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Introduction
Radar Wind Profiler (RWP) is the most suitable remote sensing
instrument for measuring the height profile of wind vector with high time and
height resolutions in all weather conditions. RWP depends upon the scattering
of electromagnetic (EM) energy by minor irregularities in the refractive index
(RI) of atmosphere. The RI is a measure of speed at which EM waves propagate
through a medium. A spatial variation in RI encountered by radio wave causes
a minute amount of the energy to be scattered in all directions. In the
atmosphere, minor irregularities in the RI exist over a wide range of sizes. RI
depends primarily upon the temperature, pressure and humidity of the
atmosphere. The atmosphere is in a constant state of agitation, which produces
irregular, small scale variations in the temperature and moisture over relatively
short distances. The wind, as it varies in direction and speed, produces
turbulent eddies, which produce variations in the RI of air that initiate the
scattering. As the irregularities (in RI) are carried by the wind, they prove to be
good tracers of the wind.
Due to their small aperture, UHF profilers operating around 900-1300
MHz are most suitable for measuring the winds in the boundary layer and lower
troposphere regions. Unlike the VHF wind profiling radars, UHF radars are very
sensitive for hydrometeors due to the small wavelength. Therefore these
profilers are very much useful in studying convection, precipitation etc. UHF
radar is a potential tool to carry out research studies such as ABL Dynamics
(Winds, Turbulence structure), Seasonal and Inter-annual variations, Interaction
between the ABL and the free troposphere, Precipitating systems, Bright band
Characterization, Rain/Cloud drop size distribution etc. It is also useful in the
operational Mountain meteorology and civil aviation and identification of
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Atmospheric ducts. It also acts as a supplementary tool to large VHF MST
radars by providing the atmospheric data in 0-5 km height range.
2. Technique
Radar Wind Profilers (RWP) derive information on the dynamical
atmospheric phenomenon by making use of variations in amplitude and
frequency of radio waves which are transmitted from radar system, back
scattered by the atmosphere and received
by the radar system again. RWP is coherent-pulse-Doppler radar whose target
is the atmosphere.
Doppler beam swinging (DBS) technique is used in RWP to derive the wind
vector. In this technique the narrow radar beam is switched in three or five
non-coplanar directions, as shown in figure-2, in a fixed sequence. The mean
Doppler obtained in all beam directions are used to compute the three
components of the winds as follows
Where xi , yi , zi are angles of ith
beam with X, Y and Z axis respectively and
VDiis the radial wind measured in beam direction i.
=
ZiDi
YiDi
XiDi
iii
iii
iii
z
y
x
V
V
V
ziziyizixi
ziyiyiyixi
zixiyixixi
V
V
V
cos
cos
cos
1
2
2
2
coscoscoscoscos
coscoscoscoscos
coscoscoscoscos
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Figure-2: Doppler Beam Swinging (DBS) Technique
3. NARL LAWP Radar System
Most important feature of NARL LAWP is the active array configuration. In this
configuration, which is shown in figure-3, each element of the planar microstrip
patch antenna array is fed directly by dedicated low power solid-state
transceiver module consisting of a power amplifier (PA) and LNA connected to
the common antenna port through a circulator. A transmit/receive (T/R) switch
switches the input port between the PA and LNA. These transceiver modules
are made with commercially available communication components, making
them low cost and affordable. Signal-to-Noise Ratio (SNR), thereby the range
performance is significantly improved as the feed loss is eliminated. This
configuration reduces the antenna size significantly (at least by a factor of 4-6)
when compared to a conventional passive array system for the given range
performance and makes the wind profiler compact and transportable. The
second important feature of this system is the utilization of a low power two-
dimensional passive multi-beam forming network, which simplifies the beam
formation. This network distributes the radar exciter output signal and feeds
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the transceivers with appropriate amplitude and phase distribution. Beam
switching is done by controlling a solid state single-pole-multi-through switch.
Main advantage of the passive beam forming network is that it avoids the need
for periodic phase calibration. In the receive mode, the outputs from the
transceivers are appropriately weighted, phase shifted, combined in the beam
forming network and delivered to the receiver. Other features include pulse
compression scheme and direct IF digital receiver. Pulse compression scheme,
incorporated into the system, enhances the height coverage without affecting
the range resolution. Direct IF digital receiver is used for achieving better
dynamic range, flexibility and programmability.
(i) Antenna Array:
(a) Transportable (8x8x array) system:
The planar array consists of 64 microstrip patch ANT elements arranged in a
8x8 matrix over an area of 1.4 m x 1.4 m. The patch elements are fabricated
using 125-mil RT/Duroid 5870 substrate. Patch element, designed for linear
polarization, is rectangular in shape having dimensions of 73.3 mm x 73.1 mm
with a ground plane of 92 mm x 92 mm size and incorporated with mounting
holes at the four corners so as to be fitted on to an aluminum ground panel.
Designed with coaxial probe feed, the antenna elements found to have return
loss better than 15 dB over the band width of 15 MHz. Cross polarization level
of the patch elements is measured to be better than 23 dB. Gain of the
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elements is measured to be 6.5 dB with a half power beam width more than 70
in cardinal planes. 8 elements, fitted on to an Aluminum ground panel along H-
plane, form the basic linear array panel. A preliminary Radome, fabricated with
a fiber reinforced plastic (FRP), is fitted on to the linear array panel for
environmental protection. Figure 6ashows photograph of the microstrip patch
antenna and figure 6b shows 8-element linear array panel with (top) and
without (bottom) preliminary Radome. The effect of the FRP Radome is found to
be negligible on the radiation characteristics. An inter-element spacing of
0.73, where is the operating wavelength, is chosen to have an optimal
compromise between the required minimum beamwidth and maximum grating-
free steer angle. 8 such linear panels are laid along E-plane to construct the full
planar array. Dummy narrow Aluminum panels are used as spacers between
the linear antenna panels in order to realize the same inter-element spacing
along the E-plane. The antenna array is installed on the roof of the shelter,
where instrumentation is kept. The bottom side of the
linear array panels and dummy panels, which are contiguously laid, acts as a
firm composite ground plane for the entire planar array. Figure 7 shows the
picture of the planar array mounted on the roof of the shelter. Figure 8 shows
the entire planar array covered by a secondary FRP Radome. An aluminum
grounded clutter suppression fence is installed surrounding the antenna array.
Radiation pattern is shown in figure-9.
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(ii) Transceiver Modules:
Solid-state Transceiver modules (TMs), each with a peak output power of 10 W,
are used to feed the antenna elements directly. 64/256 TMs are employed for
transportable/main LAWP systems. These TMs, placed below the antenna
ground plane, are designed to have uniform amplitude and phase
characteristics. The functional block diagram of a single TM is shown in figure
16. Each TM consists of a Tx/Rx (T/R) switch at the input, Tx section, Rx front-
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end (FE) section and a circulator (CIR) and bi-directional coupler (BDC) at the
output. The Tx section consists of a phase trimmer, a driver amplifier (DA) and
a final gain-controlled power amplifier (PA) module. Rx FE section comprises of
a passive diode limiter (LIM), a blanking (BLK) switch, low noise amplifier (LNA)
with gain adjustment pad and a phase trimmer. Commercially-off-the-shelf
available components are used to build the TMs to make them low cost.
Mitsubishi RA18H1213G MOSFET CW 15 W amplifier is modified for pulsed
application and used as PA. The gate voltage is switched in synchronization
with the Tx pulse to avoid the continuous drain current flow there by avoiding
the heat dissipation. Advanced Semiconductor Business Solutions make ALN
1280, which has noise figure (NF) of 0.7 dB, is employed as LNA. This device,
which has a gain of 28 dB and output compression point of 16 dBm is used for
DA also. Varactor diodes are used in the phase trimmers of both Tx and Rx
sections. RCPL E3NG and Anaren 11305-20 are used as CIR and BDC,
respectively. CIR connects the output signal from the Tx section to the antenna
and the antenna to the Rx section. The Tx-Rx isolation achieved is about 90 dB
(25 dB and 65 dB due to circulator and blanking switch, respectively). BDC is
used to monitor the Tx output
during Tx time and to inject a test signal into the Rx section. The loss due to
the circulator and BDC is about 1 dB. Though TMs, produced in the mass
production, are expected to have uniform phase and amplitude characteristics,
differential errors do exist due to assembling process. A provision is made to
adjust the phase and gain to the tolerable limits in both Tx and Rx paths to
compensate these errors. Phase trimmers are kept to achieve the uniformity in
phase among all the TMs in both Tx and Rx paths. Gain can be adjusted in the
Tx section by varying the gate bias to the power amplifier, and in the Rx
section by fixing attenuator pads with appropriate value at the output of LNA.
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Tx and Rx sections are realized on FR4 substrates and mounted on the top and
bottom sides, respectively, of a milled Aluminum structure. Figure 17 shows the
picture of Tx (left) and Rx (right) sides of the TM. A control and safety circuit,
located on the Rx side, ensures safe operation of TM. Entire TM, realized in a
compact Aluminum enclosure weighing about 400 gm, with dimensions 98 mm
x 102 mm x 33 mm, shown in figure 18, operates with a single 12.5 V DC
power supply. It has three RF interface ports namely, Tx-in/Rx-out port, ANT
port and test port. The control port receives the timing signals to control the PA
gate on/off, BLK and T/R switches.
A Tx pulse at -10 dBm level is fed to the TM to generate 10 W at the ANT port.
The pulse width (PW) and the inter-pulse-period (IPP) are in the range 0.25
4.0 s and 20 200 s, respectively, with duty ratio (DR) not exceeding 10%.
In the Rx mode, the signal received from the ANT element is amplified by the
FE section with a net gain of 25 dB. Coupled ports of the BDC are monitored for
forward and reflected signal power levels. A fraction of the forward path
voltage is brought to outside for testing Tx and Rx paths. The control and
safety interlock circuit, in addition to controlling different
switches, continuously monitors the temperature, output power and VSWR and
generates interlock if any parameter goes beyond the preset limits, and cuts
off the gate bias to the PA, thus ensuring safety. The 24 cm-long coaxial cable,
used between the TM and antenna element, which introduces a loss of about
0.2 dB, is considered as an integral part of the TM in Tx and Rx path
characterization. TMs are set for the same Tx power level and insertion phase
with maximum deviations of0.5 dB and 5, respectively. Similarly in the Rx
path, the gain and insertion phase of the TMs are adjusted to be uniform with
maximum deviations of0.5 dB and 2, respectively. NF values of the Rx path
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with reference to ANT port are measured to be in the range 3.5-4 dB though
the design goal was 3 dB. TMs are suspended below the antenna panels by
fixing them to the array ground plane adjacent to the corresponding ANT feed
points. This configuration, where the TMs located very close to the ANT
elements, will result in the enhanced Tx power availability (at the antenna
plane) and keeps the overall system noise figure at minimum value. The net
result is the significant enhancement in the SNR at the receiver output when
compared to a passive array system.
The transfer curve for the TR module is shown in figure-19.
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4. Data Processing
Host PC performs data cleaning and process the time domain data to estimate
the physical parameters like mean Doppler, Doppler width and 3-D wind vector.
Time domain data is first subjected to DC removal, to eliminate any fixed DC
offset in the complex time domain signals. The in-phase and quadrature phase
signals are averaged separately and the respective mean values are subtracted
to eliminate the DC offset. However, the slowly varying (low Doppler) clutter
due to undesired echoes like reflections from trees, power lines swaying in the
wind etc., will be still present. A technique called de-trending (May and Strauch,
1998) is used, in the time domain, to remove the slowly varying (low Doppler)
clutter signals. In this technique, a long series (1024 points) of time domain
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data is divided into small segments of length (256 or 128 points). A linear trend
is approximated to each segment as a straight line and is removed from the
original data. This removes the clutter effectively. Figures 8a and 8b illustrate
the de-trending process for a single range bin and its efficacy in cleaning the
time domain data. It may be noted from figure 44a that the original in-phase
(top) and quadrature (bottom) components, shown in gray shade, are
associated with strong DC shift as well as strong but slowly varying clutter
signals. It can be clearly seen that the DC and clutter, present in the original
signal, are completely removed (black lines) after de-trending process.
Corresponding Doppler spectrum is shown in figure 44b, before (top) and after
(bottom) de-trending. The strong peak with narrow width, which is
corresponding to DC and clutter is removed in the de-trending process. After
de-trending, the time series data is subjected to either rectangular or Hamming
or Hanning windowing. In the routine operation, Hanning window is selected for
its optimal performance. The complex time
domain data, after windowing, is converted into Doppler spectrum by applying
complex FFT and computing the power spectrum. Incoherent integration is then
performed, if necessary, where several successive spectra are averaged to
improve detectability. Cutter, alternately, can be removed in the frequency
domain also by taking out a significant number of points on both sides of the
zero Doppler (Barth et al, 1994) and these points are replaced by the value
obtained by averaging the two points bracketing the area being removed and
by interpolating. This is illustrated in figure 45. The number of points to be
replaced is dynamic for each range-bin. Any of the two clutter removal
techniques (either in time domain by de-trending or in frequency domain by
interpolating) can be selected. Presently frequency domain technique is
followed in the processing. Gadanki site is surrounded by mountains, which
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contaminate the data by strong DC and clutter. Figure 8c shows the stacked
range Doppler spectra for uncoded modebefore (left) and after (right) the DC
and clutter removal. It can be clearly seen that the clutter signal is masking the
atmospheric signal. In the processed spectra the atmospheric signal is clearly
visible and a continuous trace can be seen through out the height range.