USER’S MANUAL__________________________________________________________________
242 _________________________________________________________________ M211322EN-D
6.6.2 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" suffix), 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
difference as a first approximation of the unfolded result. The standard
algorithm uses that difference to unfold the velocity from the most recent
ray, which yields a lower variance estimate than the difference 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, the RVP900 also 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
whenever 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 different PRTs, and which avoids the SNR sensitivity of
the DPRT-1 width estimator.
6.7 Dual PRF Velocity Unfolding
For a radar of wavelength λ operating at a fixed sampling period τ
s
=
1/PRF, the unambiguous velocity and range intervals are given by:
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 λ for a given τ
s
. If the unambiguous
range interval is made sufficiently large by increasing τ
s
, then the resulting
velocity span may be unacceptably small.
The 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
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