SEPTENTRIO PolaRx5S, User Manual

Page: 59 / 107

5.3. SBF2 I SMR PROGRAM
SI
=
10
log
(
P max
) − 10
log
(
P min
)
10
log
(
P max
) + 10
log
(
P min
)
Where Pmax is conventionally de fi ned as the power amplitude of the third peak down from
the maximum excursion over the 3000 samples in the last minute, and Pmin is the power
amplitude of the third level up from the minimum excursion.
Columns 30, 44 and 58 provide a modi fi ed scintillation index where only the numerator is
computed (it is expressed in dB).
Spectral Slope and Strength
Columns 31, 45 and 59 provide the opposite of the slope (often denoted ‘ p ’ ) of the phase
PSD in log-log axes, where the PSD is obtained by computing the FFT of the 3000 detrended
phase samples in the last minute. The slope is computed by
fi tting a linear polynomial to the
PSD over the 0.1 to 25 Hz frequency range.
The fi gure below illustrates the way the slope is computed. I n the example depicted,
p
is
about 1.7.
Next to the spectral slope, the spectral strength (often noted ‘ T ’ ) is provided in columns 60
to 62. The spectral strength is the detrended phase power spectral density at 1.0 Hz, i.e. the
intercept of the linear polynomial described above.
59