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Appleton Sales In November 1982, a U.S. weather satellite spotted a The 1 25-mile-per-hour winds of Hurricane Hugo begin to lash the coast of the southeastern United States under the constant gage of a U.S. weather satellite. Satellite images warned residents of Charleston (upper left, of the approaching Hugo, still 210 miles away in this picture, giving them several days to prepare for the storm and evacuate the city. 0 :~\~\f~t Q1 ~dWdll ~1IU W~11~U it start moving north. SucIdenly the storm turned east and raked across the islands less than 24 hours later, leaving nearly a quarter billion dollars of damage in its wake. But only one life was lost. Hurri- cane warnings, based largely on satellite pictures, had alerted most people in time to take sheller. Twenty-five years ago, there were no operational satellites for weather forecasting or for any other routine application. Today, however, satellite systems girdle the earth watching for storms, relaying communica- tions, mapping uncharted terrain, and helping ships and airplanes navigate any- where on the globe. I Satellites started to become routine in April 1965, when the first nonexperimental commercial satellite flew into space and parked in orbit over the Atlantic. Early Bird carried circuits for 240 telephone calls or television service between Europe and North America. Today, several generations of satellites later, Early Bird's descendants handle up to 120,000 calls at once or share the capacity with TV broadcasts, computer data, and electronic mail. They are part of a worldwide communications network launched by the International Telecommuni- cations Satellite Organization, or Intelsat, which now has more than 100 member nations. Communications satellites have dramat ically altered the nature of our global com- I munity. Twenty-five years ago, reliable communications existed only between countries in the developed world. But communications satellites and the develop ment of small, portable television cameras have linked people together around the globe. In 1969, for example, communications satellites brought TV coverage of the first manned lunar landing live from space to nearly a fifth of the world's population. Today they bring instant coverage of news and events to every corner of the earth. Scores of civilian and military communi- cations satellites sit in a ring around the earth 22,300 miles above the equator. At that altitude they complete one revolution every 24 hours and therefore remain stationary relative to the surface of the earth below. Satellites in this geostationary orbit provide high sky platforms for relaying signals to and from ground stations, vehicles, boats, and planes scattered over the surface below. The idea for a system of radio-relay satellites in geostationary orbit maintained by astronauts was described by science fiction writer and futurist Arthur C. Clarke E N G I N E E R I N G A N D T H E A D VA N C E M E N T O F H U M A N W E L F A R E

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In 1945. Astronaut custodians, however, were impractical and, when Early Bird went up In the 1960s, shown to be us necessary. Transis- tors and other solid-state components of the satellite's Innards were far more reliable than the vacuum tubes of 1945-v~ntage radios and small enough to fit into a tiny satellite. Early Bird lasted more than three years without maintenance, and todays communications satellites frequently last until their fuel runs out, about a decade. A country divided by mountains, jungles, water, or distance is now often united by a domestic satellite commun~ca- tions system. In 1965 the Soviet Union established the Molniya system using satellites In oblong orbits looping over the northern latitudes. These satellites were not geostationary. However, one always rose above the country just as another set. Now more than 40 countries have established their own satellite communications systems, often by constructing ground stations and renting circuits on commercial satellites. Before the era of routine satellite use, weather observations were available for less than 20 percent of the globe. Much of the unobserved region covered the vast oceans of the Southern Hemisphere and the tropics, where a storm could churn into a hurricane with no one knowing until it sank a ship or washed away a town. But since 1966, when the first fully operational weather satellite A P P L I C A T I O N S A ~ E L ~ I T E S I An astronaut retrieves a communications satellite, Westar ill, that failed to go into high geostationary orbit after being launched into low earth orbit. The space shuttle brought the satellite back to earth, where it was refurbished. Pending U.S. government approval, the satellite will be launched aboard a Chinese rocket and will provide # communications link for East Asia. Communications satellites relayed live television coverage of moon missions to millions of people around the world. Here, Apollo ~ 5 astronauts collect rock I samples. 1 1

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logo dish antennas near Bogota, Colombia, aim at a communications satellite in geostationary orbit above the equator. 2 system was launched, no major tropical storm has gone undetected anywhere in the world. Two U.S. weather satellites circling from pole to pole scan the earth from low orbit twice a day. Soviet satellites also routinely survey global weather from polar orbit. High-flying satellites in geostationary orbit monitor wide reaches of the planet and generate the weather pictures seen on television. The United States launched the first such operational satellite, GOES (Geostationary Operational Environmental Satellite), in 1974 and now maintains one above the western Atlantic and another over the eastern Pacific. A European satellite hovers over Africa, an Indian satellite over the Indian Ocean, and a Japanese satellite over the western Pacific. These five provide continuous coverage of weather throughout the equatorial and midlatitude regions of the earth. Radiometers carried by weather satel- lites measure reflected solar energy and infrared energy radiating from the earth and atmosphere. Computers then turn the data into images of clouds and weather patterns. Radiometers also measure radiation at precise wavelength intervals, generating information that allows scientists to deter- mine atmospheric temperature, water vapor, and ozone levels. Today's radiometers are sensitive enough to sound the atmosphere from geostationary satellites more than 22,000 miles up in space. Weather satellites frequently carry equipment to relay data collected by thou- sands of remote automatic ground stations from instruments such as rain or river-flow gauges. And, since 1982, increasing numbers of U.S. weather satellites, as well as Soviet navigation satellites, have also been carrying search-and-rescue radio equipment, some of it supplied by Canada and France. The devices pick up distress calls from ships and planes in trouble, leading to the rescue so far of more than 1,200 persons. Images generated by weather satellites have been used to study snow cover, geolog- ic faults, and other large ground features. But the need for more detailed information gathered in a systematic, repetitive way led to the development of specialized terrain- observation satellites. These orbiters create high-quality images that can be used to help study worldwide crop production, search for minerals and petroleum, monitor environ- mental problems such as desert creep and deforestation, and measure snowmelt to predict water resources and control flooding. In 1972 the United States launched the first in a series of Landsat satellites, which scrutinize the entire earth from near-polar orbit, observing each spot at the same local time every 18 days. All Landsats have carried multispectral sensors that measure solar energy and infrared radiation at four or more wave- lengths. Every type of terrain reflects and absorbs sunlight and emits infrared radiation in ways that reveal its identity and condition. Objects often show up better at one wave- length than another, which gives this technique an advantage over conventional photography. The data are transmitted back to one of several ground stations, processed into images, and made available to the public. France and the Soviet Union have launched similar satellites and make their images available commercially. The French I SPOT system produces particularly high quality images. Infrared satellite images do not show as I much detail as visible images, but they yield thermal information that the others cannot provide. Water, for example, that collects in faults usually holds more heat than sur- rounding rock. Infrared sensors detect this heat differential, revealing an otherwise hidden geologic formation. Infrared imaging is also useful for monitoring pollution, observing volcanic activity, and leading I E N G I N E E R I N G A N D T H E a D VA N C E M E N T O F H U M A N W E L FA ~ E

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~ fishing boats to warm waters where fish I might be gathered. More recently, experimental terrain- observation satellites have been carrying microwave radar, which penetrates all types of weather, day or night. Microwave radar i can, to a certain extent, penetrate leaves and is used to map jungle terrain. It can also penetrate dry sand and has been used to trace buried watercourses in the desert. Radar return signals also tell much about the land that is useful for mineral and petroleum exploration. The military forces of the United States, the Soviet Union, and other countries use their own terrain-observation satellites to keep track of foreign troop movements, electronic communications, military installa- tions, and rocket launchings. The satellites are hardier than their civilian counterparts, as well as more sophisticated, capable, and flexible. These "eyes in the sky" have greatly reduced the possibility for surprise attack and have thus helped maintain peace among world powers for the past quarter century. In the future, satellites may be used to monitor the health of the entire earth bio- sphere. A major step in that direction will be taken as part of the International Space Year I in 1992. A "Mission to Planet Earth" is being organized using earth-observation satellites and other means to study such problem i areas as polar ozone holes, deforestation, ocean productivity, landcover change, and global warming caused by the "greenhouse effect." The precision of their orbits makes satellites ideal instruments for navigation. They have all but replaced the sextant since about 1968, when the Navy completed a system of navigation satellites, called Transit, for ballistic missiles fired from submarines. Transit employs satellites in near-polar orbits that are spaced to provide global coverage. I Using data encoded on satellite signals, a computer-aided receiver on a ship or plane can determine its location to within less than I a quarter mile. Several satellites have already been launched for a new navigation system the Global Positioning System (GPS), or Navstar that will have 18 satellites plus 3 spares when completed. Using GPS, receivers in ships, planes, ground vehicles, and portable packs will be able to determine their locations to within 10 meters. The heart of the system is an atomic clock in each satellite that loses or gains just one second every 33,000 years. A plane, for example, receives signals from four satellites, each signal telling the time it was emitted and the position of the satellite. The plane's comput- erized receiver calculates how far each signal has traveled, then uses this data to pinpoint its position in space. In 1988 a satellite was launched to track long-haul trucks in the United States. A freight company using the system immedi- ately found one of its trucks 300 miles off course. More advanced systems used by some military aircraft show pilots a map of the local area. Cars may someday be equipped to receive satellite signals that provide drivers with a street map of the city being traveled. Indeed, satellites may become our standard means of navigation, whether on land, in the air, or at sea. A P P ~ ~ ~ AT g O N 5 AT E ~ ~ ~ ~ E 5 I Satellite image reveals extensive clear cutting in Tongass National Forest on the northern tip of Prince of Wales Island, Alaska. Recently cleared areas appear pink, cleared areas that have started to recover are light green, and undisturbed forests are dark green. Technicians prepare a Nawstar navigation satellite for launch. The 12 spike antennas will form the satellite's signal into a cone covering the entire hemisphere below. 13