Satellite

“ Space had become an adventure, not just for lonely astronauts, but for everyone right in his living room.
Project Telstar, of course, had more serious objectives:
to prove that a broadband communications satellite could transmit telephone messages, data, and television; to test, under the stresses of an actual launch and the hazards of space, some of the electronic equipment that had been developed for satellite communications; to measure the radiation that a satellite would meet in space; to find out the best ways to track a moving satellite accurately ”

“ And, in 1962, Project Telstar demonstrated to the whole world that an active repeater satellite could send telephone calls, data, and television across the ocean.
Bringing satellite communications almost to reality has required more than putting a man-made satellite into orbit around the earth. Just as important have been the invention and development of many remarkable new devices: the transistor, the solar cell, the traveling wave tube, the horn-reflector antenna, the waveguide, the solid-state maser, and the electronic computer—to mention only some of the more important. ”

“ When the satellite is in the opposite direction from the sun, the same action tends to draw the primary away from the satellite. In both cases, relative to the primary, the action is the same. When the satellite is in quadrature the convergence of the lines of attraction toward the centre of the sun tends to bring the two bodies together. When the orbit of the satellite is inclined to that of the primary planet round the sun, the action brings about a change in the plane of the orbit represented by a rotation round an axis perpendicular to the plane of the orbit of the primary. ”

“ The great advantage of the satellite was its ability to provide a continuous flow of information for long periods of time. The first science satellites were the forerunners of later vehicles that would demonstrate the direct benefits that satellites could offer to such varied fields as weather observation and communication. ”

“ We collect on the ground most of the information to calculate a satellite’s orbit, using optical instruments or radar equipment. Following a satellite through the sky is called tracking; in the early days after the first Sputniks, some of this tracking was done with the naked eye or with very simple telescopes by the Moonwatch teams. Many of you may have observed Echo I on a clear night without any kind of instrument. ”

“ Satellite I, situated at a mean distance of 261,000 miles from Jupiter's center—about 22,000 miles farther than the moon is from the earth—is urged by its master's overpowering attraction to a speed of 320 miles per minute, so that it performs a complete revolution in about forty-two hours and a half. ”

“ They tell us how fast we must move a precision tracker to follow the satellite through the sky, how much time a satellite will spend above the horizon, and how long will be the time from one chance of seeing it to the next. However, in the Echo project we were not merely concerned with planning our experiments from hour to hour; we also needed to know how the satellite would move for weeks and perhaps months in advance. When you study the motion of a satellite over such a length of time, you discover that its circular orbit will not remain the same as it was at launch. ”

“ The rings may be held to be formed of a multitude of tiny satellites, travelling nearly in one plane, each pursuing its own course around Saturn, according to the laws of satellite motion, though of course disturbed by the attraction of its fellow-satellites. ”

“ The many complications of satellite communications can be divided into several groups. First of all, there are the problems involved in fitting satellite communications into an already established world communications system. There are, next, many problems, both small and large, in designing the right kind of satellite. There are the problems of launching a satellite and getting it into the proper orbit. There are the problems in making sure it stays in the right orbit once it gets there. ”

“ The planet would, in fact, always bear the same face towards an observer situated on the satellite, just as the moon always turns the same face to the earth. It is, however, certain that the new satellite is somewhat outside this critical position. ”

“ Medium Earth Orbit (MEO) is at altitudes around 20,000 km, within a broad range. This is the regime of global navigation satellite systems such as GPS and GLONASS, but also recently of some internet communications systems such as O3B. Satellites in MEO take around 12 hours to orbit the Earth, and each satellite can communicate with a fairly large portion of the Earth’s surface. From the perspective of a person on Earth, a spacecraft in MEO will move across the sky much more slowly, and remain in the field of view for an hour. ”

“ The population of Earth-orbiting satellites is dramatically increasing with the advent of commercial satellite constellations that form global consumer communication networks. The impact of these satellite constellations on astronomy and the night sky depends strongly on the brightness of their constituent satellites, which is a complex function of time, attitude, orbital position, and wavelength. ”

“ Those who have good telescopes will think on the fortune which might conceivably have smiled on them, if perchance the satellite had presented itself on one of those rare occasions when it might have come within the reach of instruments less powerful than that by which it was actually discovered. The mathematical astronomers to whom the problem of Jupiter has ever been an attractive though a very difficult subject will find that the new satellite imparts an entirely fresh aspect to the question. ”

“ There could be no return to any kind of organized society while men had to scavenge for wood to warm themselves and to run their primitive machines.
Then someone had remembered the satellite.
It had been designed, before the war, to collect solar energy and beam it to a receiving station on Earth. The satellite packages had been fired into a 24-hour orbit, circling the Earth over a fixed point on the Equator. The receiving station, built on the southeastern coast of the United States, saw the five units as a single second-magnitude star, low on the horizon all year, every year. ”

“ Further in the future, studies are going on of a proposed Intermediate Altitude Communications Satellite for military use in the 6,000- to 10,000-mile-high range (beyond that of Telstar and Relay) and Advanced Syncom, a synchronous satellite of increased capacity. Work is also continuing to acquire new technical knowledge that will be needed in the future—such as various methods of keeping satellites stabilized in space and new ways of supplying power, including improved solar cells and the use of radioisotopes. ”

“ Yet if a planet is fortunate enough to have one satellite or more, the astronomer's method of weighing the planet is exceedingly simple; and all the major planets have satellites except the two interior ones, Mercury and Venus. As the satellite travels round its primary, just as the moon does round the earth, two elements of its orbit need to be ascertained, and only two. ”

“ In 1945, of course, the very idea of getting a satellite out into space seemed utterly fantastic, and satellite communications could only be classified as science fiction. Ten years later, although Sputnik I had not yet been launched, artificial satellites were close to reality. At that time, John R. Pierce of Bell Telephone Laboratories made 14 the first serious study of what would have to be done to build a working satellite communication system—assuming it ever became possible to put satellites into orbit. And Bell Laboratories has been interested in satellite communications ever since. ”

“ The advent of the earth satellite provided scientists with a new and valuable research tool. Science satellites have been used for such tasks as solar and astronomical observations, biology experiments, or atmospheric investigation. ”

“ Terrestrial cloud cover and weather conditions are obtained from one of the IESIS geostationary satellites and navigational data are transmitted from a global positioning system already in place. Each satellite adjusts its attitude as required, turns detectors on and off, modifies sensor resolution, and takes both active and passive observations of the Earth. Data then are processed by comparison to predicted observables as derived from the world model. ”

“ Launching and Tracking Problems How big a rocket booster would be needed? From what part of the world should a satellite be launched? What kind of orbit should it go into? (See table below) How far up should the satellite go? How can a satellite be tracked once it is in orbit? How do we predict future orbits of a satellite? Orientation and Control Problems How can orders be given to a satellite while it is in orbit? If it is to stay in a fixed attitude, how can it be kept there? What can be done to keep a satellite properly stabilized? Can optical measurements be made on a satellite? ”

“ Communications satellites can be grouped into two broad categories. Passive vehicles reflect signals from one ground station to another. Active satellites accept ground signals and either amplify and rebroadcast them immediately or record messages for later transmission.
The Echo satellite balloons typified the passive category of communications spacecraft. These satellites “bounced” radio signals from one ground station to another. Uninflated Echo payloads were carried into orbit packed in special storage containers. ”

“ The satellite will come up to the margin of the planet, looking like a brilliant little bead of light as it joins itself to it (a particularly exquisite sight) , will glide across the margin, and after a longer or shorter period will become invisible, being merged in the greater brightness of the central portions of Jupiter's disc, unless it should happen to traverse one of the dark belts, in which case it may be visible throughout its entire journey. ”

“ Each satellite transmits signals on two radio frequencies, superimposed on which are navigation and system data. Included in this data are predicted satellite ephemeris, atmospheric propagation correction data, satellite clock error information, and satellite health data. This segment normally consists of 21 operational satellites with three satellites orbiting as active spares. The satellites orbit at an altitude of 20,200 km, in six separate orbital planes, each plane inclined 55° relative to the equator. The satellites complete an orbit approximately once every 12 hours. ”

“ Captain Eaton said, “especially for us, since we usually cruise in that area above earth called the ‘satellite zone.’ For this reason, every person on pilot duty is responsible for knowing the position of every satellite within dangerous range of the Carefree. This requires constant study and figuring of orbit paths. It really is the biggest job the pilot has to do, because generally the Carefree is on automatic pilot and runs itself, you might say.” ”

“ To these determinations respecting the orbits of the satellites, Dr. Herschel adds his estimate of the distances to which they must move from the body of the planet before they become visible by his 20-feet telescope. The first requires to be at more than half its greatest elongation. The second also becomes invisible when it is within half its greatest elongation. If there be an interior satellite, as large as the first, he imagines it would be visible through so small a part of its orbit, as not to be seen for two nights together. ”

“ But of course, if the planet has an atmosphere thousands of miles deep, laden more or less heavily with cloud-masses, it might quite readily happen that a satellite should be seen apparently through the planet, — not, of course, through the middle of the planet, but through parts lying thousands of miles within the apparent outline. ”

“ The shadow of the new satellite on the globe of the planet, though no doubt it would be an extremely small point, would still nevertheless be intensely black in comparison with the surrounding surface, and therefore it might be expected that it ought to be comparatively easy to see when sufficient optical power was available. ”

“ Thus, it is most efficient if the onboard world model simply consists of descriptions of those niches the IESIS satellite (s) will encounter along an orbital swath on the next revolution. The model is uploaded from the ground prior to each pass. If swath width in kilometers is W, repeat time (in days) for the satellite to cover the entire equatorial
Figure 2.12,- Ground-based world model data processing operations. ”

“ The first or nearest Satellite to Jupiter is the most advantageous for this purpose, because it’s motion is quicker than the motion of any of the rest, and therefore it’s immersions and emersions are more frequent. ”

“ With such a glass more details are visible in the dark belts and along the bright equatorial girdle than can be correctly represented in a sketch before the rotation of the planet has altered their aspect, while the shadows of the satellites thrown upon the broad disk, and the satellites themselves when in transit, can be seen sometimes with exquisite clearness. ”

“ It seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet Mercury.
Next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of Satellites. The name satellite is derived from a Latin word signifying an attendant; for the bodies so-called move along always in close proximity to their respective "primaries," as the planets which they accompany are technically termed. ”