GPS pinning parameters
I sent an email to SkyTraq, and they replied by sending me Application Note AN0025 v5 (Uploaded with permission).

There is a slight difference from the datasheet in how this note names the parameters:


Pinning speed PSPEED in km/h
Pinning threshold PTH in s (pinning cnt in datasheet)
Unpinning speed UPSPEED in km/h
Unpinning threshold UPTH in s (unpinning cnt in datasheet)
Unpinning distance UPDISTANCE in m


When the speed is below PSPEED for longer than duration PTH, it enters a "position pinned state", where the GPS module is considered stationary. The reported location will not move around at all, contrary to the normal behaviour where the location jumps around in a stochastic fashion.

If there is a 3D fix, it will return to the normal "unpinned" state when the speed exceeds UPSPEED for longer than UPTH. If the fix is only 2D or intermittent, it takes longer (see pp.4–5). In either case, if the position of the GPS module has drifted by more than UPDISTANCE, it will also "unpin".

The application note says nothing about the purpose of this feature. I have an unconfirmed suspicion that pinning the location might improve the GPS time estimate on stationary devices.