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Introduction to GPS The Global Positioning System - Part 3

GPS Errors and Biases GPS pseudorange và vận chuyển pha đo được cả hai bị ảnh hưởng bởi một số dạng sai sót ngẫu nhiên và những thành kiến (lỗi hệ thống). Những lỗi này có thể được phân loại như những người có nguồn gốc ở các vệ tinh, những người có nguồn gốc ở người nhận, và những người được do để tuyên truyền tín hiệu (khúc xạ khí quyển) [1]. Hình 3.1 cho thấy các lỗi khác nhau và những thành kiến . Các lỗi có nguồn gốc ở các vệ tinh bao gồm thiên văn, hay quỹ đạo,. | 3 GPS Errors and Biases GPS pseudorange and carrier-phase measurements are both affected by several types of random errors and biases systematic errors . These errors may be classified as those originating at the satellites those originating at the receiver and those that are due to signal propagation atmospheric refraction 1 . Figure 3.1 shows the various errors and biases. The errors originating at the satellites include ephemeris or orbital errors satellite clock errors and the effect of selective availability. The latter was intentionally implemented by the U.S. DoD to degrade the autonomous GPS accuracy for security reasons. It was however terminated at midnight eastern daylight time on May 1 2000 2 . The errors originating at the receiver include receiver clock errors multipath error receiver noise and antenna phase center variations. The signal propagation errors include the delays of the GPS signal as it passes through the ionospheric and tropospheric layers of the atmosphere. In fact it is only in a vacuum free space that the GPS signal travels or propagates at the speed of light. In addition to the effect of these errors the accuracy of the computed GPS position is also affected by the geometric locations of the GPS satellites as seen by the receiver. The more spread out the satellites are in the sky the better the obtained accuracy Figure 3.1 . 27 28 Introduction to GPS Ephemeris orbital error Selective availability Ionospheric delay Tropospheric delay Geometric 1 Clock error effects 1 Clock error Multipath error System noise Antenna phase center variations Figure 3.1 GPS errors and biases. As shown in Chapter 2 some of these errors and biases can be eliminated or reduced through appropriate combinations of the GPS observables. For example combining L1 and L2 observables removes to a high degree of accuracy the effect of the ionosphere. Mathematical modeling of these errors and biases is also possible. In this chapter the main GPS error sources are .