Vaisala RVP900 - DPRT-2 Mode; Dual PRF Velocity Unfolding

7.7.2 DPRT-2 Mode
The trigger consists of alternating short and long period pulses, where the ratio of the
periods is determined by the velocity unfolding ratio that has been selected. Doppler data
are extracted from both the short and long pulse pairs (hence the "-2"
sux), and unfolded
velocities are made available on each ray based on the combined PRT data from that ray
alone. DPRT-2 mode is intended for rapidly scanning radars where the ray-to-ray spatial
continuity assumptions of the traditional Dual-PRF algorithms do not apply.
The DPRT-2 velocity unfolding algorithm uses a
modified version of the standard Dual-PRF
algorithm. Both start by computing a simple velocity
dierence as a first approximation of
the unfolded result. The standard algorithm uses that dierence to unfold the velocity from
the most recent ray, which yields a lower variance estimate than the
dierence itself. The
DPRT-2 algorithm is similar, except that the folded velocity from both PRTs are unfolded
independently and then averaged together.
In addition to the above, RVP900 computes the DC average of the (I,Q) data within each
bin. This is used as a simple estimate of clutter power, so that corrected reflectivities are
available in DPRT-2 mode when a non-zero clutter filter is selected. DPRT-1 mode is the
same in this respect. However, the DPRT-2 widths use an improved algorithm based on the
two
dierent PRTs, and which avoids the SNR sensitivity of the DPRT-1 width estimator.
7.8 Dual PRF Velocity Unfolding
For a radar of wavelength ʎ operating at a fixed sampling period Ʈ
s
= 1/PRF, the
unambiguous velocity and range intervals are given by:


=

4

 

= 


2
where c is the speed of light. Often these intervals do not fully cover the span of velocity
and range that one would like to measure. The problem is generally worse for short
wavelength radars, since that unambiguous velocity span is directly proportional to ʎ for a
given Ʈ
s
. If the unambiguous range interval is made
suciently large by increasing Ʈ
s
, then
the resulting velocity span may be unacceptably small.
RVP900 provides a built-in mechanism for extending the unambiguous velocity span by a
factor of two, three, or four beyond that given above. The technique, called Dual PRF
velocity unfolding, uses two pulse periods rather than one, and relies on the extra
information thus obtained to correct (that is, unfold) the mean velocity measurement from
each individual period. The Dual PRF trigger pattern consists of alternating (N+k)-pulse
intervals where the period in each interval is either Ʈ
l
(for the low-PRF) or Ʈ
h
(for the high-
PRF). Here N is the sample size, and k represents a delay that permits the clutter
filter to
equilibrate to the new PRF after each change. The clutter
filter impulse response lengths
vary according to which filter is selected.
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