外文翻譯---基于接收機(jī)的一種新差分gps修正（英文）

1、* Corresponding author. Tel.#34-52131408; fax: 34-52131413. E-mail address: jlmartinez@ctima.uma.es (J. L. Martm Hnez)Control Engineering Practice 8 (2000) 253}258A new method of generating di!erential GPS correctionsJ.

2、L. MartmHnez*, M. A. MartmHnez, A. Garcm Ha-CerezoDepartment of System Engineering and Automation, University of Maaccepted 27 July 1999AbstractIn this paper, a simple method is proposed for calculating positions on the

3、ground with a di!erential global positioning system that does not use conventional correction data. The method employs two low-cost GPS receivers, one of them being at a “xed location. In order to coordinate their operat

4、ion, both receivers only use three of the highest satellites in sight. The precision obtained with this technique is similar to that calculated by more sophisticated methods. ? 2000 Elsevier Science Ltd. All rights reser

5、ved.Keywords: Global positioning system; Satellite selection; Position errors; Dilution of precision; Error correction1. IntroductionCivil applications of the GPS are not su$ciently pre- cise, because of certain sources

6、of error that a!ect the system. These errors can be divided into two groups: those independent of, and those dependent on satellite geometry. Selective availability (SA), ephemeris errors and receiver resolution are inde

7、pendent of the satellite geometry. On the other hand, triangulation errors, atmo- spheric delays and multipath re#ections depend strongly on the satellite elevation angle. The GPS receiver relies on knowing exactly where

8、 the satellites are in space with respect to the center of the Earth (ephemeris), but errors can appear in the orbit prediction. The receiver's resolution depends on the pre- cision of the onboard clock and on the de

9、lays in the internal transmission paths. SA is the intentional changing of the satellites' on- board clock and broadcast ephemeris introduced by the Defense Department of the USA in order to limit civilian use. Thus,

10、 SA is the main source of error and appears as a continuous variation in the user's position (Sonnen- berg, 1988). A good geometry for triangulation is achieved when the satellites are further apart from each other o

11、n the horizon (see Fig. 1). Generally, PDOP (Position Dilution of Precision in 3-D) is used by GPS receivers as the automatic criterion for selecting satellites, since it repres-ents the geometrical contribution of obser

12、vation errors on GPS positioning accuracy (Langley, 1999). Atmospheric layers alter the GPS signals and can introduce signi“cant errors that arise as the radio waves pass through the charged ionosphere and water-laden tr

13、oposphere (Herring, 1996). The delay e!ects are a func- tion of the satellite elevation angle which determines the length of the signal path (see Fig. 2). If the signal from the satellite su!ers multipath re#ec- tions, t

14、he time of #ight of this signal is increased. It is very di$cult to model multipath re#ections properly because they depend heavily on the environment of the antenna. But it is known that signals from a satellite with a

15、low elevation angle are much more a!ected by multipath re#ections. GPS provides longitude, latitude, height, time, course, and velocity. The horizontal accuracy of the GPS is nearly 100 m, 156 m for heights and 340 ns fo

16、r time, anywhere in the world and in all weather conditions. The di!erential GPS is a technique that employs two receivers and is based on the assumption that they are a!ected by approximately the same errors. If one rec

17、eiver is “xed at a precisely known location, its calculated position can be compared to the known position to generate a composite error vector. This di!erential cor- rection is passed to the other receiver several times

18、 a minute employing a pre-established code. This methodology signi“cantly reduces positioning errors since the height of a satellite (at more than 20,200 km) is much greater than the distance between the receivers (100 k

19、m maximum).0967-0661/00/$- see front matter ? 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 6 1 ( 9 9 ) 0 0 1 5 8 - 6Fig. 3. GPS GN-74 receiver.Fig. 4. Information #ow.Because the separation betwee

20、n both receivers is insig- ni“cant compared to the distance of the satellites in space, both GPSs will use practically the same set of satellites in their calculations, di!ering perhaps in the lower-angle satellites, not

21、 in the higher-angle ones. More- over, satellites with higher elevation angles produce smaller ranging errors. However, they tend to give higher triangulation errors (Park, Kim, Lee & Jee, 1996). GPS receivers allow

22、the selection of a 2-D or 3-D position if required. While a minimum of four satellites are needed to position in 3-D mode, only three are required to work in 2-D mode. Since in many applica- tions the receiver moves on t

23、he ground, a 2-D con“gura- tion has been adopted. If the GPS receiver is working in single mode, more than the minimum number of satellites required for posi- tioning will be employed to increase the accuracy of the meas

24、urements. However, when the receiver works in coordination with another, it is better to make the calcu- lations with the minimum number of satellites in order to avoid ambiguities in the error obtained. Moreover, it wou

25、ld help the calculations if the unit selected three of the satellites with higher elevation angles that have smaller horizontal dilution of precision in 2-D (HDOP) values. In summary, the methodology proposed in this pap

26、er consists of subtracting the two simultaneous measure- ments provided by the base station and the mobile re- ceiver from each other, which were obtained using the same set of three satellites. This technique causes mor

27、e individual errors but these are approximately identical in both receivers.3. ImplementationTwo GN-74 units have been employed as low-cost GPS receivers (Furuno Electric, 1995). These consist of a small board with a ser

28、ial port employing the RS-232 protocol (see Fig. 3). It has eight channels for tracking upto eight satellites. This receiver uses the NMEA-0183 communication protocol (National Marine Electronics Association, 1992). Sinc

29、e it is impossible to modify the receiver's “rmware, an external computer connected to the GPS serial port has been used to select the three highest satellites with the best HDOP at each moment, to mask the other sat

30、ellites in view and to calculate the position (see Fig. 4). The characteristics of the receiver's “l(fā)ter have been set to accurate positioning and large smoothing. Data com- munications are implemented via an RS-232 i

31、nterface, 4800 baud, half-duplex radio modem for wireless data transmission with a maximum range of 4 km (see Fig. 5).4. Experimental resultsTo check the e$ciency of this method, a large number of experiments have been c

32、arried out. Positions were recorded every two seconds over 31 minutes and plotted with respect to the actual position. Figs. 6}8 show the subtraction of the data collected at the same time by the GPS receivers when they

33、are in a “xed location and separated by a short distance, using the same set of three, four and “ve satellites, respectively. As can be observed, the “rst test provides a better perfor- mance. The position obtained emplo

34、ying the proposed tech- nique has been compared to the position calculated by GN-74 using conventional RTCM correction data from the virtual base station Omnistar via satellitesJ. L. Martn &nez et al. / Control Engin