Friday, August 31, 2012

மாவீரன் அலெக்ஸாண்டரின் கடைசி ஆசைகள்......

மாவீரன் அலெக்ஸாண்டர் எல்லா நாடுகளையும் கைப்பற்றி விட்டு தாய்நாடு திரும்பும் வழியில் நோய்வாய்ப்பட்டார். பல மாதங்கள் ஆகியும் அவருக்கு அந்த நோய் தீரவில்லை. சாவு தன்னை நெருங்குவதை உணர்ந்தார் அவர்.

ஒருநாள் தன்னுடைய தலைமை வீரர்களை அழைத்து, "என்னுடைய சாவு நெருங்கி விட்டது. எனக்கு மூன்று ஆசைகள் உள்ளன. அவற்றை நீங்கள் கண்டிப்பாக நிறைவேற்றி வைக்க வேண்டும்" என்று கட்டளையிட்டார்.

அவர்களும் அவற்றை நிறைவேற்றுவதாக வாக்களித்தனர்.

முதல் விருப்பமாக, "என்னுடைய சவப்பெட்டியை எனக்கு சிகிச்சையளித்த மருத்துவர்கள் மட்டுமே தூக்கி வர வேண்டும்."

இரண்டாவது, 'என்னைப் புதைக்கும் இடத்திற்குச் செல்லும் வழியானது நான் சம்பாதித்து வைத்த, விலைமதிப்பற்ற கற்களால் அலங்காரம் செய்யப்பட வேண்டும்."

மூன்றாவதாக, "என் கைகள் இரண்டையும் சவப்பெட்டிக்கு வெளியில் தெரியுமாறு வைக்க வேண்டும்."

வீரர்களுக்கு இந்த ஆசைகள் வித்தியாசமாகத் தெரிந்தன. என்ன காரணமாக இருக்கும் என்று கேட்கப் பயந்தார்கள்.

அதில் ஒருவன் தைரியமாக முன் வந்து, "அரசே..! நாங்கள் தங்கள் ஆசையை கண்டிப்பாக நிறைவேற்றுகிறோம். ஆனால், இதற்கான காரணத்தை தாங்கள் விளக்க வேண்டும்" என்று கேட்க, அலெக்ஸாண்டர் அதற்கு விளக்கமளித்தார்.

1. என்னுடைய சவப்பெட்டியை மருத்துவர்கள் தூக்கிச் செல்வதால், உலகில் உள்ள எல்லோரும் ஒரு விஷயத்தை அறிந்து கொள்வார்கள். மருத்துவர்களால் எந்த ஒரு நோயிலிருந்தும் ஒரு உயிரை நிரந்தரமாகக் காப்பாற்ற முடியாது. மரணத்தை அவர்களால் நிறுத்த முடியாது.

2. வாழ்க்கையில் எவ்வளவு பணமும், நாடுகளும், இன்ன பிற செல்வங்களும் சம்பாதித்தாலும், அவற்றை உன்னுடன் எடுத்துச் செல்ல முடியாது. அது சவக்குழி வரை மட்டும் தான்..! மனிதர்கள் வீணாக அதன் பின்னால் செல்ல வேண்டாம் என்பதைத் தெரியப்படுத்துவதற்காக..!

3. உலகையே வென்றவன் சாகும் போது கைகளில் ஒன்றுமில்லாதவனாகத்தான் இருக்கிறான் என்று அறிந்து கொள்வதற்காக....!!!
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How Do Satellites Work?


Satellite Communication Basics
The basic elements of a satellite communications system are shown in Figure 1 below. The process begins at an earth station--an installation designed to transmit and receive signals from a satellite in orbit around the earth. Earth stations send information in the form of high powered, high frequency (GHz range) signals to satellites which receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. The area which receives a signal of useful strength from the satellite is known as the satellite's footprint. The transmission system from the earth station to the satellite is called the uplink, and the system from the satellite to the earth station is called the downlink.

Figure 1: Satellite Communication Basics
Satellite Frequency Bands
The three most commonly used satellite frequency bands are the C-band, Ku-band, and Ka-band. C-band and Ku-band are the two most common frequency spectrums used by today's satellites. To help understand the relationship between antenna diameter and transmission frequency, it is important to note that there is an inverse relationship between frequency and wavelength--when frequency increases, wavelength decreases. As wavelength increases, larger antennas (satellite dishes) are necessary to gather the signal.
C-band satellite transmissions occupy the 4 to 8 GHz frequency range. These relatively low frequencies translate to larger wavelengths than Ku-band or Ka-band. These larger wavelengths of the C-band mean that a larger satellite antenna is required to gather the minimum signal strength, and therefore the minimum size of an average C-band antenna is approximately 2-3 meters in diameter as shown in Figure 2.

Figure 2: C-band Satellite Antenna
Ku-band satellite transmissions occupy the 11 to 17 GHz frequency range. These relatively high frequency transmissions correspond to shorter wavelengths and therefore a smaller antenna can be used to receive the minimum signal strength. Ku-band antennas can be as small as 18 inches in diameter, as commonly seen in the RCA DSS and Sony DSS systems. Figure 3 below shows the Ku-band antenna of the Sony DSS system.

Figure 3: Ku-band Satellite Antenna
Ka-band satellite transmissions occupy the 20 to 30 GHz frequency range. These very high frequency transmissions mean very small wavelengths and very small diameter receiving antennas.
Geosynchronous Earth Orbit (GEO) Satellites
Today, the overwhelming majority of satellites in orbit around the earth are positioned at a point 22,238 miles above the earth's equator in a special type of geosynchronous earth orbit (GSO) known as Geostationary earth orbit (GEO), or what is sometimes called Clarke orbit. This is in honor of Arthur C. Clarke, the man who first suggested in 1945 that satellites in geosynchronous orbits could be used for communications purposes. As shown in Figure 4, it is at the precise distance of 22,238 miles that a satellite can maintain an orbit with a period of rotation around the earth exactly equal to 24 hours. Since the satellites revolve at the same rotational speed of the earth, they appear stationary from the earth’s surface. That's why most earth station antennas (satellite dishes) don't need to move once they have been properly aimed at a target satellite in the sky. The mathematical derivation of the Clarke orbit is a straight-forward calculus problem.(1)

Figure 4: The Clarke Orbit

Medium Earth Orbit (MEO) Satellites
During the last few years, technological innovations in space communications have given rise to new orbits and totally new systems designs. Medium earth orbit (MEO) satellite networks have been proposed that will orbit at distances of about 8000 miles. Signals transmitted from a MEO satellite travel a shorter distance which translates to improved signal strength at the receiving end. This means that smaller, more lightweight receiving terminals can be used. Also, since the signal is traveling a shorter distance to and from the satellite, there is less transmission delay. Transmission delay is the time it takes for a signal to travel up to a satellite and back down to a receiving station. For real-time communications, the shorter the transmission delay the better. For example, a GEO satellite requires .25 seconds for a round trip. A MEO satellite requires less than .1 seconds to complete the job. MEOs operate in the 2 GHz and above frequency range.

Low Earth Orbit (LEO) Satellites
Proposed LEO satellites are divided into three categories: little LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of only 500 to 1000 miles above the earth. This relatively short distance reduces transmission delay to only .05 seconds and further reduces the need for sensitive and bulky receiving equipment. Little LEOs will will operate in the 800 MHz (.8 GHz) range, big LEOs will operate in the 2 GHz or above range, and Mega-LEOs will operate in the 20-30 GHz range. The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and the capability of real-time, low delay video transmission. Microsoft Corporation and McCaw Cellular (now known as AT&T Wireless Services) have partnered to deploy 840 satellites to form Teledesic, a proposed Mega-LEO satellite network.

High Altitude Long Endurance (HALE) Platforms
Experimental HALE platforms are essentially highly efficient and lightweight airplanes carrying communications equipment that will act as very low earth orbit geosynchronous satellites. These crafts will be powered by high efficiency turbine engines or a combination of battery and solar power. At an altitude of only 70,000 feet, HALE platforms will offer transmission delays of less than .001 seconds and even better signal strength for very lightweight hand-held receiving devices.(2)

Orbital Slots
With more than 200 satellites up there in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space? To tackle that problem, international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit where communications satellites can be located. These locations are specified in degrees of longitude and are known as orbital slots. In response to the huge demand for orbital slots, the FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites.

 

Satellite Basics

  A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.
In addition to communications satellites, there are other types of satellites:
  • Weather satellites: These satellites provide meteorologists with scientific data to predict weather conditions and are equipped with advanced instruments
  • Earth observation satellites: These satellites allow scientists to gather valuable data about the earth's ecosystem
  • Navigation satellites: Using GPS technology these satellites are able to provide a person's exact location on Earth to within a few meters


How Satellites Work


Not so long ago, satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, and everyday telephone calls. In many other instances, satellites play a background role that escapes our notice:
  • Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
  • Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
  • The most reliable taxi and limousine drivers are sometimes using the satellite-based Global Positioning System (GPS) to take us to the proper destination.
  • The goods we buy often reach distributors and retailers more efficiently and safely because trucking firms track the progress of their vehicles with the same GPS. Sometimes firms will even tell their drivers that they are driving too fast.
  • Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal (read this page for details).
In this article, we will show you how satellites operate and what they do. You'll get to see what's inside a satellite, explore the different kinds of orbits and find out why the intended use of the satellite affects the choice of orbit. We'll even tell you how to see and track a satellite yourself!

What Is a Satellite?



A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth's original, natural satellite, and there are many man-made (artificial) satellites, usually closer to Earth.
  • The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.
  • Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).
  • Approximately 23,000 items of space junk -- objects large enough to track with radar that were inadvertently placed in orbit or have outlived their usefulness -- are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to the count. This online catalog of satellites has almost 26,000 entries!
Although anything that is in orbit around Earth is technically a satellite, the term "satellite" is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellites.

Whose Satellite Was the First to Orbit Earth?

The Soviet Sputnik satellite was the first to orbit Earth, launched on Oct. 4, 1957.
Because of Soviet government secrecy at the time, no photographs were taken of this famous launch. Sputnik was a 23-inch (58-centimeter), 184-pound (83-kilogram) metal ball. Although it was a remarkable achievement, Sputnik's contents seem meager by today's standards:
On the outside of Sputnik, four whip antennas transmitted on short-wave frequencies above and below what is today's Citizens Band (27 MHz). According to the Space Satellite Handbook, by Anthony R. Curtis:
After 92 days, gravity took over and Sputnik burned in Earth's atmosphere. Thirty days after the Sputnik launch, the dog Laika orbited in a half-ton Sputnik satellite with an air supply for the dog. It burned in the atmosphere in April 1958.
Sputnik is a good example of just how simple a satellite can be. As we will see later, today's satellites are generally far more complicated, but the basic idea is a straightforward one.

Personal Reflection: Sputnik, Oct. 4, 1957

Sputnik's transmissions died along with its battery after only three weeks, but its effects have been felt for decades. As a fifth-grader, I witnessed the stir caused by the launch of Sputnik. News reports showed that many people in the United States were embarrassed to see the Soviet Union achieving a scientific first, as well as frightened that a foreign country had placed something overhead (read these Sputnik stories). Soviet rocket development seemed well ahead of the United States' efforts. The push toward getting an American satellite into space started immediately. American schools and universities were soon stocked with new science books. One side effect that had a direct impact on many students like me was an increase in science homework, giving a personal dimension to the national wake-up call.

How Is a Satellite Launched into an Orbit?


 All satellites today get into orbit by riding on a rocket. Many used to hitch a ride in the cargo bay of the space shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into orbit regularly.
For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.
After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest.
How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth's circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 kilometers). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 kilometers). To travel around that circumference in 24 hours, a point on Earth's surface has to move at 1,038 mph (1,669 kph). A launch from Cape Canaveral, Florida, doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A, one of its launch facilities, is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth's rotational speed there is about 894 mph (1,440 kph). The difference in Earth's surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate -- fatter around the middle -- not a perfect sphere. For that reason, our estimate of Earth's circumference is a little small.)
Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the Feb. 11, 2000, lift-off of the space shuttle Endeavour with the Shuttle Radar Topography Mission required launching a total weight of 4,520,415 pounds (2,050,447 kilograms). It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.
Once the rocket reaches extremely thin air, at about 120 miles (193 kilometers) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.

Inertial Guidance Systems

A rocket must be controlled very precisely to insert a satellite into the desired orbit. An inertial guidance system (IGS) inside the rocket makes this control possible. The IGS determines a rocket's exact location and orientation by precisely measuring all of the accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals, the gyroscopes' axes stay pointing in the same direction. This gyroscopically stable platform contains accelerometers that measure changes in acceleration on three different axes. If it knows exactly where the rocket was at launch and it knows the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.

Orbital Velocity and Altitude

A rocket must accelerate to at least 25,039 mph (40,320 kph) to completely escape Earth's gravity and fly off into space (for more on escape velocity, visit this article at kidsplanet.com).
Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit. With satellites, the object is not to escape Earth's gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity's pull on the satellite and the inertia of the satellite's motion -- the satellite's tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 kilometers). Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line (read this page for details on orbits).
The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity. At an altitude of 124 miles (200 kilometers), the required orbital velocity is just over 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 kilometers) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permits the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called "geostationary." Geostationary orbits are ideal for weather satellites and communications satellites.
The moon has an altitude of about 240,000 miles (384,400 kilometers), a velocity of about 2,300 mph (3,700 kph) and its orbit takes 27.322 days. (Note that the moon's orbital velocity is slower because it is farther from Earth than artificial satellites.)
In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite can stay in orbit for centuries (take the moon as an example).
Satellites usually start out in an orbit that is elliptical. The ground control station controls small onboard rocket motors to provide correction. The goal is to get the orbit as circular as possible. By firing a rocket when the orbit is at the apogee of its orbit (its most distant point from Earth), and applying thrust in the direction of the flight path, the perigee (lowest point from Earth) moves farther out. The result is a more circular orbit.

What Is a Satellite Launch Window?

A launch window is a particular period of time in which it will be easier to place the satellite in the orbit necessary to perform its intended function.
With the space shuttle, an extremely important factor in choosing the launch window was the need to bring down the astronauts safely if something went wrong. The astronauts had to be able to reach a safe landing area with rescue personnel standing by. For other types of flights, including interplanetary exploration, the launch window must permit the flight to take the most efficient course to its very distant destination. If weather is bad or a malfunction occurs during a launch window, the flight must be postponed until the next launch window appropriate for the flight. If a satellite were launched at the wrong time of the day in perfect weather, the satellite could end up in an orbit that would not pass over any of its intended users. Timing is everything!

What Is Inside a Typical Satellite?

Satellites come in all shapes and sizes and play a variety of roles. For example:
  • Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.
  • Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
  • Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
  • Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.
  • Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.

  • Rescue satellites respond to radio distress signals (read this page for details).
  • Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
  • Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include relaying encrypted communication, nuclear monitoring, observing enemy movements, early warning of missile launches, eavesdropping on terrestrial radio links, radar imaging, photography (using what are essentially large telescopes that take pictures of militarily interesting areas)
Despite the significant differences between all of these satellites, they have several things in common. For example:
  • All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch.
  • All of them have a source of power (usually solar cells) and batteries for storage. Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets (read this page for details). Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.
  • All of them have an onboard computer to control and monitor the different systems.
  • All of them have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
  • All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.
The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.

What Are the Types of Satellite Orbits?

There are three basic kinds of orbits, depending on the satellite's position relative to Earth's surface:
  1. Geostationary orbits (also called geosynchronous or synchronous) are orbits in which the satellite is always positioned over the same spot on Earth. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The "satellite parking strip" area over the equator is becoming congested with several hundred television, weather and communication satellites! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite's signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.
  2. The scheduled space shuttles used a much lower, asynchronous orbit, which means they passed overhead at different times of the day. Other satellites in asynchronous orbits average about 400 miles (644 kilometers) in altitude.
  3. In a polar orbit, the satellite generally flies at a low altitude and passes over the planet's poles on each revolution. The polar orbit remains fixed in space as Earth rotates inside the orbit. As a result, much of Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography.

How Are Satellite Orbits Predicted?

Special satellite software, available for personal computers, predicts satellite orbits. The software uses Keplerian data to forecast each orbit and shows when a satellite will be overhead. The latest "Keps" are available on the Internet for amateur radio satellites, too.
Satellites use a variety of light-sensitive sensors to determine their position. The satellite transmits its position to the ground station.

Satellite Altitudes


 Looking up from Earth, satellites are orbiting overhead in various bands of altitude. It's interesting to think of satellites in terms of how near or far they are from us. Proceeding roughly from the nearest to the farthest, here are the types of satellites whizzing around Earth:

80 to 1,200 miles -- Asynchronous Orbits

Observation satellites, typically orbiting at altitudes from 300 to 600 miles (480 to 970 kilometers), are used for tasks like photography. Observation satellites such as the Landsat 7 perform tasks such as:
  • Mapping
  • Ice and sand movement
  • Locating environmental situations (such as disappearing rainforests)
  • Locating mineral deposits
  • Finding crop problems
Search-and-rescue satellites act as relay stations to rebroadcast emergency radio-beacon signals from a downed aircraft or ship in trouble.
The space shuttle was the familiar manned satellite, usually with a fixed duration and number of orbits. Manned missions often have the task of repairing existing expensive satellites or building future space stations.

The "glass cockpit" on space shuttle Atlantis, March 1, 2000
Photo courtesy NASA
Teledesic, with the financial backing of Bill Gates, promises broadband (high-speed) communications using many planned low Earth orbiting (LEO) satellites.

3,000 to 6,000 miles -- Asynchronous Orbits

Science satellites are sometimes in altitudes of 3,000 to 6,000 miles (4,800 to 9,700 kilometers). They send their research data to Earth via radio telemetry signals. Scientific satellite applications include:

6,000 to 12,000 miles - Asynchronous Orbits

For navigation, the U.S. Department of Defense built the Global Positioning System, or GPS. The GPS uses satellites at altitudes of 6,000 to 12,000 miles to determine the exact location of the receiver. The GPS receiver may be located:
As consumer prices for GPS receivers come down, the familiar paper map may face tough competition. No more getting lost leaving the rental car agency at an unfamiliar airport!

Taken while the Clementine spacecraft was orbiting the moon
Photo courtesy NASA

Advanced Communications Technology Satellite, launched in 1993, used multiple antennas for narrow-beam transmissions.
Photo courtesy NASA

22,223 Miles - Geostationary Orbits

Weather forecasts visually bombard us each day with images from weather satellites, typically 22,223 miles over the equator. You can directly receive many of the actual satellite images using radio receivers and special personal-computer software. Many countries use weather satellites for their weather forecasting and storm observations.
Data, television, image and some telephone transmissions are routinely received and rebroadcast by communications satellites. Typical satellite telephone links have 550 to 650 milliseconds of round-trip delay that contribute to consumer dissatisfaction with this type of long-distance carrier. It takes the voice communications that long to travel all the way up to the satellite and back to Earth. The round-trip delay forces many to use telephone conversations via satellite only when no other links exist. Currently, voice over the Internet is experiencing a similar delay problem, but in this case due to digital compression and bandwidth limitations rather than distance.
Communications satellites are essentially radio relay stations in space. Satellite dishes get smaller as satellites get more powerful transmitters with focused radio "footprints" and gain-type antennas. Subcarriers on these same satellites carry:
  • Press agency news feeds
  • Stock market, business and other financial information
  • International radio broadcasters moving from short-wave to (or supplementing their short-wave broadcasts with) satellite feeds using microwave uplink feeds
  • Global television, such as CNN and the BBC
  • Digital radio for CD-quality audio

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