to the earth.
LEO-based telecommunication systems provide underdeveloped countries and territories with the ability to acquire satellite telephone service in areas where it otherwise would be too costly or even impossible to lay land lines.
Low earth orbit is defined as an orbit within a locus extending from the earth’s surface up to an altitude of 1,200 miles. Attributing to their high speeds, data transmitted through LEO is handed off from one satellite to another as satellites generally move in and out of the range of earth-bound transmitting stations. Due to low orbits, transmitting stations are not as powerful as those that transmit to satellites orbiting at greater distances from earth’s surface.
Most communication applications use LEO satellites because it takes less less energy to place the satellites into LEO. Moreover, they need less powerful amplifiers for successful transmission. As LEO orbits are not geostationary, a network of satellites are required to provide continuous coverage. However, as a result of the popularity of this type of satellite, studies reveal that the LEO environment is getting congested with space debris. NASA keeps track of the number of satellites in the orbit, and estimates that there are more than 8,000 objects larger than a softball circling the globe. Not all of these objects are not satellites, but rather pieces of metal from old rockets, frozen sewage and broken satellites.
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