Computer Networks And The Communications Satellites Computer Science

Add: 19-01-2017, 16:42   /   Views: 110

A spacecraft in orbit around the earth to receive and retransmit radio signals.

Communications satellites amplify and sort or route these signals.

In earlier days they functioned much like ground microwave repeaters but with greatly increased coverage.

Whereas a ground repeater relays signals between two fixed locations, a communications satellite interconnects may locations, fixed and mobile, over a wide area.

With the advent of on-board processing, switching and rerouting of signals has been added to the functionality of some communications satellites, making them "switchboards in the sky."

HISTORY

The first artificial satellite was the soviet sputnik 1, launched on October 4, 1957, and equipped with an-board radio-transmitter that worked on tow frequencies, 20.005 and 40.002MHz.

The first American satellite to relay communications was project SCORE in 1958, which used a tape recorder to store and forward voice messages.

It was used to send a Christmas greeting to the world form U.S.

President Dwight D.

Eisenhower.

NASA launched an Echo satellite, in 1960; the 100-foot(30 m) aluminized PET film balloon served as a passive reflector for radio communications.

Courier 1B, built by philco, also launched in 1960, was the world's first active repeater satellite.

COMMUNICATIONS SATELLITES

Geostationary communications satellite

Communications satellites have a quiet, yet profound, effect on our daily lives.

They link remote areas of the Earth with telephone and television.

Modern financial business is conducted at high speed via satellite.

The newspaper USA Today is typeset and transmitted to printing plants via satellite.

Radio signals near the microwave frequency range are best suited to carry large volumes of communications traffic.

They are not deflected by the Earth's atmosphere as lower frequencies are.

Basically, they travel in a straight line, known as line of sight communication.

If someone in San Francisco tried to beam a microwave signal directly to Hawaii, it would never get there; it would disappear into space or dissipate into the ocean.

Over short distances, we can erect microwave towers every 25 miles or so to act as "repeaters" to repeat and boost the signal.

Think of a geostationary communications satellite as a repeater in the sky.

OSCAR SATELLITES

An OSCAR satellitehttp://octopus.gma.org/i_space.gif

The satellite UOSAT-11 is one of dozens of amateur satellites orbiting the earth.

Sputnik, the world's first artificial Earth-orbiting satellite, transmitted a beacon on 20.005 MHz which was monitored by thousands of hams and Short Wave Listeners (SWL).

Since 1957, many OSCAR (Orbiting Satellites Carrying Amateur Radio) satellites have been constructed by ordinary people interested in satellites communications.

OSCAR 1, launched in December of 1961, weighed 10 pounds and transmitted a 15 milliwatt beacon for about 3 weeks.

OSCAR 13, launched in the summer of 1988, provides reliable, near-global communications.

Interestingly enough, the OSCAR series of satellites are actually ballast for larger primary NASA payloads.

It's simpler and cheaper to ballast a rocket with dead weight rather than reduce the thrust.

As a result, it is possible to add secondary payloads of homemade satellites to multi-million dollar NASA missions at minimal costs.

There are currently nineteen OSCAR satellites orbiting our planet with various communications capabilities and functions.

Most are used by ordinary amateur radio operators for educational, scientific, and purely recreational purposes.

Anyone interested in knowing more about the OSCAR series of satellites in encouraged to contact the Radio Amateur Satellite Corporation (AMSAT).

EXPERIMENTAL SATELLITE COMMUNICATIONS

For the purpose of test and evaluation of new technologies a number of satellites have been designed and operated for technical experiments.

Various experiments have also been conducted using these satellites for demonstrating different applications of communications satellites.

Prominent among these experimental satellites are: