Inside GNSS Media & Research

MAR-APR 2018

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52 Inside GNSS M A R C H / A P R I L 2 0 1 8 Following are three examples that illustrate how to use the interactive controls. Example 1: Measurement error vs C/N O . e tool lets you select any subset of the visible satellites. In this case we've cho- sen three with different C/N 0 : Strong, Medium, Weak ( Example 1 ). You can read the computed raw pseudorange errors directly from the plot, including mean and standard deviation. You can clearly see two things: 1) the relationship of C/N 0 to pseudor- ange errors 2) the increased filtering that the receiv- er does to track the weaker signals, combined with the fading caused by multipath, especially on weak signals — note the increasing time constants of the errors. Example 2: Iono, and Tropo analysis In this example we process data collected at a known point, under open sky, away from any buildings or reflecting surfac- es. We disable Iono and Tropo models by unchecking the respective boxes on the control panel, and we set the Reference PVT to the known true position of the receiver ( Example 2 ). e pseudorange error plots will now include the sum of Ionosphere, Tropo- sphere, and Orbit errors. For GPS and Galileo, the typical orbit error in the broadcast ephemeris are now less than 0.5 meters (see Additional References), so the rest of the pseudorange error will be dominated by Iono+Tropo delay. ere is one thing le to do: specify which satellite to use for computing the clock error. Ordinarily the analysis program will automatically select sev- eral satellites for this. But, in order to observe relative error of iono and tropo we select a single reference satellite for computing the receiver clock. e pseu- dorange error on this satellite will be zero, by definition, and the errors on the remaining satellites will be the dif- ference between their errors and the reference satellite. We choose the high- est satellite, and specify it in a text file CustomParam.txt: FIGURE 2 The basic interactive tools allow you to adjust the plots (including selecting specific satellites to plot), enable/disable iono and tropo models, set start and end times to process windows of data, and set the reference position (PVT) used for computing clock errors and measurement residual errors. EXAMPLE 1 Measurement error vs. C/N 0 ANALYSIS TOOLS

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