Geoscience Reference
In-Depth Information
locations spaced by
n
PRT
c
/2 are compared to determine the correct range of the Doppler
estimates and presence of overlaid echoes. If one of the overlaid powers is larger than user
specified threshold (typically 5 dB) the corresponding Doppler spectral moment is assigned
to the correct range whereas the values at location of the other overlaid echoes are censored.
If the powers are within 5 dB, the variables at all locations where the overlay is possible are
censored. Because the Doppler spectral moments are computed and recorded only to the
distance of at most twice the unambiguous range the censoring is also done to that distance.
There is a special VCP (Zittel et al., 2008) with three scans at same elevation on five
consecutive lowest elevations whereby velocities from three PRFs (No. 4, 6, and 8 in table 2)
are combined to increase the
v
a
and display it
up to the distance of 175 km.
3.2.2 Scans at mid and high elevations
At elevations between 1.6
o
and 7
o
a “batch” sequence is transmitted. It is a dual PRF in
which the first few (3 to 12) pulses are at the lowest PRF and the rest (between about 25 and
90) are transmitted at one of the four highest PRFs (shortest PRTs, Table 2). The lowest PRF
pulses are for surveillance, reflectivity measurements, and censoring and assignment of
range to Doppler spectral moments; just the same as in the lowest scans. To improve
accuracy of the reflectivity estimates powers from the Doppler sequence (high PRF) are
included in the averaging provided there is no contamination by overlaid echoes. Beyond 7
o
elevation uniform PRTs are transmitted because the tops of storms at locations where
overlay can occur are below the radar beam.
3.2.3 Phase coding
To mitigate range overlay some volume scanning patterns at the lowest elevations (<2
o
)
have transmitted sequences encoded with the SZ(8/64) phase code (Sachidananda & Zrnic,
1999). The concept is depicted in Fig. 6 and explained in the caption. The prescribed phases
Ψ
k
(i.e., switching phases) are applied to the transmitted pulses. Formally this is represented
by multiplication of the sequence with the switching code
a
k
= exp(j
Ψ
k
). The first trip return
signal is made coherent by multiplying it with the conjugate
*
a
= exp(-j
Ψ
k
). With this
*
multiplication the 2nd trip signal is phase modulated by the code
. The 2nd trip can
be made coherent by multiplying the incoming sequence with
a
k-1
*, in which case the 1st trip
signal is modulated by the code
c
k
*. The code,
a
k
is designed such that the modulation code
c
k
has a phase shift given by
caa
k
k
1
k
2
. The special property of this code is that
its autocorrelation is unity for lags in multiples of 8 (lags 8
n
;
n
=0,1,2,...), and is zero for all other
lags. Therefore the power spectrum has only 8 non-zero coefficients separated by
M
/8
coefficients. The SZ(8/64) switching code is given by
8
k
4
k
k
1
k
k
2
a
exp[
j
(
m
/ 8)];
k
0,1, 2...63.
(5)
k
m
0
It has periodicity of 32 hence the number of samples
M
must be an integer multiple of 32. From
(5) it is obvious that the phase sequence consists of a binary sub multiple of 360
o
hence it is
generated without round-off errors using standard binary phase shifters. Because the
