Robert Ehrlich wrote:

After this discussion we are far from the original question, i.e.
can we infer anything about the polar of a glider just from GPS
fligth logs of this glider?
... solution proposed ...

After a bit thinking about that my conclusion is that the above
solution doesn't work. Even if the system I was talking about
is overdetermined, there is always an undetermination on the couple
vertical speed of airmass/sink speed of glider relatively to airmass,
any combination of both with the same sum satisfies in the same way
the equations, so no valuable information on the polar of the glider
can be obtained unless we add some information on the airmass, either
by some other data, or by some further modelling (e.g. assuming the
total vertical movement of the airmass is zero, which is not realistic,
or that it is some given percentage of the average lift, or anything we may
think about it)