Wrong distance and average speed displayed when a distance sensor is working intermittently

[dr-m]

Describe the bug
Sometimes, the magnetic speed/cadence sensor on my bicycle can drift too far away from the wheel magnet, for example, reporting a speed of less than 10 km/h instead of the correct speed.

This of course is an operator error, but the handling of this could be better. For example, it would be nice to have an alarm sound if the GPS speed starts to significantly differ from the speed calculated from the wheel sensor. I think that this should be much easier to implement than #1950.

To Reproduce

1. Start riding with a speed/distance sensor configured, and initially working.
2. During the recording, move the sensor or the wheel magnet so that they will lose contact completely or intermittently.
3. Observe that only one (too small) speed and distance are being displayed, based on the sensor reading. Some other programs, such as AAT, would display both GPS and sensor speed.
4. Fix the sensor and complete the ride.

For example, if I load to https://augustinbegue.github.io/gpx-visualizer/ the GPX recording of a ride that I completed yesterday, it will report 149.25 km and 24.15 km/h average moving speed, which I find plausible. OpenTracks reports only 143.93 km and 23.4 km/h. I suspect that the sensor connection was bad for at most 20 kilometers before I noticed and fixed it. I did not yet try out https://mytourbook.sourceforge.io/mytourbook/ to visualize where exactly the connection started to fail.

For an earlier ride in the opposite direction, the sensor must have been in place all the time, because the summary in OpenTracks does not differ that much from what independent GPX analysis tools would report.

Technical information

• Device: Sony Xperia 1 ii (XQ AT51)
• OS: Android 12
• OpenTracks version: v.4.21.1irreproducible

[dennisguse]

If there are multiple sensor present that provide similar data, the current implementation is really simple: just use the one that is technically "better" aka measures in a better way.
Basically, GPS is in these cases just used for location and altitude.
Speed, distance etc are taken from the Bluetooth sensors (as those measure more precisely if they are working).

TBH: switching sensors is easy. The really complicated part is determining which sensor is more correct.
And there are a lot of edge cases...