EXPLORATIONS - Global Positioning System
时间：2006-03-06 16:00:00
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EXPLORATIONS - Global Positioning System
By Paul Thompson
Broadcast: Wednesday, May 05, 2004
(MUSIC)
VOICE ONE:
This is Steve Ember.
VOICE TWO:
And this is Phoebe Zimmermann with the VOA Special English program, EXPLORATIONS. Today we tell about a device1 you can hold in your hand. It permits you to find your way across mountains, through deserts and across oceans without ever getting lost.
(MUSIC)
VOICE ONE:
Captain Josiah Cressy
Let us begin back on June twenty-seventh, Eighteen-Fifty-One. We are on the sailing ship, "Flying Cloud," in the Atlantic Ocean. The Flying Cloud is under the command of Captain Josiah Cressy.
For many days now, Captain Cressy has made the huge ship travel at speeds that were not thought possible. The crew is frightened by the speed. No ship in history has ever traveled this fast. The crew also is frightened because the ship is sailing in some of the most dangerous waters in the world.
The Flying Cloud is near Cape2 Horn at the end of the South American continent. The weather has been bad for several days. The person responsible for guiding the ship must be able to see the Sun or a star to know the position of the ship. The stormy weather has prevented this for several days.
VOICE TWO:
The Flying Cloud
The person who plans the directions for a sailing trip is usually the ship's captain. On the Flying Cloud, however, the captain's wife does this job.
Her name is Eleanor Cressy. She is famous as an expert navigator. She has planned this trip through the dangerous waters near Cape Horn. Many underwater rocks are found here. To guide the ship safely, she must know where the ship is at all times.
Eleanor Cressy must use a method called "Dead Reckoning3" because she has not been able to see the Sun or stars to find the ship's true position. Dead Reckoning is extremely difficult. It is part science, and part estimate4.
To find the position of the Flying Cloud, Missus Cressy must use the last known position of the ship. She also has to consider the ship's direction, its speed and the movement of waves or the ocean current.
Here, near Cape Horn, she is permitted no mistakes. Many ships have sunk in this part of the world. Hour after hour, Eleanor Cressy uses mathematics to find the ship's Dead Reckoning position. She does this again and again, carefully searching for mistakes. The lives of the crew and the future of the ship are her responsibility.
VOICE ONE:
Eleanor Cressy demonstrates5 her great skill at navigation6 the next day. On the morning of June twenty-ninth, Captain Cressy can see Cape Horn, just eight kilometers to the north. The ship is exactly where Eleanor said it should be. It is safe and will continue on to San Francisco, California, faster than any sailing ship ever.
(MUSIC)
VOICE TWO:
Eleanor Cressy's job in Eighteen-Fifty-One was important and extremely difficult. The job of navigator did not change much until the middle of the Twentieth Century. At sea or on land, finding7 the correct direction to travel has always been a problem.
However, within the past several years, the problem of navigation has greatly changed. Dead Reckoning navigation is a thing of the ancient past. Now, people do not need the skills of Eleanor Cressy to navigate8. They can use a simple device that will permit them to navigate anywhere in the world. It uses a technology called "Global Positioning System," or G-P-S.
VOICE ONE:
Artist Picture of a GPS Satellite
G-P-S is able to show your exact position on Earth. Weather does not affect the device. Many G-P-S devices9 can be held in the hand. Some are larger, and meant to be placed in ships, automobiles10, trucks, airplanes or other aircraft.
Whatever the size, the device works11 much the same way. The G-P-S device is a radio receiver. It receives information from twenty-four satellites in orbit12 around the Earth. The satellites are placed so that a G-P-S device on the surface of the Earth can receive information from at least four of the satellites at any time.
VOICE TWO:
A satellite sends information, including the exact time at which it is operating. It also sends information about the position of other satellites.
This information travels from the satellite to the G-P-S device at almost the speed of light. But the satellite is far enough away to permit the device to measure the distance.
The G-P-S device uses the time the information was sent to find its distance from the satellite. The device measures the exact distance to four satellites to establish its position on Earth. The G-P-S device can do this second by second, minute by minute, day after day and arrive an the correct answer all the time.
You can place the device in a ship, a car or other moving vehicle. Then you can watch the position information change as the vehicle moves. In fact, the G-P-S device will give both your direction and the speed you are traveling.
VOICE ONE:
The United States government owns the twenty-four satellites that provide the information for G-P-S. The Department of Defense13 controls the satellites.
The first G-P-S satellite system was called NAVSTAR. It was launched14 in February, Nineteen-Seventy-Eight. The NAVSTAR satellites were created to provide extremely correct navigation information to American military ships and aircraft.
A few years later, President Ronald Reagan signed a document that permitted information from NAVSTAR to be used by anyone. He did this after a Korean Air Lines flight was lost. The Korean airplane had flown by mistake into airspace over the Soviet15 Union in nineteen-eighty-three. It was shot down by Soviet military aircraft. President Reagan said the American satellite navigation system would help prevent such accidents in the future.
It costs nothing to use the satellite information. All you need is a G-P-S device to receive the information.
The least costly16 G-P-S devices sell for about one-hundred dollars. The smallest devices can be held in the hand. Devices for aircraft or ships are larger. They may cost several thousands of dollars.
VOICE TWO:
After President Reagan permitted the public use of American navigation information, several electronics17 companies began making the G-P-S devices. However, there was a problem. The Defense Department would not let the satellites send the exact information to the public. Defense officials made sure the satellites sent information with mistakes. This was done so the information could not be used by military forces of any future enemy.
In May of two-thousand, the United States announced that such a security18 measure was no longer needed. The government turned off the equipment preventing satellites from providing the correct information.
Today, a G-P-S device that receives information from four satellites at the same time will show your correct position on Earth to within ten meters. It can also tell you the speed of your vehicle, the direction you are traveling, how far you have traveled, and the distance remaining until the end of your trip.
(MUSIC)
VOICE ONE:
G-P-S devices are popular in the United States. Many companies make and sell them. Most of the devices come with a receiver that looks like a small television. The less costly G-P-S devices provide information in black and white. More costly ones provide the information in color.
Many G-P-S devices can be linked to computers. The computers place information into the device including maps of city streets and major roads between cities.
The user19 of the device enters information using the controls. The G-P-S provides information about the direction of travel and tells how to get where you want to go. The device will correctly guide you from road to road and street to street, warning before a left and right turn must be made. The device also remembers where it has been. So it can guide the G-P-S user home again.
Recently, some companies started producing G-P-S devices that speak the directions. A person driving a vehicle does not have to look at the device for information. A person only has to listen. Here is an example. This G-P-S device is from the Garmin Company. Listen as the voice gives directions.
(SOUND)
VOICE TWO:
As you can see, the new G-P-S devices can help anyone get to where they want to go. And, they can do this just as well as the famous Eleanor Cressy did one-hundred-fifty years ago.
(MUSIC)
VOICE ONE:
This VOA Special English program was written by Paul Thompson and produced by Caty Weaver20. This is Steve Ember.
VOICE TWO:
And this is Phoebe Zimmermann. Join us again next week for another EXPLORATIONS program on the Voice of America.
(MUSIC)
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