WHY COMMUNICATION SATELLITES ?

[Music] [Applause] messages over great distances these scenes were made during the first television broadcast ever relayed through space by a communication satellite why do we use satellites like these for long distance communication to find out let's start with some simpler and more familiar ways of communicating these are all ways of communicating sending and receiving messages they are useful over short distances if we are our line of sight line of sight or straight line signals are often used by sailors on ships blinker lights on one ship can be used to send signals in a straight line to other ships but the earth is round as these two ships move apart the curve of the earth itself interferes with straight line signals the light cannot reach the other ship how then can we send and receive signals or messages over distances beyond the curve of the earth radio provides one way but radio signals also travel in straight lines these signals cannot pass through the earth however a radio signal sent from one side of the earth can be received on the other side this is done by making use of a special part of the atmosphere high above the earth this part of the atmosphere is called the ionosphere when a radio signal reaches the ionosphere it is bounced or reflected back radio signals can be sent over great distances by bouncing them up and down between the surface of the earth and the ionosphere under ideal conditions radio signals can be bounced all the way from one side of the earth to the other television is also a form of communication television signals however are not reflected by the ionosphere instead they pass right through the ionosphere and are lost in space to send television messages from coast to coast a series of towers has been built across the country these towers are about 20 or 30 miles from each other we call these towers relay stations actually there are nearly a hundred towers across the United States messages can be relayed in straight lines from one to the other around the curve of the earth but relay stations like these would be difficult to build across oceans one way to send television messages across an ocean would be to place some kind of mirror or reflector in the sky if it were high enough signals might then be aimed at it to be bounced or reflected back to receiving stations on the other side of the ocean around the curve of the earth the first such reflector was a huge shiny plastic balloon called echo with a powerful rocket echo was launched into orbit around the earth these photographs actually taken from a rocket show how EKKO was released and inflated in space while circling the earth as a satellite echo acts as a reflector to bounce back signals aimed at it from ground stations it is called echo because signals striking its shiny surface are bounced or echoed back toward receivers on the ground because the signals echo reflects must travel such great distances they lose much of their strength by the time they again reach the earth they are too weak to be used for regular television broadcasts in order to get a stronger signal satellites have been developed which carry electronic equipment this equipment inside the satellite receives television signals from the earth it then strengthens or amplifies the signals and rebroadcasts these stronger signals back to earth the first satellite of this kind was called Telstar but because television signals travel in straight lines a satellite like echo or Telstar can receive signals only when it rises above the curve of the earth and is in line with a ground station the messages the satellite can receive and send are interrupted when it disappears or sets behind the curve of the earth to avoid these interruptions a series of satellites is needed each follows the other so an approaching satellite appears before the previous satellite is out of range the signal can be transferred from one satellite to the next interruption but is it possible to have continuous communication without using so many satellites the height or altitude of a satellite determines the time it will take to make one complete orbit around the Earth a low altitude satellite will go around the earth several times a day a high-altitude satellite will go around fewer times a day the higher a satellite is placed the longer it will take for each orbit a satellite placed a little more than 22,000 miles above the equator will take exactly 24 hours for each orbit since the earth itself rotates once every 24 hours a satellite in this position always stays above the same point on the Earth's surface such a satellite can receive and send back messages continuously over an area of about 1/3 the Earth's surface the first such high-altitude satellite to be developed was called simcom here are some of its delicate but high-powered equipment is made ready for installation rocketed into their correct orbits three such satellites can provide continuous communication almost anywhere in the world because light radio and television signals travel in straight lines the curve of the earth interferes with long-distance communication radio signals can be reflected up and down between the ionosphere and the Earth's surface but television signals pass through the ionosphere and are lost in space communication satellites like echo only reflect signals back to earth other low altitude satellites like Telstar contain electronic equipment to receive and strengthen signals before sending them back to earth a series of such satellites can give continuous communication or free high-altitude satellites can relay messages without interruption to almost any point on earth satellites may soon be the principal means of communication from continent to continent here on earth and they may someday even act as relay stations to receive and send messages to bases on distant planets [Music]