Communications Satellites
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Geostationary Satellites
Satellites can be characterized by the orbits they keep. The most common type today is the geostationary satellite (GEOS), as shown in Figure 1.
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Figure 1 Satellite orbits (not to scale).
If the satellite is in a circular orbit 35,838 km above the earth's surface and rotates in the equatorial plane of the earth, it will rotate at the same angular speed as the earth and will remain above the same spot on the equator as the earth rotates. This configuration has many advantages to recommend it:
Because the satellite is stationary relative to the earth, there is no problem with frequency changes due to the relative motion of the satellite and antennas on earth (Doppler effect).
Tracking of the satellite by its earth stations is simplified.
At 35,838 km above the earth, the satellite can communicate with roughly one-fourth of the earth; three satellites in geostationary orbit separated by 120x cover most of the inhabited portions of the entire earth, excluding only the areas near the north and south poles.
On the other hand, there are problems:
The signal can get quite weak after traveling over 35,000 km.
The polar regions and the far northern and southern hemispheres are poorly served by geostationary satellites.
Even at the speed of light, about 300,000 km/sec, the delay in sending a signal from a point on the equator beneath the satellite 35,838 km to the satellite and 35,838 km back is substantial.
The delay of communication between two locations on earth directly under the satellite is in fact (2 x 35,838)/300,000 = 0.24 sec. For other locations not directly under the satellite, the delay is even longer. If the satellite link is used for telephone communication, the added delay between when one person speaks and the other responds is increased twofold, to almost 0.5 sec. This is definitely noticeable.
Another feature of geostationary satellites is that they use their assigned frequencies over a very large area. For point-to-multipoint applications such as broadcasting TV programs, this can be desirable, but for point-to-point communications it's very wasteful of spectrum. Special spot and steered-beam antennas, which restrict the area covered by the satellite's signal, can be used to control the "footprint" or signaling area. To solve some of these problems, orbits other than geostationary have been designed for satellites. Low-earth-orbiting satellites (LEOS) and medium-earth-orbiting satellites (MEOS) are important for third-generation personal communications.
Low- and Medium-Earth-Orbiting Satellites
The original AT&T satellite proposal was for low-earth-orbiting satellites, but most of the early commercial satellites were geostationary. Nevertheless, low-earth orbits have advantages, and many recent satellite proposals are based on them. The idea is to use constellations of inexpensive low-earth-orbiting satellites, sometimes called lightsats. They orbit at altitudes of about 320 to 1,100 km above the earth's surface. Therefore, the propagation time is much smaller. Moreover, their signal is much stronger than that of geostationary satellites for the same transmission power. Their coverage can be better localized so that spectrum can be better conserved. For this reason, this technology is currently being proposed for communicating with mobile terminals and with personal terminals that need stronger signals to function. On the other hand, to provide broad coverage over 24 hours, many satellites are needed. Sixty-six are being proposed by Motorola for its Iridium system.
A number of commercial proposals have been made to use clusters of LEOs to provide communications services. These proposals can be divided into two categories:
Little LEOSs: Intended to work at communication frequencies below 1 GHz, using no more than 5 MHz of bandwidth, and supporting data rates up to 10 Kbps. These systems are aimed at paging, tracking, and low-rate messaging. Orbcom is an example of such a satellite system. It was the first (little) LEO in operation, with its first two satellites launched in April of 1995. These are some of its stats:
Designed for paging and burst communication and optimized for handling small bursts of data from 6 to 250 bytes in length.
Used by businesses to track trailers, railcars, heavy equipment, and other remote and mobile assets. It can also be used to monitor remote utility meters and oil and gas storage tanks, wells, and pipelines, or to stay in touch with remote workers anywhere in the world.
Uses the frequencies 148.00 to 150.05 MHz to the satellites, and 137.00 to 138.00 from the satellites, with well over 30 satellites in low-earth orbit. Supports subscriber data rates of 2.4 Kbps to the satellite and 4.8 Kbps down.
Big LEOSs: Frequencies above 1 GHz and supporting data rates up to a few megabits per second. These systems tend to offer the same services as those of small LEOSs, with the addition of voice and positioning services. Globalstar is one example of a Big LEO system. These are some of its stats:
Its satellites are fairly rudimentary. Unlike Iridium, it has no onboard processing or communications between satellites. Most processing is done by the system's earth stations.
Uses CDMA as in the CDMA cellular standard.
Uses the S-Band (about 2 GHz) for the down link to mobile users.
Tightly integrated with traditional voice carriers. All calls must be processed through earth stations.
Satellite constellation consists of 48 operating satellites and 8 spares, in 1,413 km orbits.
A LEO satellite can be "seen" by a point on earth on the order of minutes before the satellite passes out of sight. If intermediate orbits are used—higher than the LEOS and lower than GEOS—a point on earth can see the satellite for periods on the order of hours. Such orbits are called medium-earth-orbiting satellites (MEOS). These orbits are on the order of 10,000 km above the earth, and require fewer handoffs. While propagation delay to earth from such satellites (and the power required) is greater than for LEOS, they are still substantially less than for GEOS. ICO Global Communications, established in January 1995, proposed a MEO system. Launches began in 2000; 12 satellites, including two spares, are planned in 10,400 km orbits. The satellites will be divided equally between two planes tilted 45x to equator. Proposed applications are digital voice, data, facsimile, high-penetration notification, and messaging services.