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GNSS Solutions

March 14, 2015

How does a GNSS receiver estimate velocity?

Equations 1 – 11

Q: How does a GNSS receiver estimate velocity?

A: Stand-alone single-frequency GNSS receivers represent the largest slice of the commercial positioning market. Such receivers operate mainly in single point position (SPP) mode and estimate velocity either by differencing two consecutive positions (i.e., approximating the derivative of user position) or by using Doppler measurements related to user-satellite motion.

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By Inside GNSS
January 16, 2015

Why are carrier phase ambiguities integer?

It is well known that carrier phase ambiguities are integer values. Intuitively, this is hard to understand with a common counter-argument progressing along these lines: even if the receiver measures the instantaneous phase of the incoming signal (thus removing any fractional cycle component at the receiver end), the phase of the signal at the satellite cannot be guaranteed to be zero, so how can the ambiguity be integer?

In this article we explain why the carrier phase ambiguities are indeed integer.

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By Inside GNSS
July 21, 2014

GNSS Position Estimates

Q: How do measurement errors propagate into GNSS position estimates?

A: Not surprisingly, GNSS positioning accuracy is largely dependent on the level of measurement errors induced by orbital inaccuracies, atmospheric effects, multipath, and noise. This article discusses how, specifically, these errors manifest as position errors.

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By Inside GNSS
May 6, 2014

How do you compute a relative position using GNSS?

Q: How do you compute a relative position using GNSS?

A: In many applications the absolute position of an object is less important than the relative position to other objects in the vicinity. An obvious example is a collision avoidance system, wherein the proximity of nearly vehicles is much more important than whether the vehicles are located on street “X” or “Y.”

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By Inside GNSS
January 19, 2014

Ionospheric Scintillation

Ionospheric scintillations are rapid temporal fluctuations in both amplitude and phase of trans-ionospheric GNSS signals caused by the scattering of irregularities in the distribution of electrons encountered along the radio propagation path. The occurrence of scintillation has large day-to-day variability. The most severe scintillations are observed near the poles (at auroral latitudes) and near the equator (within ± 20 degrees of geomagnetic equator).

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By Inside GNSS
November 18, 2013

Multipath vs. NLOS signals

Q: How Does Non-Line-of-Sight Reception Differ From Multipath Interference?

A: It is well known that GNSS signals may be reflected by buildings, walls, vehicles, and the ground. Glass, metal, and wet surfaces are particularly strong reflectors.

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By Inside GNSS
July 17, 2013

Receiver Architecture

Q: What is a maximum likelihood vector tracking receiver?

A: Vector tracking in GNSS receivers is based on the idea that instead of tracking each satellite’s signal separately, all signals are ultimately related to the position and velocity of the user antenna and thus can be tracked collectively. (See the discussion in this column in Sept/Oct 2012 issue of Inside GNSS by Drs. Lashley and Bevly for more information.)

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By Inside GNSS
May 23, 2013

Calculating Time Offsets

Q: How do you deal with timing differences between GNSSs? 

A: All GNSSs inherently depend on precise timekeeping to measure the satellite/receiver time of flight of signal propagation with sufficient accuracy to compute ranges/distances for multilateration calculations. Each GNSS ground segment therefore dedicates considerable effort to maintaining a highly stable atomic time scale as well as the corresponding offset to global standards such as UTC (Coordinated Universal Time). 

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By Inside GNSS