Orbits

Orbits are usually elliptical, but nearly all artificial satellites aim for one as circular as possible so that they can remain at the same height. They generally fall into three categories, classified as Low, Medium, and Geosynchronous Earth Orbit. The last category is the most useful for communications, though some newer systems have aimed for others.

Each of the different orbits shown in Figure 12.1 and Table 12.1 has slightly different properties. Higher orbits require more powerful rockets and take longer to reflect a signal back to Earth. Lower orbits cover a smaller area per satellite and fall back to Earth more quickly because of atmospheric drag. A very thin atmosphere extends thousands of kilometers into outer space, and friction from air molecules can make low orbits decay.

The orbital period of a satellite depends only on its distance from Earth and increases as it gets higher. The lower a satellite is, the faster it moves. For example, the space shuttle uses a relatively low orbit of no more than 400 km, circling Earth in around ninety minutes; the moon is a thousand times more distant and takes nearly a month.

Table 12.1. Communications Satellites Orbits

Orbit	Altitude	Latency	Min. Number of Satellites for Global Coverage
GEO	35,785 km	0.5 s	3
MEO	5,000–12,000 km	0.1 s	10
LEO	100–1000 km	0.05 s	48
Highly Elliptical	500–27,000 km	variable	15

GEO

The Geosynchronous Earth Orbit (GEO) was discovered by Arthur C. Clarke at the same time as he outlined the principles of satellite communication. Clarke calculated that at 35,784 km, the orbital period is exactly 24 hours. This means that if a satellite is placed at exactly this altitude above Earth's axis of rotation—the equator—it should not drift east or west at all.

The geostationaryorbit, another of Clarke's ideas, is a special and very useful case of the geosynchronous orbit. It has to be perfectly circular so that the satellite doesn't drift north and south or move up and down. A geostationary satellite appears to hang in the sky, so people don't have to track orbits or worry about them disappearing over the horizon. Users simply point their dish at a fixed spot and leave it there. With only three satellites in geostationary orbit, an operator can cover the entire planet.

The first satellite to reach geostationary orbit was Syncom 3, launched by NASA (National Aeronautics and Space Administration) in 1964. Positioned above the Pacific, it carried live television pictures of the Tokyo Olympic Games to U.S. viewers before being commandeered by the U.S. military for use in the Vietnam War. Since then, an increasing number of mainly communications satellites have joined it. They are too high to experience atmospheric drag, so old geostationary satellites remain in space, slowly drifting into more inclined orbits as a result of the moon's gravity. Over the years, they have formed an artificial ring around Earth, called the Clarke Belt.

The main problem with geostationary satellites is that they must be positioned over the equator, making it difficult for people to use them at latitudes in the far North or South. The dish requires a clear line of sight to the satellite, which can be blocked by buildings if it lies low on the horizon. For example, a geostationary satellite can't reach many parts of Manhattan unless the dish is placed high up on a skyscraper. In the Antarctic, even low-lying hills can block the field of view.

The dish size increases with latitude, because the signal strength is made much weaker the further North or South the antenna is positioned. This happens for the same reason that less heat from the sun reaches the poles than the tropics— it loses energy passing through the atmosphere. As Figure 12.2 shows, the amount of atmosphere blocking its path is thicker at higher latitudes.

GEO users far from the equator are also more likely to suffer from sun outages when satellites literally fly too close to the sun. During the March and September equinoxes, the sun passes behind the satellite as seen from the dish, temporarily drowning out the signal with its own radiation. These outages occur up five times each equinox, lasting between one and ten minutes, depending on the location and dish size. The bad news is that they're completely unavoidable, but the good news is that they're entirely predictable, so users can plan ahead. Many say that despite these outages, satellites are still more reliable than Earth-bound networks. They don't suffer from power failures or fiber cuts when someone accidentally chops through a cable.

GEO's other disadvantage is its great height. Even at the speed of light, signals take a fraction of a second to get to the Clarke Belt and back, producing a noticeable delay in conversation and playing havoc with Internet protocols. They also require powerful rockets to launch, and are too far away for a malfunctioning satellite to be repaired in orbit or brought back to Earth.

MEO

Middle Earth Orbit (MEO) satellites are those at an altitude of between around 5,000 and 15,000 km above the atmosphere and the most dangerous of the Van Allen radiation belts that encircle the Earth. MEO allows the whole world to be covered with relatively few satellites compared to lower orbits—around ten, with the exact number depending on the altitude. Its advantage over GEO is reduced latency, enabling conversations without noticeable delay.

Technically, MEO can extend right out to 35,000 km or more, but few satellites use non-geosynchronous orbits above about 10,000 km. While it is perfectly possible to put one higher than this, there is little reason to do so. Apart from the Clarke Belt, these higher orbits offer few advantages, but require more powerful rockets and radio equipment. Below 5,000 km, the Van Allen radiation is so strong that it will quickly destroy a satellite.

The most well known users of the MEO region are the ICO (Intermediate Circular Orbit) phone system and the GPS (Global Positioning System) satellites, both of which can be picked up with a nondirectional antenna.

LEO

The Low Earth Orbit(LEO) region extends from about 100 km to 1,000 km. Any higher would put a satellite inside the deadly Van Allen belt, and any lower, inside the thermosphere, the part of the atmosphere where friction burns meteors up.

Low orbits are the most common kind because they are so easy to reach. The earliest communications satellites, the Echoseries, used them from 1960 onward. Essentially just balloons coated in silver foil, they could reflect television transmissions across America. They needed powerful tracking equipment and giant antennas the size of houses, so sensitive that two AT&T engineers trying to find a satellite picked up radiation from the Big Bang instead. Their accidental discovery was rewarded several years later with a Nobel Prize.

This complicated tracking meant that low orbits fell out of favor for communications purposes, but they enjoyed a revival in the 1990s. When a satellite is only a few hundred miles up, its transmissions are much easier to pick up. By the late 1990s, advances in technology had enabled receivers for these satellites to be made no larger than mobile phones of a decade earlier, and many companies planned cellular networks based on satellites. The most well known was the ill-fated Iridium, though its failure has not put off others who plan similar schemes.

LEOs do have some disadvantages. Though huge by the standards of terrestrial cellular networks, their coverage is smaller than higher satellites. The number needed to cover the whole planet varies depending on altitude, but is generally a lot; proposed systems have required between 40 and 1,000. They also need to be replaced every few years because of atmospheric drag. Most include small boosters to lift them back up, but unlike the solar-powered batteries that run satellites themselves, these require actual rocket fuel. When it runs out, they fall to Earth.

The loss of satellites to drag can be significant, not to mention dangerous for anyone living underneath. If a network of 200 has a lifetime of four years, a new one needs to be launched every week. Modern rockets have a tendency to explode, so operators need to keep some spares in orbit.

The lowest orbits of all are used mostly for scientific and military purposes, as they are the easiest and safest for human engineers to reach. With the exception of the Apollo program, few astronauts have ever ventured beyond the LEO range. A mission lasting only a few days doesn't have to worry about being dragged back to Earth, while a space station can be brought supplies of rocket fuel along with food or replacement crews. Unique satellites, such as the Hubble telescope, are regularly saved from burn-up and carried up a few miles by the space shuttle, but doing this is so costly that communication satellites are cheaper to replace than salvage.

Elliptical

A newer approach is to use highly elliptical orbits, which deliberately vary their altitude from a few hundred to several thousand kilometers. The plan is that these will combine the low latency of LEO with the stability of GEO. The satellites will swoop up and down, seeming from some parts of Earth to hover for long periods in the same position.

Highly elliptical orbits are so complicated that the company Virtual Geosatellite LLC has been granted patents on the equations needed to describe them, but the idea is that, like geostationary satellites, they permit a dish to point continuously in the same direction. By the time one satellite has moved away, another will have taken its place.

The orbits can be arranged so that this "virtual geostationary" satellite appears to be anywhere, not just directly over the equator. This should make them more convenient for most areas, and means that they can use existing frequencies without interfering with transmissions to the already crowded Clarke Belt.