# RONSARD Radar: Implementation of Dual Polarization on a C-Band Doppler Weather Radar

Georges Scialom, Jacques Faroux, Manuel Giraud, Richard Ney, Raquel Evaristo, Yvon Lemaître, Nicolas Viltard

► To cite this version:

Georges Scialom, Jacques Faroux, Manuel Giraud, Richard Ney, Raquel Evaristo, et al., RONSARD Radar: Implementation of Dual Polarization on a C-Band Doppler Weather Radar. IEEE Geoscience and Remote Sensing Letters, 2009, 6 (1), pp.132-136. 10.1109/LGRS.2008.2008318. hal-00352951

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# RONSARD Radar: Implementation of Dual Polarization on a C-Band Doppler Weather Radar

Georges Scialom, Jacques Faroux, Manuel Giraud, Richard Ney, Raquel Evaristo, Yvon Lemaître, and Nicolas Viltard

*Abstract*—The French C-band meteorological Doppler radar Recherche sur les Orages et Nuages par un Système Associé de Radars Doppler (RONSARD) was recently equipped with dual polarization. This modification required, on the one hand, an additional receiver and, on the other hand, a new design for the antenna geometry in order to decrease strongly the sidelobe level. This new radar configuration allows us to choose between two complementary modes: 1) the previous single-polarization mode, still preserved with fast Fourier transform calculations of the first three momentums of the Doppler spectrum, i.e., horizontal (H) reflectivity, radial wind velocity, and wind velocity variance calculated both in precipitation areas and clear air areas (depending on the gate), and 2) the dual-polarization mode with pulse pair processing of H and vertical reflectivities and velocities, differential phase shift, and coherence coefficient. Moreover, both modes work over contiguous gates along each direction, allowing fine radial range resolution. Thanks to the flexibility between these two modes, the RONSARD radar becomes a new major facility aimed at studying the dynamics–microphysics interactions within precipitation and their environment.

*Index Terms*—Antenna with direct illumination, polarimetric C-band meteorological Doppler radar.

## I. INTRODUCTION

THE RECHERCHE sur les Orages et Nuages par un Système Associé de Radars Doppler (RONSARD) facility was originally a system of two Doppler radars developed in the late 1970s [5]. These radars were successfully used as the main instruments for several field experiments on tropical squall lines or midlatitude fronts.

Since 1991, only one of these radars has been maintained and progressively modified. A first renovation was undertaken in order to improve the observation of clear air convective structures within the atmospheric boundary layer, while preserving its main functions, i.e., observing precipitation areas. This first upgrade was achieved in 1998, leading to two field campaigns dedicated to clear air structure observations. Meanwhile, the RONSARD radar was one of the main instruments deployed in the Po Plain (Italy) during the international Mesoscale Alpine Programme (MAP) experiment dedicated to autumn alpine precipitation [1].

The second and most important upgrade was made in 2003–2006 in order to implement the dual polarization capability.

It is well known that transmitting and receiving in dual polarization [in the present case, horizontal (H) and vertical (V)] brings new pieces of information, allowing particularly 1) a better estimate of precipitation [2], [4], [8] and 2) hydrometeor identification [11].

When single polarization is used, a Doppler radar provides H (or V) polarization reflectivity $Z_{HH}$ (or $Z_{VV}$) and H (or V) polarization radial velocity $V_{HH}$ (or $V_{VV}$), sometimes along with the velocity variance, as it is the case for the RONSARD system.

With the dual polarization at transmission and reception, it additionally provides the reflectivity $Z_{VV}$ (or $Z_{HH}$), the differential reflectivity $Z_{DR}$, the radial velocity $V_{VV}$ (or $V_{HH}$), and several other parameters like the correlation coefficient at zero lag $\rho_{HV}$ and the differential phase $\Phi_{DP}$. Depending on the radar design, the linear depolarization ratio (LDR) can also be made available.

In Section II, the main changes implemented on the radar system are presented, particularly the dual polarization transmission and receiver implementation, the new antenna design, the signal processing, and the radar control. In Section III, some preliminary results obtained using data gathered during the African Monsoon Multidisciplinary Analyses (AMMA) experiment [6] are presented and commented.

## II. MODIFICATIONS CONDUCTED ON THE RADAR

The modifications suggested by Testud and Ney [9] concern the transmission/reception system since an additional receiver is required, the antenna design in order to improve its performance, and the upgrade of the signal processing along with the radar control and display system. The aim of these modifications was to be able to operate the radar in the frame of AMMA in two possible modes, i.e., 1) the new dual polarization mode for studying the microphysics and its relationship with the dynamics and 2) the previous single polarization mode with only the dynamics, giving a better sensitivity, allowing, particularly, observations in clear air.

### A. New Transmission/Reception System Architecture

A simultaneous transmission and reception of dual polarization in H and V directions (at low elevation) was installed during the 2003–2006 period. The LDR parameter is not available in this configuration since it needs alternate transmission in H and V directions, in order that V cross-polarization (respectively H) be received when H cross-polarization (respectively V) is transmitted. Alternate transmission was not retained because it requires high-speed switching between the two polarizations. LDR could have been implemented in the

<footnote>The authors are with the Centre d'Etudes des Environnements Terrestre et Planétaires, Institut Pierre-Simon Laplace, 78140 Vélizy, France (e-mail: georges.scialom@cetp.ipsl.fr).</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. Block diagram of the dual polarization/monopolarization transmission/reception architecture.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. New receiver block diagram.</figcaption></figure>

single polarization mode, thanks to the existence of the dual receiver, but it was not identified as a priority for the scientific objectives of AMMA. LDR implementation could be envisaged in the future. Now, other polarimetric parameters are still available, namely, $Z_{HH}$, $Z_{VV}$, $Z_{DR}$, $\rho_{HV}$, and $\Phi_{DP}$, allowing, particularly, hydrometeor classification. The block diagrams of the dual transmission and reception units are shown in Figs. 1 and 2, respectively.

Fig. 1 shows the whole new transmission and reception architecture which allows either dual or single polarization operation. The radar has two transmission and reception channels at 5.6 GHz, in the H and V polarizations, respectively. For reception, a third channel, the reference or BURST channel, contains a sample of the transmitted signal in order to preserve its phase and ensure coherence. Each receiving channel contains a low-noise amplifier, followed by a bandpass filter to eliminate contamination from other transmitting sources, spurious spectral lines, and noise at the image frequency. Then, a mixer using a very high stability frequency local oscillator (5540 MHz) shifts the signal and the BURST to the 60-MHz intermediate frequency (IF). Fig. 2 shows the receiver architecture.

The 60-MHz IF unit contains a bandpass filter eliminating residual HF signals and an amplifier to compensate for the 50-m-long cable losses between the aerial and control cabins.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Antenna evolution: (a) Old Cassegrain antenna. (b) New antenna with direct centered illumination.</figcaption></figure>

Another major change with respect to the “old” RONSARD is that the analog-to-digital converter is carried out at 60 MHz and followed by a digital receiver performing demodulation and digital filtering, while formerly, the demodulation was carried out by a second frequency mixing and followed by a low-pass filter and analog-digital conversion.

## B. Antenna

The former RONSARD antenna [5] was a Cassegrain antenna, i.e., a set of a feed horn, a primary reflector, and the 4-m-diameter paraboloid dish as main reflector. The feed horn located at the center of the paraboloid dish illuminates a primary reflector, fixed at the paraboloid focus, which in turn illuminates the main paraboloid dish [Fig. 3(a)]. In this kind of geometry, the antenna sidelobe level is too high, particularly the first one which was 17 dB below the main lobe. For dual polarimetry applications, it was necessary to decrease this level and, thus, to modify the antenna geometry. The sidelobe level depends on 1) the illumination function provided by the feed horn, 2) the condition of antenna surface, 3) the mechanical elements illuminated by the beam, and 4) the refractions on the antenna edge. Point 2) is of secondary order. Point 3) was the one taken into account, and the geometry adopted was a center-fed paraboloid dish. The feed horn is smaller than the previous one and positioned at the paraboloid focal point. This way, secondary refractions are minimized and so are the sidelobes, particularly the first one. Fig. 3(b) shows the new antenna geometry. After antenna modification, the first sidelobe level lies between -23.4 and -26.5 dB under the main one, allowing retrieval of polarimetric parameters. Points 1) and 4) were notably improved by the antenna geometry change. Fig. 4 shows the new beam diagram at 2° elevation (quasi-H), pointing out the significant decrease of the level of the first sidelobe. The V diagram (not shown) provides slightly worse results (-23.4 dB).

## C. Signal Processing and Computing

For some 35 years, many components were integrated and simplified, driven by more efficient computing devices. Now, most of the computation is performed by a standard PC with an acquisition board that digitizes the 60-MHz input.
<figure><img src="image_5.png" /><figcaption>Fig. 4. New antenna beam diagram in a quasi-H plane (2° elevation).</figcaption></figure>

In its previous configuration before implementing dual polarization (until 2004), the system was able to retrieve (and record) the first three momentums of the spectrum for 512 gates, after fast Fourier transform (FFT) computing. This monopolarization configuration is still preserved and even improved since 1024 triplets of “horizontal” reflectivity $Z_{HH}$, radial velocity $V_{HH}$, and velocity variance $Var_H$ corresponding to each gate are recorded when this mode is used (“horizontal” stands for H transmitting and receiving). Velocity variance is accurately calculated and allows us not only to filter out the noise but also to discriminate between hydrometeors within stratiform precipitation [7]. The passage from 512 to 1024 range gates improves the radial definition since it is now possible to sample contiguous 100-m-wide gates, instead of sampling 100-m-wide gates every 200 m in the old configuration for a 100-km maximal range (all these numbers are doubled for a 200-km maximal range). In the single polarization configuration, the gates with clear air are processed as follows: The noise level is estimated from the part of the spectrum away from the signal maximum and subtracted from the signal. Moreover, it is possible to integrate the spectrum to improve sensitivity. Generally, two or four integrations are sufficient to extract the clear air signal.

When in dual polarization mode, the computation is based on pulse pair processing (PPP), and the velocity variance is disregarded due to insufficient available computing time since, in that mode, seven other parameters have to be recorded. Dual polarization parameters, namely, $Z_{HH}$, $Z_{VV}$, $Z_{DR}$, $V_{HH}$, $V_{VV}$, $\rho_{HV}$, and $\Phi_{DP}$ are thus calculated for 960 range gates. Due to real-time computing limitations, 960 range gates were chosen instead of 1024.

A comparison of $V_{HH}$ and $V_{VV}$ is used to filter out the noise, using the fact that, in meteorological signals, they should be equal to each other within a threshold chosen as $\pm 0.5 \text{ m} \cdot \text{s}^{-1}$. When their difference exceeds $0.5 \text{ m} \cdot \text{s}^{-1}$, the corresponding measurement is considered as noise.

### D. Radar Control, Data Visualization, and Recording

The radar is fully controlled by the computer which takes care of the signal processing. In particular, it controls the transmitter, receivers, and antenna motions. Sequences are also programmed and executed by the computer. Three main types of sequences are possible and can be combined, namely, con-

<table><caption>TABLE I
TECHNICAL CHARACTERISTICS OF THE RONSARD RADAR</caption><tr><td>Transmission frequency</td><td>5.6 Ghz (C band)</td></tr><tr><td>Peak power</td><td>250 kW</td></tr><tr><td>Mean power</td><td>250 W</td></tr><tr><td>Polarization</td><td>H and V</td></tr><tr><td>Antenna diameter</td><td>4 m</td></tr><tr><td>Antenna surface</td><td>12.56 m2</td></tr><tr><td>Antenna gain</td><td>44 (H); 43 (V)</td></tr><tr><td>Main lobe width (&lt;-3dB)</td><td>0.9° (H and V)</td></tr><tr><td>1st sidelobe level (H)</td><td>-25.5 dB in horizontal plane<br>-23.4 dB in 45° slanted plane</td></tr><tr><td>1st sidelobe level (V)</td><td>-26.5 dB in horizontal plane<br>-23.4 dB in 45° slanted plane</td></tr><tr><td>Cross-polarization isolation</td><td>-26.5 dB</td></tr><tr><td>Azimuth and Elevation resolution</td><td>0.09°</td></tr><tr><td>Maximal wind in exploitation</td><td>25m/s</td></tr><tr><td>Maximal rotation velocity of antenna</td><td>18° s<sup>-1</sup></td></tr><tr><td>Range gate number (single polar)</td><td>1024</td></tr><tr><td>Range gate number (dual polar)</td><td>960</td></tr></table>

<table><caption>TABLE II
OPERATING MODE CHARACTERISTICS OF THE RONSARD RADAR</caption><thead><tr><th></th><th>Operating. Mode 1</th><th>Operating. Mode 2</th></tr></thead><tbody><tr><td>Pulse Repetition Frequency</td><td>1464 Hz</td><td>732 Hz</td></tr><tr><td>Transmitted pulse duration</td><td>0.667 µs</td><td>1.333 µs</td></tr><tr><td>Reception gate width</td><td>100 m</td><td>200 m</td></tr><tr><td>Maximal range</td><td>102.5 km</td><td>205. km</td></tr><tr><td>Unambiguous velocity range</td><td>±19.6 m/s</td><td>±9.8 m/s</td></tr><tr><td>Velocity resolution</td><td>0.6 m/s</td><td>0.3 m/s</td></tr></tbody></table>

secutive plan-position indicators (PPIs) at any elevation, range-height indicators at any azimuth, and observations at V or slantwise fixed elevation. Color displays of sequences during or after operations show two parameters among those available (three in single polarization and seven in dual polarization). Data are first stored on external disks, then burned on DVDs. Tables I and II provide the main technical characteristics and operating modes of RONSARD, respectively. In summary, there are two possible pulse-repetition frequencies, 1464 and 732 Hz, yielding 102.5- and 205-km maximal radial ranges, with $\pm 19.6$- and $\pm 9.8 \text{ m} \cdot \text{s}^{-1}$ velocity ranges, respectively. Precisions are 1 dB on reflectivity, 0.1 dB on $Z_{DR}$, 1° on $\Phi_{DP}$, 0.01 on $\rho_{HV}(0)$, and 0.1° on azimuth and elevation. Conical scans are made of consecutive PPIs at increasing elevations (typically 14 elevations). PPI duration is about 30 s for 1° azimuth increment, and total volumic sequence duration is about 7 min.

### III. FIRST RESULTS GATHERED DURING THE AMMA CAMPAIGN

The RONSARD radar was deployed in Northern Benin for three months (June 15–September 15, 2006) during the AMMA
campaign in order to make measurements within mesoscale convective systems during the active phase of the West African Monsoon. Forty-three such systems were sampled and identified either as isolated, local convection, or as part of organized systems (convective lines or squall lines).

In this section, we successively present a brief recall of polarimetric measurements and applications, an evaluation of polarimetric parameters during AMMA, and an illustration of hydrometeor (HM) classification using these parameters.

A. Recall of Polarimetric Measurements and Applications

The differential reflectivity $Z_{DR}$ is null for spherical particles since the backscattered H and V signals are similarly affected. On the contrary, if the radar signal is backscattered by spheroidal particles with eccentricity $a/b$ ($a$ = H dimension, $b$ = V dimension) significantly different from 1, then it will be positive if $a/b > 1$ and negative if $a/b < 1$. $Z_{DR}$ is independent from absolute calibration and from total particle concentration. It depends on the drop size distribution and is affected by propagation, particularly attenuation.

$\rho_{HV}(0)$ is the correlation coefficient at zero lag between the received H and V signals. It is generally close to 0.9–1, but decreases where, along a beam path, the HM have various forms and/or orientations or are of several types. $\rho_{HV}(0)$ is independent from calibration and HM concentration, and immune to propagation effects.

The differential phase $\Phi_{DP} = [\Phi_{hh} - \Phi_{vv}]$ is essentially positive for oblate HM and negative for prolate HM. Its derivative with respect to the range $r$ along the path $K_{DP} = \partial\Phi_{DP}/\partial r$ is known as the specific differential phase and is sensitive to statistically anisotropic HM only. $K_{DP}$ increases if all the HMs in a certain volume are flattened or elongated, or if the dielectric constant increases, but is not affected by randomly oriented hail. In the present case, backscatter differential phase was filtered from raw $\Phi_{DP}$ using the filter described in [3] before calculating $K_{DP}$. Reflectivity and differential reflectivity were corrected for attenuation using an appropriate algorithm [8].

B. Polarimetric Data Evaluation

The “Contoured Frequency by Altitude Diagrams” (CFAD) [10] offers a synthetic statistical view of the different radar parameters. The ordinate of CFAD is height, and the abscissa is the value of the parameter whose distribution is plotted. Fig. 5 shows the CFADs of reflectivity $Z_{HH}$ and differential reflectivity $Z_{DR}$ in a case of stratiform precipitation following isolated convection observed on July 20, 2006 at about 13:00 Coordinated Universal Time (UTC). The $Z_{HH}$ CFAD [Fig. 5(a)] clearly shows a peak of reflectivity associated to the melting layer. Below the latter, $Z_H$ slowly decreases toward the ground, and above, $Z_{HH}$ also decreases toward the top. The maximum mean reflectivity is about 29 dBZ at ground level, 30 dBZ at 4 km (the bright band peak altitude) about 500–700 m below the 0 °C isotherm. The $Z_{DR}$ CFAD [Fig. 5(b)] also shows a transition at 4-km height between a V profile in the rain region and a somewhat slanted profile in the ice region. The histogram spreading at 4 km could be the signature of hydrometeor mixing (liquid and solid water mixing). The $\rho_{HV}$ CFAD (not shown)

<figure><img src="image_6.png" /><figcaption>Fig. 5. CFADs of (a) $Z_{HH}$ and (b) $Z_{DR}$ in a case of stratiform precipitation observed on July 20, 2006.</figcaption></figure>

provides values > 0.98 in rain, then a minimum ~0.94 in the bright band about 3.5–4 km, and again, an increase to a maximum of 0.98 at 4–4.5 km, then a decrease above. The $\rho_{HV}$ values are mostly close to 1 in the rain region and decrease in the bright band, with a trend to scatter at higher altitude. Patchy echoes above 11 km are probably due to residual noise echoes not filtered during preprocessing.

C. Illustration of First Results

The hydrometeor classification within the present case of stratiform rain on July 20, 2006 at 13:00:46 UTC is briefly discussed as an illustration of the scientific results.

The used algorithm is a fuzzy-logic-based method derived from that of [11]. In this method, 11 classes of HM are pre-defined according to their polarimetric signature, namely, light rain, medium rain, strong rain, rain with large drops, rain-hail mixture, graupel-hail mixture, hail, H snow crystals, V snow
<figure><img src="image_7.png" /><figcaption>Fig. 6. Histogram showing the hydrometeor classification. Contours represent the number of gates in an H layer of 200 m thick where a given hydrometeor type was identified.</figcaption></figure>

crystals, dry snow, and wet snow. The polarimetric parameters are used to compute the probability of each species. Then, each range gate is labeled with the HM with the highest probability. The 0 °C isotherm (as deduced from radiosonde at Parakou, Benin, and Niamey, Niger) lies between 4725 and 4900 m. Fig. 6 shows the results of this classification as a function of altitude. Namely, on each 200-m-altitude slab, the number of range gates with a given most probable HM is calculated. This results into a histogram of the HM probability density function as a function of altitude. It appears that, below the 0 °C isotherm, there are essentially light or moderate rain and rain with large drops. At about 3 km, there are also some graupels and particularly wet snow. Above the 0 °C isotherm, there are essentially horizontal crystals, plus significant amount of wet snow and a few graupels. This classification deduced from the dual polarization parameters $Z_{HH}$, $Z_{DR}$, $\rho_{HV}$, and $\Phi_{DP}(K_{DP})$ is consistent with the terminal fall velocity $Vf$ values independently deduced from the Doppler wind (not shown). In particular, the presence of wet snow at 4 km could help the $Vf$ profile to increase slowly below 0 °C instead of presenting a sharp transition from ice to rain. Note that this single example of July 20, 2006 at 13:00:46 UTC is still representative of 2-h measurements within stratiform precipitation for HM classification and for CFAD evaluation of polarimetric parameters.

## IV. CONCLUSION

The French C-band meteorological Doppler radar RONSARD is presently operational under two main modes, i.e., 1) a single polarization mode in which the first three momentum of the velocity spectrum (reflectivity, radial velocity, and velocity variance in H transmission and reception) are computed by FFT in 1024 contiguous gates on each radar beam, and 2) a dual-polarization mode in which computation by PPP provides for 960 contiguous gates, reflectivity, and velocity in H and V transmission/reception modes, along with $Z_{DR}$, $\rho_{HV}$, and $\Phi_{DP}$.

The dual polarization mode is particularly suited for hydrometeor classification and, ultimately, studies of dynamics–

microphysics interaction. The preservation of the single polarization mode and the switching between both modes bring flexibility in hydrometeor identification, for example, by replacing in stratiform situations the temperature profile by either the variance profile (single polarization) or the terminal fall velocity profile (any both modes). Moreover, the clear air signal detection is also preserved under the best conditions since the single polarization signal is twice as sensitive as the dual polarization one.

In conclusion, this radar was continuously evolving in the period 1981–2006 during which it was the central instrument of major international campaigns. It will be involved in future experiments through either systematic observations in the Paris area or dedicated campaigns like the future Hymex (2010) in the Mediterranean area.

## ACKNOWLEDGMENT

Based on a French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, particularly from France, U.K., U.S., and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available on the AMMA International website http://www.ammainternational.org.

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