Vaisala RVP900, Руководство пользователя

Страница: 227 / 484

Although valid widths are obtained independently on all rays, those measured at low-PRF
are larger than those at high-PRF. This is because the dimensionless width units are with
respect to a larger velocity interval in the latter case. To compensate for this, low-PRF widths
are multiplied by either 2/3 or 3/4 before being output. This puts them in the same scale as
the high-PRF values, and thus, the widths do not vary on alternate pulses. A useful
consequence of this is that width data can be sent directly to a color display generator
without having to plot every other ray in a different scale.
A Few Words of Caution Regarding Dual PRF Processing
The unfolding algorithms make the assumption that targets are more-or-less continuous
from ray to ray. Otherwise, it would not make sense to use data from a previous ray to unfold
velocities in the current ray. You must ensure that their antenna scan rate and beamwidth
are such that each target is illuminated, at least partially, over each full
2(N+k) -pulse
interval. In practice, a certain amount of decorrelation from ray to ray is acceptable, since the
previous rays are used only to decide into which unfolded interval the current ray should be
placed. Small errors in the previous ray data, therefore, cause no error in the output.
However, large previous-ray errors would lead to incorrect unfolding.
A more subtle side effect of Dual PRF processing arises from clutter filtering because clutter
notches now appear at several locations in the unfolded velocity span, rather than just at
zero velocity. These additional rejection points come about because the original velocity
intervals are mapped some integer number of times to create the unfolded interval.
Since each original interval has a clutter notch at DC, it follows that the final expanded
velocity interval has several such notches. For example, in the 3:2 case, in addition to
removing DC the clutter filter removes velocities at - 2V
u
/3, + 2V
u
/3, and V
u
.
These clutter filter "images" are a consequence of the Dual PRF processing technique and
are not easily removed. They can cause trouble not only for the velocity unfolding itself,
but because the computed clutter corrections to be wrong at the image points.
To minimize their impact, turn the clutter filter off at far ranges where little clutter is
expected and use a narrow clutter filter minimizes the effects of the clutter filter on
weather targets.
The 4:3 and 5:4 PRF unfolding ratios are more susceptible to unfolding errors in cases where
the spectrum width is large and/or the SNR is low. You must experiment with these ratios to
determine which provides the best results for their particular application. Although the
RVP900 trigger generator can produce any trigger frequency, only the 3:2, 4:3, and 5:4
ratios can be used with the built-in unfolding algorithms. The RVP900 still permits other
PRT ratios to be explored, but the unfolding technique must then be manually programmed
on your host computer.
Example
The following example shows 7 possible oscilloscope traces (and their associated
probabilities) for the RVP900 trigger during Dual PRF operation.
The PRF ratio is 4:3, and the sample size is 50 pulses at the high PRF, and 37 pulses at the
low PRF. The signal labeled
SCOPE is the composite of these traces, and is what is shown on
an oscilloscope.
Chapter 7 – Processing Algorithms
225