测绘类外文资料翻译--自动定位技术(编辑修改稿)

测绘类外文资料翻译--自动定位技术(编辑修改稿)内容摘要：

on continuous phase observations over some extended time. Pros and Cons of GPS Some general points on the advantages of GPS are given here only. First, it covers the whole globe spatially. Hence the word ‘Global’ in is its name, and continuously in time for 24 hour a day. Second, the system offers high positioning accuracy at an affordable price to the user. Besides, GPS is convenient to use in many applications. GPS also has its disadvantages. Perhaps the biggest problem that it has is the requirement of line of sight to GPS satellites. GPS signals can perate the atmosphere but not any solid objects such as buildings, trees, and mountains. This requirement seriously limits the use as buildings, trees, and mountains. This requirement seriously limits the use of GPS in environments such as dense urban areas and underground. Another problem with GPS is the multipath effects that contribute to significant errors when a receiver is surrounded by smooth reflective surfaces. 3 GPS BASED POSITIONING METHODS IN NAVIGATION GPS was designed primarily as a tool for navigation. It has been successfully applied for the navigation of vehicles such as cars, ships and airplanes, as well as for the guidance of weaponry objects such as missiles. This section will look at the different positioning methods of using GPS in navigation. Single Receiver Positioning The simplest way to used GPS for navigation is to carry a GPS receiver whenever you go. The receiver can offer in realtime the SPS positioning accuracy, that was discussed earlier, as long as signals from four or more GPS satellites can be received. Since such an application is very basic, almost any mercial GPS receivers on the market can be used for this purpose. Many of the receiver units cost less than US$100. This simple GPS positioning method is used very monly for recreational activities such as hiking and fishing, Fieldwork in remote areas such as in the desert or forest, and travel on the sea. Differential GPS (DGPS) Positioning If the positioning accuracy described above is insufficient, an easy and affective way to increase the accuracy is to use differential GPS. In this operation mode, two GPS receivers are required, and the other carried around to where the position needs to be determined. In this case, the first receiver is called the reference station and the second the roving station. If the user is close enough to the reference station, various GPS errors, such as the effect of the atmosphere, are very similar for both of the two stations. The reference station can be used to determine the errors in the three coordinate ponents. Corrections can be worked out for the position of the roving station based on the errors determined at the reference station. The positioning accuracy can be greatly improved by doing this ‘differential’ operation. DGPS technique has been repeatedly demonstrated to achieve a positioning accuracy of 2 to 5m over baselines up to 1,000 km. If realtime positioning is required, a data munication link between the two stations is required. A mon approach is to use a pair of radios if the distance between the two stations is short, say within a few kilometers. For distances up to a few hundred kilometers, a broadcasting station can also be used to send the correction messages to the entire designated area. For example, Hong Kong has established a permanent GPS reference station on its Kau Yi Chau island. A transmitter is used to broadcast correction messages to an area of 500 km in diameter which covers the entire Hong Kong region. The claimed accuracy of the system is within 1 m. The simple differential operation described above has relatively limited coverage, and is restricted by the coverage of the terrestrial munication system. Besides, the positioning accuracy decreases as the separation munication system. Besides, the positioning accuracy decreases as the separation between the user and the reference station increases. A technique called Wide Area Differential GPS (WADGPS) can be used to overe the problems of the simple differential method. WADGPS uses a number of GPS reference stations to cover a wide area such as a whole country or continent. The GPS positioning errors are separately modeled and applied to the roving user station in a way that breaks the position dependence. Therefore the limitation in the navigation accuracy due to the userreference station separation can be largely eliminated. Positioning accuracy of about 2 m and 3 m for the horizontal and the height ponents respectively is achieved over baselines ranging from 2,000 km to 3,500 km. The limitation in the coverage of the data links is also overe by using geostationary munication satellites. GPS Aided with Additional Sensors and Information As said above, the requirement of direct line of sigh to GPS satellites makes it very difficult to use GPS in restricted areas such as high rise cities. For example, recent tests have shown that only about 30 percent of Hong Kong can receive signals from four or more GPS satellites. One way to solve the problem is to aid GPS with sensors such as passes, odometers, inertial navigation systems (INS), which basically use gyros and accelerometers to the determination of positions, or additional information such as existing maps and databases. Pseudolite, a pseudosatellite as its name implies, is a device that can transmit GPSlike signals, A pseudolite can be placed at strategic locations such as on a tall building at a road intersection to transmit signals to nearby areas. The signals form a pseudolite can be treated in the same way as those from a real satellite. GPS can also be used in bination with GLONASS (Global Navigation Satellite System), a system developed and maintained by Russia.。