Inside GNSS Media & Research

MAR-APR 2018

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Page 52 of 67 M A R C H / A P R I L 2 0 1 8 Inside GNSS 53 Having done this we run the analy- sis, and, in this example, pick five satel- lites with increasing elevations: For these five satellites we observe the expected trend of greater atmo- spheric delay with decreasing elevations. e difference between the delay for the highest satellite (PRN 32, at 80°) and the lowest (PRN 24, at 7°) is approximately sixteen meters. In general there will be other errors (such as noise, and mul- tipath) and at times they may be larger than the atmospheric delays. So not all signals will be this well behaved, but you can quite easily see the expected trend in most datasets. Example 3: Urban Multipath/Reflection analysis You can observe the individual effect of multipath and/or signal reflections on each measurement. As an example, here is a drive test in San Francisco — you can see the San Francisco Giants base- ball stadium in the image. e green dots here show the truth reference obtained from a GNSS iner- tial navigation system. We will analyze the GNSS measurements from a smart- phone chip. You analyze measurement errors from a moving receiver by supplying an NMEA trut h reference f ile. The Analysis Tool reads the NMEA file and extracts the GGA messages (for 3D position) and RMC messages (for veloc- ity). With this information the Tool can compute clock offsets and frequency, and measurement errors for pseudor- anges and pseudorange-rate. As the car drives into the city from bottom-right towards top-le, we see a sudden and dramatic change in C/N 0 for the low satellite, and at the same time, a large increase in pseudorange error ( Figure 3 ). While the error on the other satellites remains small. So, the question is: what happened to this satellite GPS PRN 22? Mu lt ipat h — yes, but not cla s- sic multipath (with line-of-sight and non-line-of-sight signals overlapping), rather, what we see here are pure reflec- tions. How do we know that? Because we can see from the skyplot, and the Google Earth image that this satellite is completely blocked by the building to the south of the car; combined with the C/N 0 and PR error plots we can see that the signal that is tracked is something reflected from across the street. As these examples show, in just a few minutes you can do fairly sophisti- cated analysis at a level that, until now, was inaccessible to anyone but the chip manufacturers themselves. EXAMPLE 2 Iono, and Tropo analysis EXAMPLE 3 Urban multipath/reflection analysis

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