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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.

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