Geoscience Reference
In-Depth Information
estimated from the change in phase of the returned signal (Doviak & Zrnic 2006). Thus
the WSR-88D is a phase sampling and measuring instrument. The change in phase of the
return from one pulse to the next 2
πf
d
T
s
is proportional to the Doppler velocity
v
as
indicated in (2).
If the phase change caused by precipitation is outside the -
π
to
π
interval it cannot be
easily distinguished from the change within this interval. These limits define the
unambiguous frequency
f
a
= 1/(2
T
s
) and through the Doppler relation (2) the
unambiguous velocity as
v
a
=
λ/
(4
T
s
).
(4)
Scatterers do cause a Doppler shift within the pulse as it is propagating and reflecting, but
this shift is very small and can not be measured reliably as the following argument
demonstrates. Consider the
τ =
1.57 μs pulse width (WSR-88D) and scatterers moving at 10
m s
-1
(36 km h
-1
). The corresponding Doppler frequency shift is 200 Hz (at 10 cm
wavelength) and it produces a phase difference of 0.11
o
(2
πf
d
τ
) between the beginning and
end of the pulse return. This tiny difference can not be measured with sufficient accuracy to
yield useful estimate.
To mitigate the ambiguity problem the WSR-88D has some options one of which is special
phase coding and processing. The result is seen in Fig. 2 where the pink ring at 137 km
indicates the unambiguous range for velocity measurements (see discussion in section 3.2.3);
it represents censored data because the ground clutter from nearby range and weather
signals from the second trip range are comparable in power and can not be reliably
separated.
Operators of the WSR-88D have at their disposal preprogrammed volume coverage patterns
(VCP - see example in Fig. 2). These are consecutive scans starting from elevation of 0.5
o
and
incrementing until a top elevation is reached. Most algorithms require a full volume scan to
generate a product. The one in Fig. 2 (bottom left) reconstructs a vertical profile of Doppler
velocities along a radial; the radar is located to the right and green colors indicate velocities
toward the radar in 5 m s
-1
increments starting with 0 (gray color). Cylindrical protrusion
below 5 km in the middle with some velocities toward the radar (red color) is indicative of a
tornado.
3. Signal processing and display
The block diagram (Fig. 3) of the WSR-88D radar is typical for pulsed Doppler radars.
Essential components are the Frequency and Timing generator, the transmitter and the
receiver. Radar and antenna controls are omitted from the figure. Intermediate frequency (if)
on the radars is 57.6 MHz, and the local oscillator (lo) frequency is adjustable to cover the
range between 2.7 and 3 GHz (the operating band, see Table 2). The power amplifier is a
klystron. The transmit/receive switch is comprised of a circulator and additional devices to
protect the receiver from the transmitted high power pulse. The low noise amplifier (LNA)
has a noise figure ~ 0.8 dB and the receiver bandwidth is 6 MHz up to the input of the
digital receiver. The digital receiver is a proprietary product of SIGMET Co (now Vaisala)
and its essence is described next.
