Vaisala RVP900, Руководство пользователя

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The output width
W
is defined as in 7.4.5.1 R0, R1 Width Algorithm (page 204) .
7.4.6 Signal Quality Index (SQI threshold)
Using the signal quality index (SQI), RVP900 can eliminate signals which are either too weak
to be useful, or which have widths too large to justify further analysis.
SQI is defined as:
��� =
�
1
�
0
The SQI is the normalized magnitude of the autocorrelation at lag 1 and varies between 0 for
an uncorrelated signal (white noise) to 1 for a noise- free zero-width signal (pure tone).
Mean velocity estimates are degraded when the spectrum, width is large or when the signal-
to-noise ratio is weak.
The SQI is a good measure of the uncertainty in the velocity estimates and is a convenient
screening parameter to compute.
For very large signal-to-noise ratios (SNR) the SQI is a function of the spectrum width only.
For a zero-width pure tone (
W=0 ), the SQI is a function of the SNR only (for example, for
W=0 , an SNR of 1 corresponds to SQI=0.5). The SQI threshold is typically set to a value of
0.4 ... 0.5.
In terms of the Gaussian model, the SQI is :
��� = ���
��� + 1�
−�2�2
2
where the SNR is the signal-to-noise ratio.
7.4.7 Clutter Correction (CCOR threshold)
The following content is based on legacy implementation in which
R
1
, and
R
2
are
defined in terms of PPP (pulse-pair-processing).
Current major modes use FFT ( DFT) processing instead.
In addition to calculating the
R
0
,
R
1
and optional
R
2
autocorrelation terms, which are based
on filtered time series data, RVP900 computes
T
0
, which is the total unfiltered power.
By comparing the total filtered and unfiltered powers at each range bin, a clutter power, and
hence a clutter correction, for that bin can be derived. The clutter correction is defined as,
���� = 10log �
� + � = 10log
1
��� + 1
where S is the weather signal power, C is the clutter power and CSR is the clutter-to-signal
ratio.
Chapter 7 – Processing Algorithms
205