Vaisala RVP900 - Figure 49 Dual PRF Concepts

l h
Result
l
3
Region III
Region II
Region I
esaC FRP hgiHesaC FRP woL
Region I
Region II
l h
Result
h
2
Figure 49 Dual PRF Concepts
This technique is illustrated in the previous figure. The figure shows how the low-PRF and
high-PRF angles are unfolded based on the dierence angle. The diagrams show phase
planes representing the large unfolded velocity interval, and the locations of vectors on
those planes. Referring
first to the right figure, the dierence angle is plotted, and the plane
is divided into two equal size regions, one of which is centered on the dierence vector. The
high-PRF angle is then divided by two and plotted. The resultant unfolded velocity angle
must either be this vector, or this vector plus . Since adding places the vector into
acceptance Region 1
where it is nearest the
dierence angle, we conclude that this is the correct unfolding.
Likewise, on the left diagram we unfold the low-PRF angle by dividing the plane into thirds
centered on the
dierence angle. The result angle is either


3
,


3
, +
2
3



3
+
4
3
depending on which one falls into the acceptance Region 1. The resultant angle is the
same in each case.
RVP900 makes
ecient use of the incoming data by unfolding velocities from both the low
and the high-PRF data, making use each time of information in the previous ray. When low-
PRF data are taken the derived velocities are unfolded by combining information from the
previous high-PRF interval. Likewise, when high-PRF data are acquired the velocities are
unfolded based on the previous low-PRF interval. Thus, when operating in the Dual PRF
mode, RVP900 outputs one data ray for each (N+k)-pulse interval. However, the velocity
data in the Dual PRF rays are unfolded, so that the [-1,+1] interval now represents either two
or three times the prior velocity range. Put another way, the data are still interpreted as
described in the section on mean velocity estimation, except that V
u
is now larger.
Width Data
The width data are
modified somewhat during Dual PRF unfolding.
RVP900 User Guide M211322EN-J
224