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GNSS Solutions

July 11, 2011

What is a virtual reference station and how does it work?

Q: What is a virtual reference station and how does it work?

A: To reach centimeter-level — or even better — accuracy of positioning typically requires use of precise dual-frequency carrier phase observations. Furthermore, these observations are usually processed using a differential GNSS (DGNSS) algorithm, such as real time kinematic (RTK) or post-processing (PP). Regardless of the specific differential algorithm, however, implicit in the process is an assumption that the quality of the reference station data is consistent with the desired level of positioning accuracy.

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By Inside GNSS
May 19, 2011

How do you compute relative position using GNSS?

Figure 1

Q: How do you compute relative positions with GNSS?

A: GNSS is well recognized as an excellent means of computing position, but many people think that GPS only provides absolute position information. However, GNSS can also provide relative position information. In this column, we will look at some of the details of how this is done.

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By Inside GNSS
March 14, 2011

GNSS Receiver Clocks

Q: Does the magnitude of the GNSS receiver clock offset matter?

A: It is well known that GNSS receiver clocks drift relative to the stable atomic time scale that ultimately defines a particular GNSS system in the first place. GNSS receiver manufacturers, however, try to limit the magnitude of the time offset to within some predefined range.

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By Inside GNSS
January 9, 2011

Differences between Signal Acquisition and Tracking

Q: Why is acquisition of GNSS signals generally more difficult than tracking and what are the limiting factors?

A: A fairly good analogy of the difference between GNSS signal acquisition and tracking can be found in the rescue of victims of a sunken ship whose location is not accurately known. The first stage of the rescue attempt typically involves an aircraft flying a search pattern, which hopefully encompasses the location where the ship went down.

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By Inside GNSS
December 2, 2010

Measuring GNSS Signal Strength

Q: What is the difference between SNR and C/N0?

A: GPS receivers built for various applications, such as handhelds, automobiles, mobile phones, and avionics, all have a method for indicating the signal strength of the different satellites they are tracking. Some receivers display the signal strength in the form of vertical bars, some in terms of normalized signal strength, and others in terms of carrier-to-noise density (C/N0) or signal-to-noise ratio (SNR).

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By Inside GNSS
August 17, 2010

A Fully Digital Model for Kalman Filters

Q: Is it possible to define a fully digital state model for Kalman filtering?

A: The Kalman filter is a mathematical method, purpose of which is to process noisy measurements in order to obtain an estimate of some relevant parameters of a system. It represents a valuable tool in the GNSS area, with some of its main applications related to the computation of the user position/velocity/time (PVT) solution and to the integration of GNSS receivers with an inertial navigation system (INS) or other sensors.

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By Inside GNSS
June 22, 2010

Generating Carrier Phase Measurements

Q: What is the carrier phase measurement? How is it generated in GNSS receivers?

A: Simply put, the carrier phase measurement is a measure of the range between a satellite and receiver expressed in units of cycles of the carrier frequency. This measurement can be made with very high precision (of the order of millimeters), but the whole number of cycles between satellite and receiver is not measurable.

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By Inside GNSS
October 7, 2007

GNSS Solutions

“GNSS Solutions” is a regular column featuring questions and answers about technical aspects of GNSS.

Readers are invited to send their questions to columnist Mark Petovello, Department of Geomatics Engineering, University of Calgary in Alberta, Canada. He will find experts to answer those questions, which will be published in Inside GNSS.

Dr. Petovello is a professor at the university. He has been actively involved in many aspects of positioning and navigation since 1997.

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By Inside GNSS