The Control Segment consists of a system of tracking stations located around the world.
The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. The models compute precise orbital data and SV clock corrections for each satellite. The Master Control station uploads orbital data and clock data to the SVs. The SVs then send subsets of the orbital ephemeris (星历表) data to GPS receivers over radio signals. User Segment
The GPS User Segment consists of the GPS receivers and the user community. GPS receivers change SV signals into position, speed, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time distribution, and other research. Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals.
Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies are examples.
Time and frequency distribution, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS. Astronomical observatories, telecommunications facilities, and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers. Research projects have used GPS signals to measure atmospheric parameters. GPS Accuracy
At present the system consists of 24 satellites at an altitude of about 20,000 km having an orbital inclination of 55 degrees. The orbits are almost circular and it takes 12 hours for a satellite to complete a pass around the Earth. GPS signals are broadcast from a cluster of 24 or more earth orbiting satellites. Because the GPS signals are derived from the atomic frequency standards on board each satellite, they are widely used as a reference for time synchronization and frequency adjustment. The real time positioning accuracy of a single receiver is normally up to 100 meters horizontally and 150 meters vertically. However, various methods have been developed which enable much higher accuracy (centimeter level). GPS Receivers
There are a variety of different types of GPS receivers on the market for commercial and public use. Prices range from $500-$30,000, reflecting the accuracy and capabilities of the instruments. For the general outdoorsman, a good GPS receiver should have 8 satellite tracking capability and be capable of receiving the GPS satellite signals through forest covering in northern Ontario shield area; for the professional user, a minimum 8 satellite tracking capability, high memory capacity, differential GPS capability, and resistance to signal weakening under forest covering is essential; for the professional surveyor requiring high level precision and accuracy capability, they should assess the project or application for which the technology is to be used with the help of an unbiased consultant, in order to determine the most cost effective and appropriate instrument. Navigational Units
Small hand held units at relatively low cost allow boaters and hikers to know their position within a few hundred meters. This accuracy is sufficient for recreational use. Mapping
A hand held or similar unit at mid-range price that is linked to a fixed broadcast base station. These units allow utility companies, municipalities and others to locate various items (telephone poles, waterlines, valves) with a positional tolerance of several meters. This is suitable for some Geographical Information Systems (GIS) mapping purposes. GPS and Policing
GPS technology offers numerous benefits to law enforcement agencies of all types. For some agencies, the navigational capabilities offered by GPS enhance efficiency and safety. These navigational applications can be used to support a variety of policing and criminal justice functions. Other agencies use GPS positioning technologies to carry out special operations or to provide enhanced personnel safety.
For example, using computerized maps of their rights given by law, cooperated with GPS, aviation personnel can determine location, speed and time.
The positioning capabilities offered by GPS may also contribute to the success of specialized law enforcement operations such as in controlling vehicles. One such program operated in Minneapolis led to a 60% reduction in auto theft after only one month. The automatic vehicle location systems can not only provide efficiency of response and help ensure officer safety, but also provide officer with accurate information concerning the best response route to an incident. What‘s more, they can provide officers information that allows the closest patrol officers to be dispatched to a particular incident.
Advanced Transportation Management Systems (ATMS) are heavily dependant upon GPS technology to provide data about the road system. GPS allows for law enforcement personnel to clear roadway blockages to ensure the safety of motorist.
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Most people associate law enforcement with the prevention, reduction, and prosecution of criminal activity. In fact, a large portion of local law enforcement resources are involved in facilitating the movement of people and vehicles in a safe manner.
In conclusion, large-volume commercial applications such as cellular phones, personal communication systems, and in-vehicle navigation systems will fuel continued development of these technologies. What was ultimately the domain of the Department of Defense is rapidly becoming available for business, private, and general government use. Policing and public safety in general, will benefit from these market forces. It is clear that there are a number of GPS applications for policing. 1. The passage mainly discusses the components of GPS.
2. GPS satellites are space vehicles which send radio signals from space.
3. Navigation, the primary function of GPS, involves data of four dimensions: X, Y, Z (position) and Time. 4. The highest positioning accuracy of a receiver can now reach meter-level.
5. For the professional surveyor requiring high level precision and accuracy capability, a good GPS receiver should have a minimum 8 satellite tracking capability.
6. GIS mapping may be done with a hand-held or similar unit at medium-range price.
7. Thanks to the GPS system, the auto theft rate in Mineapolis had been reduced by sixty percent in a month.
8. Limited geographic coverage, lack of continuous 24-hour coverage, and the limited accuracies of other related navigational instruments are the disadvantages of __________.
9. The prices of various GPS receivers are determined by __________.
10. The accuracy of a hand-held GPS unit at relatively low price is good enough for __________.
Fast Reading -- Passage 5
Are we ready for the library of the future? Librarians or providers of tech support?
Librarians today will tell you their job is not so much to take care of books but to give people access to information in all forms. Since librarians, like so many people, believe that the entire universe of commerce, communication and information is moving to digital form, they are on a reform to give people access to the Internet—to prevent them from becoming second-class citizens in an all-digital world.
Something funny happened on the road to the digital library of the future, though. Far from becoming keepers of the keys to the Grand Database of Universal Knowledge, today‘s librarians are increasingly finding themselves in an unexpected, overloaded role: They have become the general public‘s last-resort providers of tech support. It wasn‘t supposed to be this way. Today‘s libraries offer a variety of media and social-cultural events—they are ―blended libraries,” to use a term created by Kathleen Imhoff, assistant director of the Broward County Library of Fort Lauderdale, Florida. At the newly remodeled San Francisco Public Library, the computers are prominently displayed in the center of the library building while the books are all but hidden on the periphery (外围). Imhoff‘s own library has word processing and other types of software for visitors to use, Internet access, audio CDs, videotapes, concerts, lectures, books and periodicals in three forms (print, microfiche and digital).
Many libraries have found that this kind of ―blending‖ is hugely popular in their communities, and librarians explain the changes in their institutions‘ roles by pointing to the public demand for these new services. But other trends are at work, too.
Can computers really help visitors to find what they want?
For some time, libraries have been automating their back-end, behind-the-desk functions for reasons of cost and convenience, just like any other business. Now, the computers have moved out from behind librarians‘ desks and onto the floor where the visitors are. This means that, suddenly, library-goers will have to know how to use those computers.
This sounds reasonable enough until you take a close look. Unfortunately, the same technology that cuts costs and relieves librarians of work behind the scenes increases it for the public—and for the librarians at the front desk who have to help the public figure out how to use the technology. The unhappy result: People are simply not finding the information they seek.
If you are just coming to the library to read a book for pleasure and you know what a card catalog is and you have some basic computer skills, then you are going to be OK. But if you are trying to find some specific information—say, whether software in the classroom helps kids learn better or the causes of lung cancer or the basic procedure for doing a cost-benefit analysis of computer
systems (three topics I have actually tried to look up in the San Francisco library)—then you‘re in trouble. What should a visitor of the future library be equipped with?
To begin with, library visitors must now be able to type, to use a mouse and a menu and to understand the various types of computer interfaces (terminal text, windows and browsers). It‘s also nice if you know 17 different ways to quit a program, which electronic databases you should look in for what kinds of information, the grammar
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necessary to define your search and the Library of Congress‘ controlled vocabulary. After I had been to the new San Francisco library three times, I started keeping a folder of instructions on how to do a keyword search (fi a=author, for example), since I would forget between visits.
Probably half the population has never used a computer, fewer know how to type and almost nobody knows anything about electronic databases or searching grammar. As a result, the public library is now engaged in a massive attempt to teach computer literacy to the entire country. Some librarians compare it to the adult literacy programs the library also sponsors, but this is on a far larger scale—and less closely tied to the library‘s traditional mission.
What do libraries do to help visitors to get prepared for the future libraries?
The response at each library system has been different. Some libraries actually give courses in word processing, accounting program and so on. But even at libraries where the staff has resisted becoming computer trainers, they are still forced to devote significant resources to the problem.
Such has been the case in San Francisco, where people with disabilities can sign up to use the voice-recognition program Dragon Dictate—but only if they can prove they already know how to use the software. The librarians have neither the time nor the peculiar skill (nor the time to develop the skill) to teach it to them. At the reference desks, librarians try not to spend a lot of time teaching people the basics of how to use the computer, but sometimes it‘s unavoidable. ―We try to get them started,” says business librarian John Kenney. ―We let them do as much as they can on their own and they come get us. It‘s certainly a big problem.‖
The San Francisco library offers classes on its own electronic catalog, commercial periodical indexes and the Internet twice a week as well as occasional lectures about the Internet. Although it seems odd to me that people now need to take a two-hour class before they can use the library, the classes are always full. But despite the excellent teachers, two hours is simply not enough to meet the needs of the students, many of whom have never used a computer before in their lives and many of whom simply can‘t type. When I took the class one Tuesday, the man sitting next to me said he has used the library‘s computer catalog many times, but he keeps making typing mistakes without knowing it. This unexpectedly throws him into the wrong screens and he doesn‘t know how to get back. On the floor, he repeatedly has to ask a librarian for help. Libraries‘ own trouble
―Providing technology does not mean people can use the technology,” says Marc Webb, a San Francisco librarian and one of the teachers. ―Half the voters are still trying to read English.‖ The library has also had to deal with the practical difficulties of making its catalog accessible via the Internet, a new service many libraries are starting to offer.
―It‘s absolutely overwhelming,” Webb says. ―Everyone is getting to us with multiple transports, they‘re all using different software, they have Winsock or Telnet set up differently, and suddenly the library is forced to become a hardware and software help desk. When you‘re trying to tell someone over the telephone how to set up Winsock through AOL when this is the first time they‘ve ever used a computer, it‘s very difficult.‖
1. In today‘s libraries, the librarians are people who __________.
A. take care of the books
B. conduct better-reader programs C. provide tech support for the public
D. help promote the popularity of the library
2. According to Kathleen Imhoff, libraries at the present time are __________.
A. traditional libraries B. blended libraries C. digital libraries D. high-tech libraries
3. What is true about the San Francisco Public Library?
A. There are only computer databases, without books. B. Books are no longer open to the general public.
C. Computers are more prominently displayed than books. D. The number of librarians has been decreased sharply.
4. Libraries have been automating their back-end, behind-the-desk functions in consideration of __________.
A. cost and convenience
B. advancement of technology C. the demands of readers D. the number of visitors
5. If you have some basic knowledge of card catalogue and computer skills, you will be able to __________.
A. a software that helps improve learning ability
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B. a program that can do financial analysis C. a medication for fever D. a book for pleasure
6. Which of the following programs is sponsored by public libraries?
A. Wildlife protection programs. B. Adult literacy programs.
C. Environmental reseach programs. D. Family planning programs.
7. Dragon Dictate is a sofeware which is used to help __________ in library use.
A. school children B. the blind or deaf C. elderly people D. the disabled
8. The San Fransisco library requires its visitors to take a ___________ couse before they can use the library. 9. The man sitting next to the author in the class could not open the right screen because of his __________. 10. Recently, many libraries are trying to provide the visitors with a new service: __________.
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