Rev #37
Version 8.0
April 2021
Page 89 of 121
changes are in an interval of 1 m/s. The value σ
d
= 0.1 corresponds to a very quiet air mass.
Most changes will be at an interval of 0.1 m/s.
The true model parameter σ
d
is of course unknown to the instrument. The key question is,
therefore, how do we select the corresponding parameter in the S10/100?
The only way to do this is to use the same data sensor signals for different values of wind
variance SIGWIND and study the behaviour of the vario. We can do this because we record
all sensor signals at 100Hz using the special logging function in S10/100.
Behaviour of the air mass movement for various values of SIGWIND = (0.001, 0.05, 0.1, 0.5)
in a typical flight.
Let us choose a value SIGWIND = 0.1, which looks plausible. Qualitatively, if we select a
very small value of SIGWIND = 0.001 the algorithm will average the estimate since it
considers large deviations ∆d as highly unlikely and suppresses them. We next consider the
opposite case and select a much larger value SIGWIND = 0.5. The algorithm reacts quickly
to any disturbance. We will observe a nervous reading of the needle in the instrument.
Picture above shows a segment of a typical flight.
We observe that the most likely values are around SIGWIND = 0.1. For SIGWIND = 0.5 the
random fluctuations are larger. The values smaller than 0.05 are too small since they
suppress important details of the time behaviour. It appears that the values in the interval of
0.05-0.2 are the preferred ones. But this is a qualitative and subjective observation not
based on a rigorous mathematical optimization.
In summary, the selection of the wind parameter SIGWIND is not very sensitive. It leaves
room for personal preferences of the pilot, as vario needle filter for the TEK vario. During
tests most pilots have chosen a value between 0.05-0.2.
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