GNSS-POSITIONING SYSTEMS.docx.pdf - GNSS classification...

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GNSS classification Global coverage systems: GPS (fully operational), US GLONASS (operational, currently updated), Russia Galileo (planned; by 2013), Europe Compass (planned; by 2010), China GNSS applications They can be summarized into 5 broad categories: Location = determining a basic position (e.g., emergency calls) Navigation = getting from one location to another (e.g., car navigation) Tracking = monitoring the movement of people and things (e.g., fleet management) Mapping = creating maps of the world Timing = bringing precise timing to the world J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 1
GNSS applications - examples J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 2
How GNSS works? Position determination (I) GNSS systems use the concept of Time-Of-Arrival (TOA) of signals to determine user position. Minimum 4 satellites needed in order to determine the user coordinates x u , y u , z u (horizontal, vertical & height). Position determination (II) Position determination by triangulation/multilateration principle: The position of the user is found at the intersection of 4 spheres, each with radius equal to the pseudorange measurements i . If less than 4 satellites are visible), => uncertainties in the position (i.e., intersection is J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 3
larger than one point). Satellite positions are known. In the above system of 4 equations, the unknowns are: x u , y u , z u and t. If more satellites are visible => Least Square-based solutions (similar with wireless TOA location). Position determination (III) Triangulation principle: J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 4
Code measurements (Pseudorange) Pseudorange measurements i may be affected by various errors. Typically, pseudoranges are obtained from time delays and Doppler shift estimates; also carrier-phase based positioning is possible Accurate clocks needed (e.g., GPS satellites are transmitting synchronously) => some reference (universal) time is available. The receiver clock is generally not synchronized - an exact time of reception cannot be determined. The Pseudorage measurement is defined as: c=speed of light J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 5
i = c(t R - t T ) = r + c t; t R , t T =times of reception/transmission r=true range t=clock offset Reception time is computed (measured) by the GNSS receiver through the acquisition and tracking processes Relating pseudoranges to user position and time Many solutions available: closed-form, linearization methods, least squares solutions. Several details (weighting for different errors, relativistic effects, ephemeris corrections, propagation effects, etc.) need to be accounted for. J. OMEKE 2021 EECQ 2233 ENG. SURVEYING 1B Lecture 3 6
Sources of error in satellite positioning Signal multipath: This occurs when the GNSS signal is reflected off objects such as tall buildings or large rock surfaces before it reach the receiver. This increases the travel time of the signal, thereby causing errors (similar with cellular systems).

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