|
Fuzzy Point Averaging of the GPS Position Components
 M R Mosavi Assistant Professor Department of Electrical Engineering Iran M_Mosavi@iust.ac.ir I. Introduction The Global Positioning System (GPS) is a radio based navigation system that gives three dimensional coverage of the Earth 24 hours, a day in any weather conditions. The satellites orbit the Earth every 12 hours at approximately 12,600 miles above the Earth. The satellites continuously transmit information towards the Earth. The information is transmitted on two frequencies: L1 (1575.42 MHz), and L 2 (1227.60 MHz). With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude. GPS can be a powerful tool that assists researchers locates points of interest. While GPS provides an easy way to collect latitude and longitude, it is important to remember that there are errors inherent in any GPS collected point. In order to use GPS most effectively, users need to decide on a strategy for dealing with the errors. The goal of this paper is to obtain better accuracy than simple point averaging from a low cost GPS receiver. The theoretical backgrounds for better accuracy are based on the principle of averaging and fuzzy logic schemes. This paper is organized as follows. GPS error sources are presented in Section II. GPS accuracy is described in Section III. The fuzzy processings are provided in Section IV. Experimental results are presented in Section V. Conclusions are in Section VI. II. GPS Error Sources There are many sources of possible errors that will degrade the accuracy of positions computed by a GPS receiver. The travel time of GPS satellite signals can be altered by atmospheric effects; when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space. Sunspot activity also causes interference with GPS signals. Another source of error is measurement noise, or distortion of the signal caused by electrical interference or errors inherent in the GPS receiver itself. Errors in the ephemeris data (the information about satellite orbits) will also cause errors in computed positions, because the satellites weren't really where the GPS receiver "thought" they were (based on the information it received) when it computed the positions. Small variations in the atomic clocks (clock drift) on board the satellites can translate to large position errors; a clock error of 1 nanosecond translates to 1 foot or 0.3 meters user error on the ground. Multipath effects arise when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna. When this happens, the receiver gets the signal in straight line path as well as delayed path (multiple paths). Selective Availability, or SA, occurred when the Department of Defence (DOD) intentionally degraded the accuracy of GPS signals by introducing artificial clock and ephemeris errors. When SA was implemented, it was the largest component of GPS error, causing error of up to 100 meters. SA is a component of the Standard Positioning Service (SPS), which was formally implemented on March 25, 1990, and was intended to protect national defense. SA was turned off on May 1, 2000. III. GPS Accuracy GPS accuracy has a statistical distribution, which is dependent on two important factors. The expected accuracy will vary with the error in the range measurements as well as the geometry or relative positions of the satellites and the users. Satellite geometry can also affect the accuracy of GPS positioning. This effect is called Geometric Dilution of Precision (GDOP). GDOP refers to where the satellites are in relation to one another, and is a measure of the quality of the satellite configuration. It can magnify or lessen other GPS errors. In general, the wider the angle between satellites, the better the measurement. Most GPS receivers select the satellite constellation that will give the least uncertainty, the best satellite geometry. GPS receivers usually report the quality of satellite geometry in terms of Position Dilution of Precision, or PDOP. PDOP refers to Horizontal (HDOP) and Vertical (VDOP) measurements (latitude, longitude and altitude). A low DOP indicates a higher probability of accuracy, and a high DOP indicates a lower probability of accuracy. Figure 1 shows ‘Poor DOP’ and ‘Good DOP’.
Page 1 of 4
| Next | |