Polarimetric Phased-Array Radar for Weather Measurement: A

Polarimetric Phased-Array Radar for Weather Measurement: A Planar orCylindrical Configuration?

GUIFU ZHANG

School of Meteorology, and School of Electrical and Computer Engineering, and Atmospheric Radar Research Center,

University of Oklahoma, Norman, Oklahoma

RICHARD J. DOVIAK AND DUSAN S. ZRNIC

School of Meteorology, and School of Electrical and Computer Engineering, University of Oklahoma, and NOAA/National

Severe Storms Laboratory, Norman, Oklahoma

ROBERT PALMER

School of Meteorology, and Atmospheric Radar Research Center, University of Oklahoma, and NOAA/National

Severe Storms Laboratory, Norman, Oklahoma

LEI LEI

School of Electrical and Computer Engineering, and Atmospheric Radar Research Center, University of

Oklahoma, Norman, Oklahoma

YASSER AL-RASHID

Lockheed Martin Corporation, Moorestown, New Jersey

(Manuscript received 22 March 2010, in final form 10 September 2010)

ABSTRACT

This paper suggests a cylindrical configuration for agile beam polarimetric phased-array radar (PPAR) for

weather surveillance. The most often used array configuration for PAR is a planar array antenna. The planar

configuration, however, has significant deficiencies for polarimetric measurements, as well as other limita-

tions, such as increases in beamwidth, decreases of sensitivity, and changes in the polarization basis when the

beam scans off its broadside. The cylindrical polarimetric phased-array radar (CPPAR) is proposed to avoid

these deficiencies. The CPPAR principle and potential performance are demonstrated through theoretical

analysis and simulation. It is shown that the CPPAR has the advantage of a scan-invariant polarization basis,

and thus avoids the inherent limitations of the planar PPAR (i.e., PPPAR).

1. Introduction

In addition to military applications for target rec-

ognition and tracking (Brookner 2008), phased-array

radar (PAR) technology has recently been successfully

introduced to the weather community. The nation’s first

phased-array weather radar, the National Weather Radar

Testbed (NWRT) operating at a wavelength of 9.38 cm,

was developed in Norman, Oklahoma, through a joint

effort of a government–university–industry team (Zrnic

et al. 2007). The NWRT demonstrated that its pulse-to-

pulse electronic beam-steering capability enables meteo-

rological measurements that are as accurate in shorter

storm surveillance times as those achieved with a conven-

tional dish antenna with a mechanically steered beam. The

shorter surveillance times result in faster data updates and

offer the capability to observe detailed evolutions of se-

vere storm phenomena (Yu et al. 2007; Heinselman et al.

2008). The NWRT also has a hybrid capability to both

mechanically and electronically steer the beam. This ca-

pability has allowed multipattern measurements of the

Corresponding author address: Dr. Guifu Zhang, School of

Meteorology, University of Oklahoma, 120 David L. Boren Blvd.,

Suite 5900, Norman, OK 73072.

E-mail: [email protected]

JANUARY 2011 Z H A N G E T A L . 63

DOI: 10.1175/2010JTECHA1470.1

Unauthenticated | Downloaded 02/17/22 08:23 AM UTC

same meteorological volume to successfully mitigate both

stationary and moving clutter (Zhang et al. 2011). Fur-

thermore, the NWRT uses an antenna from the AN/

SPY1-A monopulse radar of the Aegis system (Sherman

1988), which has sum and difference channels; these can

be combined to implement a spaced-antenna interferom-

etry (SAI) technique for crossbeam wind measurement

(Zhang and Doviak 2007) and subvolume inhomogeneity/

object detection (Zhang and Doviak 2008). It has been

also theorized that the AN/SPY1-A auxiliary channels

could support the implementation of adaptive clutter

cancellation techniques (Le et al. 2009).

While PAR technology has recently received wide-

spread attention in the weather community, weather radar

polarimetry has matured to a point that it is being imple-

mented on the national network of Weather Surveillance

Radar-1988 Doppler (WSR-88D) radars (Doviak et al.

2000) using its conventional dish antenna. Polarimetric

radar provides multiparameter measurements that reveal

detailed microphysics of storms in addition to hydrome-

teor classification, accurate precipitation estimation, and

improved weather nowcasts. Therefore, the weather com-

munity and the nation expect that the future multifunction

phased-array radar (MPAR) will retain all of the capa-

bilities of the polarimetric WSR-88D (Smith et al. 2008).

It is the polarimetric capability that the second MPAR

symposium (online at http://www.ofcm.noaa.gov/mpar-

symposium) identified as the most challenging technical

issue that the community is facing.

The challenge comes from the fact that highly accurate

polarimetric radar measurements are required to provide

meaningful information. However, biases inherent to pla-

nar polarimetric phased-array radar (PPPAR) exist and

can be larger than the intrinsic values if the beam is di-

rected away from the planar array’s broadside. For ex-

ample, the intrinsic ZDR values range only from about

0.1 dB for drizzle and dry snow to 3–4 dB for heavy rain

and large drops. Thus, it is desirable that the measurement

error for ZDR be of the order of 0.1 dB (Zhang et al. 2001;

Brandes et al. 2003). However, the ZDR bias for a PPPAR

can be a few decibels (Zhang et al. 2009a). Hence, it is

crucial for the success of the MPAR project that the sys-

tem configuration for a PPPAR is selected correctly and

designed optimally.

Currently, the possible antenna array configurations

include linear array, planar array, circular/cylindrical ar-

ray, and spherical array. The linear array needs one me-

chanic rotation for weather surveillance like the rapid

Doppler on Wheels (rapid DOW; see Wurman 2003) and

the proposed design for the Center for Collaborative

Adaptive Sensing of the Atmosphere (CASA) instrument

(Hopf et al. 2009). For the planar array, multiple faces

(normally four) are needed (e.g., the SPY-1A), but the

planar array has sensitivity loss and polarization bias if

the beam points away from the broadside (Zhang et al.

2009a). A circular or cylindrical configuration has been

used for direction finding and communications (Royer

1966; Raffaelli and Johansson 2003). For satellite com-

munication applications, the spherical array is optimal and

flexible in its use of the antenna aperture size and in its

symmetry (Tomasic et al. 2002). For weather applications,

however, the spherical array cannot provide the high

cross-polar isolation required to accurately measure pre-

cipitation.

In this study, we examine cylindrical polarimetric phased-

array radar (CPPAR) for practically scan-invariant weather

measurements. The CPPAR has azimuth scan-invariant

properties and has very minor dependence on elevation at

low elevation angles. Because the WSR-88D scan strategy

has coarser elevation sampling at higher elevation angles,

and because the CPPAR’s angular resolution coarsens as

beam elevation angle increases (thus filling angular gaps

created by coarser sampling), the gradual decrease in ele-

vation resolution is a beneficial feature. In section 2 the

meteorological measurements with a PPPAR are simulated

and the PPPAR’s deficiencies are revealed. In section 3, a

cylindrical array configuration is described and the CPPAR

performance is quantified through a theoretical analysis and

simulation. An example design and simulation results are

given in section 4. Summary and discussions are provided in

the last section.

2. Issues with a PPPAR

For a PPPAR, three or four faces are normally used to

cover the 3608 in azimuth. Because the antenna faces and

their broadside directions are fixed, the beam and po-

larization characteristics change depending on the elec-

tronic beam direction. To obtain an intuitive impression,

we show KOUN (a prototype dual-polarized WSR-88D)

measurements of reflectivity and differential reflectivity

in Fig. 1. Also shown are the simulated measurements

that would be made by a four-face PPPAR comprising an

array of crossed dipoles, and those if each face had the

same size aperture as KOUN. Crossed Hertzian dipoles

are used for sake of simplicity without having to specify

the design and size of the array elements. Although other

types of antenna elements (e.g., a patch) can be simu-

lated in a similar way, detection performance would be

worse because these elements have increased directive

gain and thus have a larger scanning loss. Furthermore,

ZDR bias would still exist, but it is correctable (Zhang

et al. 2009a).

It can be seen that the reflectivity and differential re-

flectivity measurements are the same as those of KOUN at

the four broadside directions. However, the measurements

64 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 28


are considerably different at beam positions away from

the broadsides: (i) ZH sensitivity is lost and (ii) ZDR has

significant bias. There is a 7.5-dB sensitivity loss for the

horizontally polarized beam if it is 458 from the broad-

side (it would be a 15-dB loss if the three-face PPPAR

were used and the beam was 608 from the broadside).

This is due to the combined effect of the decreased gain

of the horizontal dipole radiation pattern and the de-

creased gain of the array caused by the smaller projected

aperture in directions away from broadside. In addition

to the 7.5-dB sensitivity loss for horizontally polarized

waves, vertically polarized waves also have some loss

(i.e., 1.5 dB) for beams steered to 458 from broadside.

Although the biases in the reflectivity factor and differ-

ential reflectivity seen in Figs. 1c,d are correctable, sen-

sitivity loss can only be compensated by increased power

or aperture size.

The scan-variant measurements are unacceptable to

meteorologists unless the data are calibrated to remove

the radar system effects (Zhang et al. 2009a). Although

the ZDR bias can be compensated, the sensitivity lost and

worsening angular resolution is costly to recover. One

might think to use the vertical polarization for the prime

reflectivity measurements, but there will still be loss due to

the projected area being diminished, and the azimuthal

resolution will be worse byffiffiffi2p

. Furthermore, the lost sen-

sitivity and angular resolution for other polarimetric mea-

surements (ZDR, rhv, and fDP) is also costly to recover

because the antenna size and/or transmitter power both

need to be increased.

For meteorological applications, the four-face PPPAR’s

gain and beamwidth changes are most significant for beams

that are electronically steered in azimuth. To compensate

for the 7.5-dB loss (i.e., for horizontally transmitted waves

at the azimuth limits of 6458), without increasing trans-

mitted power, the antenna aperture would need to be in-

creased by a factor of 5.66 in the horizontal direction; this

is clearly a prohibitively expensive solution. Alternatively,

to ensure that the beamwidth at 6458 is practically equal

to the beamwidth in elevation, and is no worse than the

beamwidth of the WSR-88D, the antenna size in the

horizontal direction must be increased by a factor offfiffiffi2p

;

FIG. 1. A comparison of images obtained with (a),(b) a mechanically steered beam of the polarimetric KOUN radar

and that obtained with (c),(d) a simulated four-face PPPAR and its electronically steered beam.



however, in this case the transmitter power would need to

be increased by a factor of 4 to not lose detection capa-

bility. Thus, the aperture size of the planar PPAR needs to

be 8.54/cos(108) m in the vertical plane, and 12.08 m in the

horizontal, which is significantly larger than the WSR-88D

reflector diameter. The dimension in the vertical plane

assumes that the array face is tilted 108 from the vertical to

provide a vertical beamwidth at zero degrees of elevation

to match the WSR-88D’s beamwidth. Clearly there is

motivation to decrease the array size while matching the

WSR-88D capabilities, and with this in mind the cylin-

drical array is examined.

3. Configuration and formulation for CPPAR

Realizing the deficiencies of a PPPAR for weather

measurements, we propose a CPPAR (see Zhang et al.

2009b).

As sketched in Fig. 2, there are M 3 N dual-polarized

radiating elements arranged azimuthally (M) and axially

(N) on the surface of a cylinder. Multiple simultaneous

beams are formed with each beam generated from a sector

of the cylindrical surface with the broadside direction along

the bisector of the illuminated sector. Using CPPAR, po-

larization orthogonality is preserved in all directions.

To study the radiation characteristics of a CPPAR, we

choose a coordinate system with its z direction along the

cylinder’s axis (Fig. 3). An array element (mn: mth row,

nth column), comprised of crossed h and v dipoles is lo-

cated at fn, zm on the cylindrical surface at rmn 5 axR

cosfn 1 ayR sinfn 1 azzm, where R is the cylinder’s ra-

dius, the row height zm ranges from 2D/2 to 1D/2, where

D is the axial length of the cylindrical array (equal to the

diameter D of the WSR-88D), and the bold unit vectors

represent the Cartesian coordinates (ax, ay, az) (Fig. 3). Az-

imuth location fn is measured relative to the x axis and is

fn 5 nDf, n 5 1, 2, 3, . . . . The electric field at r 5 axr sinu

cosf 1 ayr sinu sinf 1 azr cosu, transmitted by the mnth q

(i.e., q 5 h or v) dipole, is (Ishimaru 1997, section 2.4)

E(q)mn(r) 5 � k2e�jkjr�r

mnj

4p« r� rmn

�� M(q)mn(u, f), (1a)

where k 5 2p/l, l is the radar wavelength, « is the

permittivity for an assumed uniform precipitation-free

atmosphere,

M9(q)mn (u, f) 5 ar 3 [ar 3 M(q)

mn] (C m21), (1b)

where M9(q)mn is the moment of dipole q at location mn,

and ar is the unit vector along r.

Using the far-field approximation, we have the elec-

tric field at r radiated by the mnth q dipole

E(q)mn 5

E(h)mn

E(v)mn

" #’ Aejk[z

mcosu1Rsinucos(f�f

n)] M9(h)

mn

M9(v)mn

" #,

(2a)

where

A [�k2e�jkr

4p«r. (2b)

In this paper superscripts (h) and (v) are used to identify

the horizontal and vertical dipoles, repsectively. Follow-

ing the procedure of Zhang et al. (2009a), we can express

the electric fields in the plane of polarization (Doviak and

Zrnic 2006, their Fig. 8.15) at r as

E(h)mn 5 E

(h)tmne(h)

n and (3a)

FIG. 2. The CPPAR with a pair of dipoles for each array element.

FIG. 3. Coordinate system for CPPAR element radiation.



E(v)mn 5 E

(v)tmne(v), (3b)

where E(h)tmn and E

(v)tmn are the fields respectively trans-

mitted by the h and v dipoles along the normal to the plane

of the dipoles (i.e., the crossed dipole’s broadside di-

rection) located at fn, zm. Thus,

E(h)tmn 5 Aejk[z

mcosu1R sinucos(f�f

n)]M(h)

mn, (3c)

with a like expression for E(v)tmn and en

(h) is

e(h)n 5 a

y9� [a

x9sinucos(f� f

n)

1 ay9

sinusin(f� fn) 1 a

zcosu] sinusin(f� f

n),

(3d)

a form analogous to Eq. (5a) of Zhang et al. (2009a), but

one that accounts for the fn angular rotation about z of

the coordinate x, y axes to x9, y9 for the mnth element.

Here, e(v) is

e(v) 5 az

sin2u� [ax9

cosf 1 ay9

sinf] sinu cosu, (3e)

which is identical to that given by Eq. (5b) of Zhang et al.

(2009a). Note that en(h) is a function of dipole location but

e(v) is not and, as pointed out by Zhang et al. (2009a), en(h)

is not orthogonal to e(v).

To form a beam pointing in the (u0, f0) direction,

a phase shift

cmn

5�k[zm

cosu0

1 R sinu0

cos(f0� f

n)] (4)

is applied to each of the mn elements that are used to

form the beam. The phase shifts given by (4) produce

a beam in the (u0, f0) direction.

The incident horizontal and so-called ‘‘vertical’’ (i.e.,

the vertical field lies in the vertical plane, but is only

vertical at the 908 zenith angle) fields Eihmn and Eivmn in

the plane of polarization are given by (Zhang et al. 2009a)

Eihmn

Eivmn

� �5 AP

mn

jM(h)mnj

jM(v)mnj

" #exp jc(0)

mn

h i, (5a)

where c(0)mn 5 k zm[cosu� cosu0] 1 R[sinucos(f� fn)

�� sinu0cos (f0 � fn)]g, and

Pmn

5cos(f� f

n) 0

�cosusin(f� fn) sinu

� �(5b)

is a matrix that projects the elements’ broadside electric

field to the plane of polarization at r, and accounts

for h dipole orientation at fn. In this analysis we assume

that each dipole radiates only into the outward hemisphere

with an equator in the plane of the crossed dipole element.

Magnitude signs are placed around the dipole moment to

emphasize that the dipole phase is incorporated into C(0)mn.

Although the subscript index m does not appear in the

matrix, it is attached to Pmn to emphasize that the pro-

jection applies to the mnth h and v dipoles. The subscript

h and v on Eihmn and Eivmn denotes these are the hori-

zontal and vertical fields transmitted by the mnth dipoles

and incident on the scatterer; note that Eivmn has contri-

butions from both the h and v dipole moments, whereas

Eihmn depends only on the h dipole’s moment.

Radiation patterns with specified sidelobe levels and

beamwidths can be achieved with a proper weight [w(q)mn]

applied to each element. Hence, the total incident field

at r is the weighted contributions from all of the active

elements used to form the beam at (u0, f0). This field can

be expressed as

Ei5

Eih

Eiv

� �5 A�

m,nP

mnW

mn

jM(h)jjM(v)j

" #exp jc (0)

mn

, (6)

where the weighting matrix Wmn is applied to each ele-

ment, and the angular dependence of the broadside field

generated by the mnth h and v dipole moments is in-

corporated into Wmn; that is, all dipoles have M(h) 5 M(v),

which is taken to be the dipole’s source excitation mod-

ulated by Wmn. Here, Eih is the total horizontal field

generated by all the h and v dipoles that are used to form

the beam. Because the h dipoles change orientation de-

pending on their azimuth fn, the weighting vector can be

expressed as

Wmn

5

1

cos(f0� f

n)

0

01

sinu0

26664

37775w(i)

mn, (7a)

where the upper-left matrix element 1/cos(f0 2 fn)

compensates for the projection loss of the H dipole–

radiated field onto the horizontal polarization direction

along the beam’s boresight. The boresight always lies in

the plane containing the bisector of the angle encom-

passing the azimuth sector containing the elements

forming the beam; in effect, the boresight of the CPPAR

is always in the broadside direction. Alternatively,

fn

5 nDf 5 f0

6 n9Df 5 (n0

6 n9)Df,

(n9 5 0, 1, 2, . . . , Na) (7b)

is the location of the active dipoles in an angular sector

(e.g., 1208 for a three-beam CPPAR) centered on f0 with

(2Na 1 1) active array elements in the azimuthal span of

(n0 2 Na, n0 1 Na).



Likewise, the lower-right matrix element 1/sinu0 com-

pensates for the projection loss of the V dipole–radiated

field onto the vertical direction; this correction is normally

close to unity because the elevation angle (p/2 2 u0) for

weather measurements is typically small.

The scalar weight w(i)mn is for isotropic radiators; these

weights are selected to control the sidelobe levels. The

WSR-88D antenna pattern is mimicked by selecting

w(i)mn 5

1� 4[R2 sin2(f0� f

n) 1 z2

m]/D2� �

1 b

1 1 b

!

3 cos(f0� f

n). (8)

The term in the parenthesis is equivalent to the WSR-

88D illumination taper but applied to those mnth dipoles

whose projection onto the vertical plane bisecting the

cylinder lies within the pD2/4 area, where D is the di-

ameter of the WSR-88D dish antenna (dipoles outside this

circular area, but lying within the angular sector of ele-

ments forming the beam, have zero weight); the cos(f 2

f0) term accounts for the change of the density of the

array elements projected onto the vertical plane and the

term b 5 0.16 accounts for edge illumination of the WSR-

88D reflector (Doviak et al. 1998). Although w(i)mn mimics

the illumination taper on the WSR-88D antenna for the

boresight direction, the analogy no longer exists for azi-

muths in off-boresight directions. This is because the ac-

tive elements on the cylinder have a density that lacks the

symmetry of the dish antenna about the vertical bisector

of the circular area.

On the beam’s boresight (i.e., u 5 u0, f 5 f0), the ra-

diated fields from all the elements are in phase so the

phase term in (6) disappears and the incident wave field

becomes

Ei5 A�

m,nP

mnW

mnjM(q)j. (9)

FIG. 4. A table of the specifics of sample designs for a CPPAR with two, three, or four beams

and a PPPAR with three and four faces.



Because the active elements and the weighting factor w(i)mn

are symmetric about f0 and zm 5 0, there is no on-axis

cross-polar radiation. That is, the vertically polarized wave

field caused by the horizontal dipole at f0 2 n9Df cancels

that field from the dipole at the opposite azimuth f0 1

n9Df. This cross-polar null on the axis is important for

accurate polarimetric radar measurement of precipitation

(Wang and Chandrasekar 2006; Zrnic et al. 2010). This is

one of the main reasons for using a CPPAR commutating

scan in which the beam direction changes in azimuth by

shifting a column of active elements and maintaining the

weight’s symmetry about the beam center. This way, the

beam characteristics of the CPPAR are scan invariant; this

is not so for the PPPAR.

Given the field incident on a hydrometer, the scat-

tered wave field can be expressed as [Doviak and Zrnic

2006, section 8.5.2.1]

ES

5E

sh

Esv

� �5 S9E

i3

exp(�jkr)

r, (10)

where S9 is the backscatter matrix of a hydrometeor and

includes propagation effects (Zhang et al. 2009a).

Although (10) can give the H, V electric fields at any

receiving array element, we need to determine the fields

parallel to the respective dipole axis. The fields parallel

to the dipole axes are obtained by projecting Esmn

onto

the respective dipole directions, and with the proper

weighting and phase shifts. In this case the total received

wave field is expressed as

Er5 �

m,nW

mnPt

mnEsmn

e� jcmn

5 �m,n

WtmnPt

mnS9Pmn

Wmn

M(q)

3 � k2

4p«

!exp(�2jkr)

r2. (11)

4. Sample design to mimic the WSR-88D

The operational WSR-88D radar has high performance

for meteorological observations: it has a dish antenna with

a diameter of 8.54 m, a beamwidth of about 18, and the

first sidelobe below 226 dB. It is desirable for the MPAR

to have either similar or better performance. Figure 4

shows a table of the specifics of sample designs for a

CPPAR with two, three, or four beams; each mimics the

NEXRAD beamwidth at the largest electronic scan angle,

and element separations used are 1.0, 0.75, and 0.5 wave-

length. Considering the trade-off for maximizing the ef-

fective aperture and the number of beams, it is efficient to

use either three or four simultaneous beams for a CPPAR,

consistent with what is recommended by Josefsson and

Persson (2006, chapter 3). It is relatively easy to control

sidelobes with the four beams and short distance for ele-

ment separations. For comparison, a PPPAR of three and

four faces, with a beamwidth at its largest scanning azi-

muth angle (608/458) that matches the WSR-88D, is also

shown in the table. The number of elements for a PPPAR

is calculated based on elements occupying only the area

inside the ellipse filled with array elements.

In the case of three beams, a 1208 sector of a CPPAR is

used to form a beam. This would require a cylinder of

8.54/sin(608) 5 9.88-m diameter and 8.54-m height. This is

significantly smaller than the 17.1 m (i.e., 8.54 3 2) major

axis of the elliptical array for a three-face PPPAR that

matches, at the extremes of electronic steering of 608, the

WSR-88D resolution; furthermore, there is no need to

FIG. 5. (a) Copolar and (b) cross-polar one-way power density

patterns as a function of azimuth and zenith angle.



increase the total power by a factor of 16 (12 dB) to

compensate for the loss of detection capability in these

directions. For the four simultaneous beams, a 908 sector

is used to form a beam. A cylinder of 12.1 m (;8.54 3ffiffiffi2p

) diameter is needed. The cylinder’s diameter is the

same as the major axis of the elliptical array of the four-

face PPPAR, and the total number of elements for the

CPPAR is the same as for the PPPAR. However, the

elements at the corners of the CPPAR can be used for

sidelobe blanking and pattern synthesis, whereas the

PPPAR would have to have extra elements for such

functions. Additionally, the total power of the CPPAR

does not need to be increased by a factor of 4 (6 dB).

Assuming the element spacing to be the wavelength of

10 cm, there would be 380 array columns and a total of

32 680 elements covering the cylinder. Commutating

one column, the CPPAR beam moves 0.958 about the

beamwidth. If element spacing is reduced to one-half a

wavelength (i.e., 5 cm), 760 array columns would be

needed to cover the cylinder; this significantly increases

the number of total elements to 130 720. Nevertheless,

this will allow for oversampling at a 0.4748 angular

spacing and lower sidelobes. Such fine angular sampling

can also be achieved with the one-wavelength spacing of

elements, but then the phase of each column would need

to be shifted by half the angular increment between the

array elements.

Calculated CPPAR one-way-radiated power density

patterns for the above-mentioned four-beam case and

their comparisons with theoretical WSR-88D pattern are

in Figs. 5–8. Figure 5 shows 3D copolar and cross-polar

patterns for the CPPAR with tapering and polarization

compensation. The cross-polar radiation is everywhere at

least 45 dB below the copolar peak, indicating that the

CPPAR has high performance for preserving polarization

purity. In Figs. 6–8 the copolar patterns are on the two

planes through the boresight: the patterns on the hori-

zontal plane are shown in the upper panels, and those on

the vertical plane are in lower panels.

Figure 6 shows the copolar patterns whereby the dipoles

have no equivalent tapering of the WSR-88D illumination,

density adjustment, and polarization compensation. Be-

cause the WSR-88D pattern is for the tapered illumination,

CPPAR has higher sidelobes in Figs. 6a,c. This is also true

FIG. 6. One-way power density patterns for the four-beam configuration and element spacing of 0.5l without

tapering, density adjustment, and polarization compensation.



for the near sidelobes as seen in the zoomed-in plots on the

right in Figs. 6b,d. The pattern sidelobes for horizontal

polarization are a little lower than for vertical polariza-

tion because of the natural tapering caused by changes in

orientation of the horizontal dipoles as a function of fn.

Figure 7 shows the CPPAR pattern results if Eq. (8),

without the cosine term, is applied to the dipoles within

the angular sector forming the beam. The sidelobes are

substantially reduced except near 6908 azimuth angles.

This is due to the nonsymmetrical density of the active

array elements seen from the off-broadside directions.

Nevertheless, the level is 50 dB below the copolar peak

and for two-way patterns that are of interest for meteo-

rological applications, the sidelobe level is 100 dB below

the copolar peak. This low sidelobe level is due to the

applied tapering. It is also noted that the difference be-

tween the two polarizations is now very small because the

main contribution to the radiation field comes from array

elements near the broadside where there is not much

difference between the H and V polarizations. If density

adjustment [i.e., the cosine term in Eq. (8)] and polari-

zation compensation are applied, the results become even

better (Fig. 8). The main lobes are almost identical to the

WSR-88D reference pattern, which is crucial for high-

quality polarimetric radar measurements. Although side-

lobes still exist, the farther sidelobes are mostly lower

than those of WSR-88D. This is because of the natural

tapering in CPPAR.

5. Summary and discussions

In this paper, we have compared the planar and cylinder

array configuration of PPAR for weather measurements,

and we formulated a theory for studying the CPPAR

performance. Because our main objective is to draw at-

tention to some of the unique properties of the cylindrical

phased array for weather observations with polarimetric

radar, and not to provide a detailed design for a specific

CPPAR, we have assumed ideal array elements with

given excitation. It is known that a PPPAR has issues of

scan-dependent beam properties, including changes in

beam and polarization characteristics, polarization cou-

pling, sensitivity loss, and complications in calibration. To

compensate for the sensitivity loss, the four-faced PPPAR

FIG. 7. Power density patterns for the four-beam configuration and element spacing of 0.5l with tapering, but no

adjustment for density of projected elements and no polarization compensation.



antenna would have to have a diagonal dimension dou-

bled the size of the WSR-88D and an increase of power by

a factor of 4.

The CPPAR configuration can also make azimuth scan-

invariant, high-accuracy weather measurements without

changing the beam and polarization characteristics while

maintaining a manageable antenna size. Compared with

PPPAR, CPPAR has the following advantages:

(i) Scan-invariant polarimetric radar measurements

with the same beamwidth and polarization char-

acteristics in all azimuth angles for each elevation

allow for easier calibration and data interpretation.

(ii) Polarization purity: dual-polarized (H and V) wave

fields are orthogonal in all direction, and hence

maintain high-quality polarimetric data. Compen-

sation is only needed for horizontal and vertical

polarizations separately, but cross-polarization iso-

lation is maintained.

(iii) High efficiency of utilizing radiation power: Only

certain array elements are activated and properly

weighted to achieve the desired beams. The elements

on the broadside are mostly activated and weighted

higher; hence, less scan loss results from the element

radiation pattern.

(iv) Efficient use of spectrum: For example, the side-

by-side and back-to-back beams might use the

same frequency because they are always 908 (1208)

apart in the case of four (three) beams.

(v) The antenna aperture for fast data update or for

multifunctionality with simultaneous multibeams

is used optimally.

(vi) Flexibility to choose the number of beams (e.g.,

two, three, or four) and assign different task among

beams: For example, if four beams are generated,

two beams can be used for weather surveillance

and the other two for aircraft tracking, making it

a candidate for the future MAPR. This flexibility

can be combined with multiple frequencies used in

currently proposed PPPAR, that is, one band of

frequencies for the weather function and another

band for aircraft surveillance.

(vii) There is no need for the face-to-face matching as

is required for a PPPAR, where each face is an

FIG. 8. Power density patterns for the four-beam configuration and element spacing of 0.5l with tapering, element

density correction, and polarization compensation.



individual radar system that could have different

characteristics that need to be matched.

While CPPAR has the above-mentioned advantages, it is

not without problems. These include complexity for the

system design and development, difficulty in controlling

the sidelobes, and synchronization of all the elements to

form multiple beams. There are other common PPAR

issues such as polarization mode selection, cross-polar

isolation requirement, waveform design (coding), and so

forth. Although these issues are challenging, they are

solvable with further study and by using advanced tech-

nology. Hence, the challenges can be viewed as good re-

search opportunities for the weather radar community to

advance its radar technology with potential for new sci-

entific findings.

Acknowledgments. This work is supported with fund-

ing provided by NOAA/NSSL under the Cooperative

Agreement NA17RJ1227/NA080AR4320886 and a NSF

Grant ATM-0608168.

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