COMMUNICATIONS SATELLITES

Why Satellites for Communications

By the end of World War II, the world had had a taste of "global communications." Edward R. Murrow's radio broadcasts from London had electrified American listeners. We had, of course, been able to do transatlantic telephone calls and telegraph via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required to transmit television pictures required that they operate at much higher frequencies than radio stations. For example, the very first commercial radio station (KDKA in Pittsburgh) operated ( and still does) at 1020 on the dial. This number stood for 1020 KiloHertz - the frequency at which the station transmitted. Frequency is simply the number of times that an electrical signal "wiggles" in 1 second. Frequency is measured in Hertz. One Hertz means that the signal wiggles 1 time/second. A frequency of 1020 kiloHertz means that the electrical signal from that station wiggles 1,020,000 times in one second.

Television signals, however required much higher frequencies because they were transmitting much more information - namely the picture. A typical television station (channel 7 for example) would operate at a frequency of 175 MHz. As a result, television signals would not propagate the way radio signals did.Both radio and television frequency signals can propagate directly from transmitter to receiver. This is a very dependable signal, but it is more or less limited to line of sight communication. The mode of propagation employed for long distance (1000s of miles) radio communication was a signal which traveled by bouncing off the charged layers of the atmosphere (ionosphere) and returning to earth. The higher frequency television signals did not bounce off the ionosphere and as a result disappeared into space in a relatively short distance.

Consequently, television reception was a "line-of-sight" phenomenon, and television broadcasts were limited to a range of 20 or 30 miles or perhaps across the continent by coaxial cable. Transatlantic broadcasts were totally out the question. If you saw European news events on television, they were probably delayed at least 12 hours, and involved the use of the fastest airplane available to carry conventional motion pictures back to the U.S. In addition, of course, the appetite for transatlantic radio and telephone was increasing rapidly. Adding this increase to the demands of the new television medium, existing communications capabilities were simply not able to handle all of the requirements. By the late 1950s the newly developed artificial satellites seemed to offer the potential for satisfying many of these needs.

Low Earth-Orbiting Communications Satellites

In 1960, the simplest communications satellite ever conceived was launched. It was called Echo, because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio and TV signals transmitted to the satellite would be reflected back to earth and could be received by any station within view of the satellite.

Unfortunately, in its low earth orbit, the Echo satellite circled the earth every ninety minutes. This meant that although virtually everybody on earth would eventually see it, no one person, ever saw it for more than 10 minutes or so out of every 90 minute orbit. In 1958, the Score satellite had been put into orbit. It carried a tape recorder which would record messages as it passed over an originating station and then rebroadcast them as it passed over the destination. Once more, however, it appeared only briefly every 90 minutes - a serious impediment to real communications. In 1962, NASA launched the Telstar satellite for AT&T.

Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit. During another part of its orbit it could see both Japan and the U.S. As a result, it provided real- time communications between the United States and those two areas - for a few minutes out of every hour.

Geosynchronous Communications Satellites

The solution to the problem of availability, of course, lay in the use of the geosynchronous orbit. In 1963, the necessary rocket booster power was available for the first time and the first geosynchronous satellite , Syncom 2, was launched by NASA. For those who could "see" it, the satellite was available 100% of the time, 24 hours a day. The satellite could view approximately 42% of the earth. For those outside of that viewing area, of course, the satellite was NEVER available.

However, a system of three such satellites, with the ability to relay messages from one to the other could interconnect virtually all of the earth except the polar regions. The one disadvantage (for some purposes) of the geosynchronous orbit is that the time to transmit a signal from earth to the satellite and back is approximately ? of a second - the time required to travel 22,000 miles up and 22,000 miles back down at the speed of light. For telephone conversations, this delay can sometimes be annoying. For data transmission and most other uses it is not significant. In any event, once Syncom had demonstrated the technology necessary to launch a geosynchronous satellite, a virtual explosion of such satellites followed.Today, there are approximately 150 communications satellites in orbit, with over 100 in geosynchronous orbit. One of the biggest sponsors of satellite development was Intelsat, an internationally-owned corporation which has launched 8 different series of satellites (4 or 5 of each series) over a period of more than 30 years. Spreading their satellites around the globe and making provision to relay from one satellite to another, they made it possible to transmit 1000s of phone calls between almost any two points on the earth. It was also possible for the first time, due to the large capacity of the satellites, to transmit live television pictures between virtually any two points on earth. By 1964 (if you could stay up late enough), you could for the first time watch the Olympic games live from Tokyo. A few years later of course you could watch the Vietnam war live on the evening news.