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Narrator Listen to part of a lecture about GPS. Now get ready to answer the questions. You may use your notes to help yo
Narrator Listen to part of a lecture about GPS. Now get ready to answer the questions. You may use your notes to help yo
admin
2013-04-25
21
问题
Narrator
Listen to part of a lecture about GPS.
Now get ready to answer the questions. You may use your notes to help you answer.
What can be inferred about the use of GPS nowadays?
What is GPS? Its full name is the Global Positioning System and a satellite-based navigation system. Generally speaking, it is made up of a network of 24 satellites placed into orbit by the U. S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. Nowadays, GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.
How does it work? At first, GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. Secondly, GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares two times, one is the time a signal was transmitted by a satellite; the other is the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position, here three dimensions are latitude, longitude and altitude. Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
How accurate is GPS? Today’s GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin’s 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin GPS receivers are accurate to within 15 meters on average.
How about the Newer Garmin GPS receivers? With WAAS, that is Wide Area Augmentation System, its capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS, which corrects GPS signals to within an average of three to five meters. The U. S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.
GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite keep them flying in the correct path.
Here are some other interesting facts about the GPS satellites, also called NAVSTAR by the official of U. S. Department of Defense. the first GPS satellite was launched in 1978; a full constellation of 24 satellites was achieved in 1994; each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit; A GPS satellite weighs approximately 2000 pounds and is about 17 feet across with the solar panels extended; transmitter power is only 50 watts or less.
选项
A、GPS can be used nearly 24 hours a day.
B、GPS can be used in certain places in the world.
C、GPS can be used in any weather conditions
D、GPS can be used with subscription or setup fees.
答案
C
解析
本题为内在关系题,要求考生能够把握要点之间的内在关系。题目问:关于目前GPS的使用可作如下哪一项推测?由原文第一段的最后两句“Nowadays, GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS”可知GPS可在任何天气、任何地方全天使用,并且不需要支付注册和安装费用。故选择C项,其他均为干扰项。
转载请注明原文地址:https://kaotiyun.com/show/mlyO777K
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