Friday, February 9, 2018

More GPS Comparison Data

Below are more results from comparing three GPS devices: the Locosys GW-60 watch, an Android phone (Moto E4) with GPS Logit, and the USB GPS dongle I had been testing. These results are from a Kona One session 2 days ago, as reported by GPSResults:
No surprise for the 2 and 10 second results. All differences for the phone are well within the +/- ranges for the watch, with maximum differences of 0.2 knots for 2 seconds, and 0.09 knots for 10 seconds. Numbers for the phone GPS are similar, but slightly higher. Note that for this session, the GW-60 watch and the GPSLogit data were recorded at 1 Hz, while the USB dongle recorded at 5 Hz.

The results for 500 m are similar, except that the phone GPS now does as well as the dongle. For the nautical mile, the results reported by GPSResults differ a lot for the phone. This is because GPSResults filtered out single data points in the middle of a few one-mile runs:
The sixth point is shown in brackets, meaning GPSResults filtered it. Since the GPSLogit data do not have accuracy estimates, and both acceleration and HDoP around this point is low, we have to assume that GPSResults considers data from 7 GPS satellites as insufficient (unfortunately, the Mac version of GPSResults does not show a setting for minimum number of satellites). Unchecking the "On" checkbox for all filters leads to results that are more similar to the watch and dongle:
However, the last 3 nautical mile runs for the phone show larger differences even without filters. This, however, appears to be an artifact of the analysis: the entire session was only about 20 nautical miles long, with just 12.9 nautical miles at speeds above 12 knots. Looking at the sections in GPS Action Replay shows that the speeds over the aligned nautical mile regions only differ by 0.01-0.05 knots; the reported larger changes are caused by slightly different regions being picked for the different units by the analysis programs. For the GPSLogit tracks, GPSResults was able to "squeeze in" a extra nautical mile run of 13.49 knots, while the same region between previously picked (higher speed) nautical miles was just a tad shorter than 1 nautical mile in the data from the two other devices. For speedsurfing competitions, this kind of artificial difference is entirely irrelevant; it only shows that some care has to be taken when analyzing the data.

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