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Precise GPS Point Positioning: the Future Alternative to Differential GPS Surveying
Ahmed El-Rabbany, Ph. ... of Civil Engineering (Geomatics Option)
Ryerson University
Positioning with GPS can be performed by either of two ways: point positioning or differential (relative) positioning. GPS point positioning employs one GPS receiver, while differential positioning employs two (or more) GPS receivers simultaneously tracking the same satellites. Surveying works with GPS have conventionally been carried out in the differential positioning mode. This is mainly due to the higher positioning accuracy obtained with the differential positioning mode compared to that of the GPS point positioning. A major disadvantage of GPS differential positioning, however, is its dependency on the measurements or corrections from a reference receiver; i. ... two or more GPS receivers are required to be available. New developments in GPS positioning show that a user with a single GPS receiver can obtain positioning accuracy comparable to that of differential positioning (i. ... This article discusses these new GPS developments and shows how this high accuracy level could be achieved.
Classical GPS Point Positioning:
GPS point positioning, also known as the standalone or autonomous positioning, involves one GPS receiver only. That is, one GPS receiver simultaneously tracks four or more GPS satellites to determine its own coordinates with respect to the center of the earth (Figure 1). To determine the receiver’s point position at any time, the satellite coordinates as well as a minimum of four ranges to four satellites are required (El-Rabbany, 2002). ... The Principle of GPS point positioning. The expected horizontal positioning accuracy of the classical approach has improved from about 100m (2 drms) when Selective Availability (SA) was on, to about 22m (2 drms) or better in the absence of SA (Shaw et al. ... To demonstrate the performance of the classical approach, GPS data collected at Algonquin, a continuously tracking site of the Canadian Active Control System (CACS) network, was processed in the point positioning mode. ... Obviously, this level of accuracy is not suitable for almost all of the surveying works. ... Epoch-by-epoch Results (L1 code pseudorange and broadcast ephemeris)
R1
R2
R3
R4
— Known: X, Y, Z (satellites)
+ R1, R2, R3, R4
— Unknown: X, Y, Z (receiver)
+ receiver clock error
— Horizontal accuracy: 22 m (95% of the time)
Improving Point Positioning Accuracy:
The accuracy of classical GPS point positioning is limited as a result of the presence of unmodelled errors and biases.
Approximate Word count = 1737
Approximate Pages = 6.9
(250 words per page double spaced)
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