Global Positioning System 12820
Global Positioning System 12820

Global Positioning System 12820

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  • Pages: 8 (3709 words)
  • Published: November 23, 2018
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As we enter the 21st century, we are constantly being bombarded with new

technologies. From the wireless community to operations that once took weeks to

recover and now only take a day or so, our world will never be the same. Another

technology that is revolutionizing the world we live in is the Global

Positioning System or GPS. The first GPS satellite was called GPS Block I.

Launched in 1978, it was a developmental satellite. Another nine Block I

satellites were launched through 1988. GPS is the only system today able to show

you your exact position on the Earth anytime, in any weather, anywhere. There

are 24 GPS satellites in orbiting the world at 11,000 nautical miles above the

Earth. Ground stations located worldwide continuously monitor them. They

transmit signals that can be detected by anyone with a GPS receiver. Using the

receiver, you can determine your location with great precision. GPS is one of

history’s most exciting and revolutionary developments and new uses for it are

constantly being discovered. But before I go any farther it’s important to

understand a bit more about navigation. Since prehistoric times, people have

been trying to figure out a reliable way to tell where they are, to help guide

them to where they are going, and to get them back home again. Cavemen probably

used stones and twigs to mark a trail when they set out hunting for food. The

earliest mariners followed the coast closely to keep from getting lost. When

navigators first sailed into the open ocean, they discovered they could chart

their course by following the stars. The ancient Phoenicians used the North Star

to journey from Egypt and Crete. According to Homer, the goddess Athena told


Odysseus to “keep the Great Bear on his left” during his travels from

Calypso’s Island. Unfortunately for Odysseus and all the other mariners, the

stars are only visible at night – and only on clear nights. The next major

developments in the quest for the perfect method of navigation were the magnetic

compass and the sextant. The needle of a compass always points north, so it is

always possible to know in what direction you are going. The sextant uses

adjustable mirrors to measure the exact angle of the stars, moon, and sun above

the horizon. However, in the early days of its use, it was only possible to

determine latitude, the location on the Earth measured north and south, from the

sextant observations. Sailors were still unable to determine their longitude,

the location on the Earth measured east or west. This was such a serious problem

that in the 17th century, the British formed a special Board of Longitude

consisting of well-known scientists. This group offered ?20,000, equal to about

a million of todays dollars, to anybody who could find a way to determine a

ship’s longitude within 30 nautical miles. The generous offer paid off. In 1761,

a cabinetmaker named John Harrison developed a shipboard timepiece called a

chronometer, which lost or gained only about one second a day – incredibly

accurate for the time. For the next two centuries, sextants and chronometers

were used in combination to provide latitude and longitude information. In the

early 20th century several radio-based navigation systems were developed, which

were used widely during World War II. Both allied and enemy ships and airplanes

used ground-based radio-navigation system

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as the technology advanced. A few

ground-based radio-navigation systems are still in use today. One drawback of

using radio waves generated on the ground is that you must choose between a

system that is very accurate but doesn’t cover a wide area, or one that covers a

wide area but is not very accurate. High-frequency radio waves (like UHF TV) can

provide accurate position location but can only be picked up in a small,

localized area. Lower frequency radio waves (like AM radio) can cover a larger

area, but are not a good yardstick to tell you exactly where you are.

Scientists, therefore, decided that the only way to provide coverage for the

entire world was to place high-frequency radio transmitters in space. A

transmitter high above the Earth sending a high-frequency radio wave with a

special coded signal can cover a large area and still overcome much of the

“noise” encountered on the way to the ground. This is one of the main

principles behind the Global Positioning System. GPS has 3 parts: the space

segment, the user segment, and the control segment. The space segment consists

of 24 satellites, each in its own orbit 11,000 nautical miles above the Earth.

The user segment consists of receivers, which you can hold in your hand or mount

in your car. The control segment consists of ground stations. There are five of

them, located around the world that make sure the satellites are working

properly. One trip around the Earth in space equals one orbit. The GPS

satellites each take 12 hours to orbit the Earth. Each satellite is equipped

with an accurate clock to let it broadcast signals coupled with a precise time

message. The ground unit receives the satellite signal, which travels at the

speed of light. Even at this speed, the signal takes a measurable amount of time

to reach the receiver. The difference between the time the signal is sent and

the time it is received, multiplied by the speed of light, enables the receiver

to calculate the distance to the satellite. To measure precise latitude,

longitude, and altitude, the receiver measures the time it took for the signals

from four separate satellites to get to the receiver. The GPS system can tell

you your location anywhere on or above the Earth to within about 300 feet. Even

greater accuracy, usually less than three feet, can be obtained with corrections

calculated by a GPS receiver at a known fixed location. To help you understand

the GPS system, let’s take the three parts of the system – the satellites, the

receivers, and the ground control – and discuss them in more detail. Then we

will look more closely at how GPS works. As I said before, the complete GPS

space system includes 24 satellites, 11,000 nautical miles above the Earth,

which take 12 hours each to go around the Earth once (one orbit). They are

positioned so that we can receive signals from six of them nearly 100 percent of

the time at any point on Earth. You need that many signals to get the best

position information. Satellites are equipped with very precise clocks that keep

accurate time to within three nanoseconds – that’s 0.000000003, or three

billionths, of a second. This precision timing is important because the receiver

must determine exactly how long it takes for signals to

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