Guralp Systems Strong Motion - Manual (Page 15)

Güralp Systems Ltd.
4.2 Types of Strong Motion Result
The following strong motion results are calculated every
second, in real-time. They are transmitted in the SM GCF block.
Windowed MMA (minimum, maximum and average)
The most basic strong motion result type is the minimum,
maximum and average (arithmetic mean). This is computed
every second and uses a 10 second sliding window (so the
result at y(t) is based on input data from u(t) to u(t-10)).
This calculation finds the minimum, maximum and average
number of counts, and then converts these numbers (in digital
counts) into ground units using the calibration info in the info
block. The result is in gal.
PGA (peak ground acceleration)
The peak ground acceleration is computed every second, and is
the magnitude of the largest acceleration recorded in a 1s
interval. The acceleration is simply the difference between the
raw sampled value of the input and the sliding average from
the windowed MMA data. The result is in gal.
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Illustration 3: Bode plot of low latency 200Hz to 20Hz pre-
decimation IIR filter. The high pass effect is present but cannot
be seen on this scale.