Doppler experiment
Introduction
Increasingly, FlySight is being used to score Speed Skydiving competitions. There has been some concern within the Speed Skydiving community that these devices may not be accurate at such high speeds. In particular, it has been suggested that the dynamic model imposes some kind of limit at 100 m/s. An engineer from u-blox has responded to this concern as follows:
“The 100 m/s figure is part of sanity check and not an absolute limit. As long as speed does not exceed 600 m/s you should still get valid position data.”
The “sanity check” is imposed only if the signal degrades past some point—i.e., if the number or geometry of satellites or signal strength fall below some threshold. If the sanity check is imposed, we should see this reflected in the accuracy estimates reported by the u-blox module.
To test FlySight’s accuracy at speeds over 100 m/s, we have developed a Doppler speed measurement system to measure the relative speed of the jumper directly. To use this system, a transmitter with very accurate frequency is placed on the jumper. A receiver is then placed on the ground. Any motion of the jumper toward or away from the receiver will cause a shift in the received frequency.
We can then use position and velocity data from FlySights on the jumper and on the ground to calculate an expected Doppler shift. This expected shift can be overlaid on the received signal to determine if the two are in agreement.
Full Text
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