Vaisala RVP900, Руководство пользователя

Страница: 189 / 484

More aggressive windows have lower side lobe power at the expense of a broader impulse
response and an increased variance of the moment estimates.
Po
w
e
r
Frequency
0
-M/2 M/2
Side Lobes
Window Width
Figure 40 Impulse Response of Typical Window
In summary of the DFT approach and spectrum windows:
• When the clutter is strong, an aggressive spectrum window is required to contain the
clutter power so that the side lobes of the window do not mask the weather targets.
The side lobe levels of some common windows are:
• Rectangular 12 dB
• Hamming 40 dB
• Blackman 55 dB
• More aggressive windows typically have a wider impulse response. This effectively
increases the spectrum width. Rectangular is narrow, Hamming intermediate, and
Blackman the widest.
• Windows effectively reduce the number of samples resulting in higher variance
moment estimates. Rectangular is the best case, Hamming is intermediate, and
Blackman provides the highest variance moment estimates.
The best approach is to use the least aggressive window possible in order to contain the
clutter power that is present. That is, an adaptive approach is the best.
7.3.3 Autocorrelation for Moment Estimations
The final spectrum moment calculation (for total power or SNR, mean velocity and spectrum
width) in all processing modes is based on autocorrelation moment estimation techniques.
Typically the first three lags are calculated, denoted as
R
0
, R
1
and R
2
. There are two ways to
calculate these, that is, time domain or frequency domain calculation.
• In PPP mode for dual polarization, the autocorrelations are computed directly in the
time domain.
Chapter 7 – Processing Algorithms
187