Development of Dual-Polarization Phased-Array Weather Radar

An increasing number of disasters caused by extreme weathers such as localized heavy rainfalls and tornados are taking place all over the world. One main cause is rapid growth of cumulonimbus clouds. Generally, lifecycle of a cumulonimbus cloud is about 30 minutes. Weather radars with rapid scanning capability are more than ever required.

Toshiba has developed single-polarization phased-array weather radars. Now we are developing a dual-polarization phased-array weather radar in a joint effort with National Institute of Information and Communications Technology (NICT), and Osaka University. Table 1 shows the schedule of development.We already developed underlying technology funded by Ministry of Internal Affairs and Communications. By early 2017 we will develop a dual-polarization phased-array weather radar as a system, and demonstrate its capabilities by 2018, funded by Government of Japan under SIP, Cross-ministerial Strategic Innovation Promotion Program.  Dual-polarization phased-array radars enable advanced prediction of heavy rainfalls and tornados with rapid, high precision 3-dimensional observation of cumulonimbus clouds. Its performance comparison with existing radars is summarized in Table 2.

We present the current status of development.

Figure 1 shows the radar cell image for our dual-polarization phased-array weather radar. There are two characteristics.

One is the design of antenna structure optimized for 2-dimensional array. Slot coupled patch antennas are aligned on a plane to constitute a radar antenna. Polarizations in the horizontal and vertical directions are shared for one antenna element. Layout of polarization slots and feeder circuits was carefully determined to attain high isolation between H/V ports. A 4x1 element prototype is shown in Fig.2. Size is 69.82mm x 15mm for X-band.

Second is the realization of a compact receiving front-end. Transmitting fan beams and receiving pencil beams are required for our phased-array radar. While part of array elements is used for both transmission and reception, about 90% of them are only for reception. The receiving front end had previously been realized as discreet elements, which makes the system bigger and costly. As the number of channels will double for dual-polarization, the cost per channel must be reduced considerably.

Due to long experiences in semiconductor technology, we succeeded in high integration of basic functions, such as frequency mixing and filtering, for the receiving front-end on a single IC Chip. With 180nm process of CMOS technology, the RF front-end for dual-polarization is realized on a 3mm x 3mm bare chip size for X-band.

Currently we are evaluating our prototypes in order to verify that the performance meets requirements for a dual-polarization phased-array weather radar. A prototype is composed of a 4x1 element of patch antennas, an RF receiving front-end as IC, and a digital back-end. Preliminary results are shown in Fig. 3 and 4. Near the front direction (plus minus 30deg), more than 30dB of XPD (Cross Polarization Discrimination) was obtained (Fig.3). XPD is usually much less than 30dB for a patch antenna. As to the gain characteristics of the RF receiving front-end, the first prototype had shortage of gain and shifted frequency of peak gain (red line in Fig.4), due to stray capacity of local lines and inaccurate modeling of power supply inductor model, respectively. By modified layout and modeling design we improved gain characteristics, more than 20dB of gain and corrected frequency of peak gain, for the second prototype (blue line in Fig.4).

To conclude, we succeeded in developing a compact, low-cost radar cell without losing performance required for a dual-polarization phased-array weather radar. Based on the preliminary results of the radar cell we will be able to develop a dual-polarization phased-array weather radar.

 

 Table 1  Schedule of DP Phased-Array Weather Radar

Years	Phase
2012-2014	1st phase: Development of underlying technology (Funded by Ministry of Internal Affairs and Communication)
2014-2018	2nd phase: Development of radar and demonstration (Funded by Cross-ministerial Strategic Innovation Promotion Program)

 

 Table 2  Comparison of Radars

	X-Band Dual Pol Radar with Parabolic Antenna	Single Pol Phased-Array Radar	Dual Pol Phased-Array Radar
Observation Range	Radius of 60km	Radius of 60km	Radius of 60km
Sensitivity	Less than 1mm/h  @ 60km	Less than 1mm/h @ 60km	Less than 1mm/h @ 60km
Temporal Resolution	5 to 10 min	30 sec	30 to 60 sec
Polarization	Dual Polarization	Single Polarization	Dual Polarization
Beam Shape	Pencil Beam for Both Transmission and Reception	Transmission: Fan Beam, Reception: Pencil Beam	Transmission: Fan Beam, Reception: Pencil Beam
Beam Scanning	Mechanical for Both AZ/EL	AZ: Mechanical, EL: Electronic	AZ: Mechanical, EL: Electronic
Antenna	Parabolic	Phased-Array	Phased-Array

 

Figure 1  Radar Cell Image

 

Figure 2  Patch Antenna

 

Figure3  Preliminary Result of Prototype (Antenna Pattern)

 

Figure4  Preliminary Result of Prototype (Gain Characteristics of Receiving Front-End)