Satellite Communication Technology

COMSATs were first developed in the late 1950's under two primary categories, passive and active. Passive communications satellites use radio reflective mirrors to deflect a signal back to earth. Alternatively, active satellites collect a radio signal, amplify the signal and send it back to earth at a much higher power (NASA.gov). Communications specialists quickly realized the value of the active satellite which would lead to the development of COMSATs for commercial use. Eventually Congress passed the Satellite Communications Act of 1962 in order to control the developing commercial satellite industry. The new Act centered on controlling the industry towards public needs and national objectives. Congress understood the power of the new technology and quickly realized it would need to be governed. This brought about the Communications Satellite Corporation which would be the governing agency for all communications satellite activity (jstor.org).

As a result of the new legislation, commercial satellites began to appear. The first generation of direct relay satellites that emerged during the mid-1960's consisted of several 24-hour satellites built by the Hughes Aircraft Company. These satellites flew at the high altitude of 20,000 miles in order to remain stationed over a single point on the earth. This would provide uninterrupted direct relay communications because the satellite would always be in view (nasa.gov). Because these satellites remain directly above the equator in order to synchronize with earth's orbit they were label Geostationary. The alternative to a geostationary satellite is a low or medium-earth orbit satellite which orbits faster than the earth turns.

Geostationary Satellite: Geostationary orbits exist at very high altitudes as the satellite must maintain a certain speed to remain in orbit and fend off Earth's gravitational pull. The following is the formula for calculating the required distance from earth for a geostationary orbit.

R = (G x M x period2/(4 x pi2) )(1/3)

where G is Newton's constant of gravity (6.61x10-11 m3kg-1s-2), M is the mass of the Earth (5.93x1024 kg), and period is 23h56m = 86160 s (nasa.gov).

Some of the disadvantages of geostationary satellites include their far distance from Earth and the comparatively delayed time it takes information to make a round trip to the satellite. In addition the limited space for satellites directly above Earth's equator can result in traffic congestion. Finally, the extreme distance of geostationary satellites makes them a poor choice for producing detailed imagery of earth (Muller, 200).

Low-Earth-Orbiting Satellites: The alternative to the geostationary satellite is the low-Earth-orbiting satellite (LEO) which provides a much stronger relay due to its close proximity to the surface. LEO satellites fly at an altitude of about 250 miles above the earth and have a total orbit time of approximately 90 minutes. This gives the satellite very limited time for line of sight communication over a specific area on earth. However, communication happens at a much higher rate leading to increased response time. As a result, a constellation of many LEO satellites are required in order to maintain constant communication as each one comes in and out of view during its orbit (Muller, 199)

Medium-Earth Orbit: A third and final category of satellites are those that orbit at a medium altitude of between 1,200 miles and 22,000 miles above the earth. Typical orbit times vary between 2 and 24 hours (Muller, 204). These Medium-Earth-Orbit (MEO) satellites are ideal for GPS navigation as they provide extended periods of visibility as they pass overhead. The current ICAO accepted system is called the Global Navigation Satellite System (GNSS) and incorporates a series of 24 MEO satellites. At a speed of 3.6km/second and a full orbit time of 12 hours, the GNSS system is built to provide a visibility of at least 4 satellites at any point on the earth at and given time (kowoma.de).

GPS receivers require continuous visibility of at least 3 satellites to provide a 2-Dimensional position and 4 satellites to provide 3-Dimentional position based on their speed and altitude. This results in an accuracy of 100 feet 95% of the time (freeflightsystems.com). Typically a 5th satellite is incorporated to back up and cross check the primary 4 (kowoma.de). Specifically, aircraft must rely on accurate signal coverage during critical phases of flight such as departure and arrival. In order to guarantee signal coverage the GPS receiver utilizes a self-check system called Receiver Autonomous Integrity Monitoring (RAIM) which predicts satellite coverage at a prescribed time in the future.

Chapter 2: Limitations to Satellite Signal

The rapid growth of satellite communication over the last three decades has brought with it several benefits for information systems. Besides navigation capabilities, satellites have expanded the role of web browsing, file transfer protocol (FTP), remote login (Telnet), video teleconferencing, e-mail, and broadcast. The benefit of satellite communication for these systems is found in the satellites ability to broadcast very large amounts of information over a very large area. Although the above applications are all feasible through satellite communications, they also have their individual requirements. The two greatest limiting factors for the transfer of information are throughput and responsiveness (isoc.org).

Throughput and Responsiveness

Throughput is a measurement of the amount of information that can be transferred through a satellite. Broadcasting and on-line information retrieval are examples of applications which require a high amount of throughput. Both of these require the transfer of large amounts of information and would be best integrated with a GEO satellite. This is because the GEO satellite remains stationary above a fixed point therefore allowing uninterrupted signal transfer. In addition, the high altitude of the GEO satellite allows it to transmit the signal over a large geographic area. The only drawback of the satellite's altitude is the time it takes to transfer information to and from Earth which is typically 400 milliseconds (isoc.org).

The second factor affecting information transfer is responsiveness. Responsiveness is a measure of how fast a signal can be transferred through a satellite. Several applications listed above such as interactive gaming and video conferencing require very fast undisrupted data transfer in order to provide effective communication. An example of poor responsiveness can be noticed in the signal latency experienced when viewing network news casters who communicate with journalist halfway around the world.