The Air Up There

One easy way to distinguish among the many new satellite systems is by how high up they are. This is also one key factor in determining how many satellites a system needs for worldwide coverage and how powerful those satellites must be. If an antenna can cover 15 degrees, for example, that same arc covers a much smaller area if the satellite is 200 miles away than if it's 20,000 miles away. However, it will require much less power to deliver a signal fr om 200 miles away than from 20,000 miles away. Satellite people have four basic terms to describe different altitudes.

GEO: Short for geosynchronous earth orbit, GEO satellites orbit at 22,238 miles above the earth's equator. At this altitude, the period of rotation of the satellite around the earth is exactly 24 hours. The satellite seems to stay above exactly the same point on the earth's surface. (As a footnote, this orbit is called a Clarke orbit, named for the author Arthur C. Clarke, who first posited in 1945 that it should be possible. According to B YTE senior contributing editor Jerry Pournelle, Clarke never got the patent he sought for figuring it out.) Most of today's satellites are GEOs, as are planned broadband systems such as Hughes' Spaceway and Loral's Cyberstar.

GEOs require few satellites to cover the entire earth's surface. However, they're saddled with a 0.24-second latency for a signal to travel from earth to satellite and back to earth again. GEOs also need to obtain specific orbital slots around the equator to keep far enough apart, each separated by 2 degrees, or about 1000 miles, according to Erwin Edelman, demonstrations coordinator at NASA's Lewis Research Center. The ITU and, in the U.S., the FCC mete out these slots.

MEO: According to Marco Caceres of the Teal Group, medium earth orbit satellites orbit at altitudes between 6250 and 12,500 miles. Unlike GEOs, their position changes relative to the earth's surface. At their lower altitudes, you need more of them to achieve complete coverage of the earth's surface, but the latency reduces substantially. Right now, according to Caceres, there aren't many MEOs, and the ones in orbit are used for positioning.

LEO: Low earth orbits promise extremely high bandwidth and low latency. Plans exist for huge constellations of hundreds of satellites that will cover the entire globe. LEOs generally orbit below 3125 miles. Most of them are much lower: only 400-1000 miles. At these altitudes, latency reduces to nearly negligible times -- hundredths of a second.

Three kinds of LEOs handle different amounts of bandwidth. Little LEOs are low-bandwidth applications (tens to hundreds of Kbps) such as paging and include systems such as OrbComm. Big LEOs can handle paging, cellular services, and some data transmission (hundreds to thousands of Kbps). Examples include GlobalStar and Iridium. Broadband LEOs (sometimes called mega-LEOs) operate in the Mbps range and include Teledesic, Celestri, and Skybridge.

HALE: High-altitude, long-endurance platforms are basically a solar-powered, lightweight airplane or lighter-than-air craft that hover over an unmoving spot some 70,000 feet above the earth's surface. Not often talked about, and right now primarily a research venture. An example of a HALE that uses blimps is Skystation.