Satellite Communication The Artificial Satellites Part 2 |
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By: John Dulaney Posted: 2006-09-18 20:51:56 |
| The first thought of an artificial satellite was to bring about a global communication facility. By that time, transatlantic telephone calls were quite established and telegraph broadcast, through under water cables, existed for the last 50 years. At about this time, a new thought came into the mind, the thought of broadcasting live television pictures. But greater amount of information were required on this technology, with its operation in a much higher frequency than the common radio broadcasts. The first ever radio broadcast was done at 1,020 Kilo Hz. Frequency is the measure of the number of times a signal repetition occurs in one second. If the signal completes one full cycle in one second, the measurement term for that signal is one Hertz, which, in short form, is 1Hz. Therefore, 1,020 Kilo Hz would mean that the signal completes 1,020,000 cycles in one second. As far as television is concerned, a much higher frequency was required to transmit pictures and such a typical transmission would need a frequency of 175 MHz. As a result, television could not be broadcasted with the frequency of that of a radio.The frequency bands required for radio and television signal transmission can directly propagate from transmitter to receiver. The signal band is quite dependable and more or less limited the line of sight. Hence, you will find the television transmitting antenna is set up on a high tower, so the purpose of the line of sight to the receiving antenna is served. The radio signals, transmitted for thousands of miles, used the charged layer of the atmosphere (ionosphere) to bounce back to Earth. But higher frequencies did not bounce and disappeared into the atmosphere, in a relatively short space.Therefore, as far as television frequency was concerned, it depended on absolute line of sight and restricted to 30 miles as the crow flies, or perhaps by co-axial cables across continents for live broadcasts. Transatlantic broadcasts were out of question and for example, news was delayed by 12 hours to reach Europe. The appetite for transatlantic radio entertainment and telephone was increasing rapidly. The demand for the new television medium could not take off with the existing communication facilities. The frequency requirement could not match the existing ones, which were simply not able to handle all of the requirements. Towards the end of 1950s, television transmission became hopeful with the newly developed artificial satellites.In 1960, the first ever communication satellite was launched. It was the simplest kind and was called Echo. It consisted of a large, 1,000 feet diameter aluminised plastic balloon. Radio and television signals transmitted to this satellite would bounce back to Earth and would be received by the earth receiving stations at the line of sight principle.Echo was launched at low Earth orbit and unfortunately at this orbit, Echo encircled the Earth every 90 minutes. Therefore, it was visible at any single point for only 10 minutes out of its 90 minutes orbital time. In 1958, Score satellite was launched, which carried a tape recorder and would record the transmitting signals, which would be played back when it passed over the destination. However, since it also had a 90 minute orbital time, no real-time communication could be achieved.In 1962, NASA launched the Telstar satellite for AT&T. This satellite was launched at such a height that it could see Europe and USA simultaneously during one point of its orbit. Therefore, this satellite was able to provide real-time communication between two points for a few minutes during its orbital path.Continuous real-time communication was not possible with low polar Earth orbital satellites. The solution to the problem was ofcourse use of geosynchronous orbit. In 1963, the rocket booster power to launch such a satellite became available and the first ever geosynchronous orbital satellite, Syncom 2, was launched by NASA. This had an orbital path which scanned the Earth's rotational speed and it stayed constantly visible through 24 hours, and it could view approximately 42% of the Earth's surface. A real-time communication became possible within that area and outside of this viewing area, no communication could be set up.However, a system of three satellites was set up, where one satellite would relay the communication to the other, which would in turn, either relay it to the third or transmit the signal to the destination it was meant for. The disadvantage with the geosynchronous orbit is that, it took 1/4th of a second to transmit a signal to the satellite from Earth and receive back the signal again, The signal needs to travel 22,000 miles up, and down again by another 22,000 miles to complete the transmit and receive.For data transmission, this signal delay was not so much significant, but with telephone communication, this delay was indeed annoying. Syncom 2 was put at a height of 22,000 miles above earth. The principle followed the cannon ball example, as illustrated in Part 1 of the article. After the success of Syncom 2, a spurt of launching such satellites took effect, especially with the success of geosynchronous satellite technology.There are 100 satellites in geosynchronous orbit today out of 150 communication satellites that have been launched. One of the well known satellite development was Intelsat, launched by an internationally-owned corporation. This organisation has launched 8 different satellites of 4 or 5 of each series. The satellites have been launched in a period of 30 years. With such a satellite network, and with relaying signals from one satellite to other, it has been possible to handle large telephone traffic between any two points on the Earth. It was also possible to transmit live television pictures between any two virtual points on the Earth. By 1964, one could watch the Olympics from Tokyo, live. A few years later, live television broadcast became a reality.Finally it arrived. A reliable toll quality voice communication was made available through satellite communication. Using the state-of-art technology, satellite application gives you the quickest reliable access to your corporate network or to the public switched telephone network (PSTN) system, disregarding where-ever you are. You just need to pick up your telephone receiver and get connected in seconds, no matter where you are calling from and where you are calling to. As for data communication, file transfers, E-mail, access to databases at a much higher speed is now conceivable, to and from any part of the world. With reliable security built-in in these systems, the signals are all digitally coded providing you with a very safe and secured communication. Today, distant learning has become the possibility with those satellites up in the sky. |
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