Measurement and Analysis of Weather Phenomena with K-Band Rain Radar

Jun-Hyeong Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

bdsfh0820@kaist.ac.kr

Ki-Bok Kong Development team Kukdong Telecom

Nonsan, Republic of Korea Kbkong@kdtinc.co.kr

Seong-Ook Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

soparky@kaist.ac.kr

Abstract—To overcome blind spots of an ordinary weather radar which scans horizontally at a high altitude, a weather radar which operates vertically, so called an atmospheric profiler, is needed. In this paper, a K-band radar for observing rainfall vertically is introduced, and measurement results of rainfall are shown and discussed. For better performance of the atmospheric profiler, the radar which has high resolution even with low transmitted power is designed. With this radar, a melting layer is detected and some results that show characteristics of the meting layer are measured well.

Keywords—K-band; FMCW; rain radar; low transmitted power; high resolution; rainfall; melting layer

I. INTRODUCTION A weather radar usually measures meteorological

conditions of over a wide area at a high altitude. Because it observes weather phenomena in the area, it is mainly used for weather forecasting. However, blind spots exist because an ordinary weather radar scans horizontally, which results in difficulties in obtaining information on rainfall at higher and lower altitudes than the specific altitude. Therefore, a weather radar that covers the blind spots is required.

A weather radar that scans vertically could solve the problem. This kind of weather radar, so called an atmospheric profiler, points towards the sky and observes meteorological conditions according to the height [1]. Also, because the atmospheric profiler usually operates continuously at a fixed position, it could catch the sudden change of weather in the specific area.

In this paper, K-band rain radar which has low transmitted power and high resolutions of the range and the velocity is introduced. The frequency modulated continuous wave (FMCW) technique is used to achieve high sensitivity and reduce the cost of the system. In addition, meteorological results are discussed. Reflectivity, a fall speed of raindrops and Doppler spectrum measured when it rained are described, and characteristics of the melting layer are analyzed as well.

II. DEVELOPMENT OF K-BAND RAIN RADAR SYSTEM

A. Antenna To suppress side-lobe levels and increase an antenna gain,

offset dual reflector antennas are used [2]. Also, separation

wall exists between the transmitter (Tx) and receiver (Rx) antennas to improve isolation between them. With these methods, leakage power between Tx and Rx could be reduced. Fig. 1 shows manufactured antennas and the separation wall.

B. Design of Tranceiver Fig. 2 shows a block diagram of the K-band rain radar.

Reference signals for all PLLs in the system and clock signals for every digital chip in baseband are generated by four frequency synthesizers. In the Tx baseband module, a field programmable gate array (FPGA) controls a direct digital synthesizer (DDS) to generate an FMCW signal which decreases with time (down-chirp) and has a center frequency of 670 MHz. The sweep bandwidth is 50 MHz which gives the high range resolution of 3 m. Considering the cost, 2.4 GHz signal used as a reference clock input of the DDS is split and used for a local oscillator (LO). the FMCW signal is transmitted toward raindrops with the power of only 100 mW. Beat frequency which has data of the range and the radial velocity of raindrops is carried by 60 MHz and applied to the input of the Rx baseband module. In the Rx baseband module, quadrature demodulation is performed by a digital down converter (DDC). Thus, detectable range can be doubled than usual. Two Dimensional-Fast Fourier Transform (2D-FFT) is performed by two FPGAs. Because the 2D FFT is performed with 1024 beat signals, the radar can have high resolution of the radial velocity. Finally, data of raindrops are transferred to a PC with local LAN via the an UDP protocol. TABLE I. shows main specification of the system.

Fig. 1. Manufactured antenna and separation wall.

2016 URSI Asia-Pacific Radio Science Conference August 21-25, 2016 / Seoul, Korea

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ALMA Band 2 Optics: Design, Constraints, Implementation and Measurements

Sivasankaran SrikanthNational Radio Astronomy Observatory

Central Development Laboratory 1180 Boxwood Estate Road

Charlottesville, VA 22903, USA e-mail: ssrikant@nrao.edu

Alvaro GonzalezNational Astronomical Observatory of Japan

2 Chome-21-1 Osawa, Mitaka Tokyo 181-8588, Japan

e-mail: alvaro.gonzalez@nao.ac.jp

Abstract—This paper describes the development, construction, and measurement of the optics of a receiver operating between 67-90 GHz prototyped by the National Radio Astronomy Observatory (NRAO). The intent is to install this receiver on the Atacama Large Millimeter/submillimeter Array (ALMA) as a new band increasing the capability of ALMA. ALMA is an international radio telescope in the Atacama Desert of northern Chile [1]. The array consists of 54 12-meter and 12 7-meter high precision antennas operating in the 0.3 to 10 mm wavelength range. The instrument provides both interferometric and total-power astronomical information on atomic, molecular and ionized gas and dust in our galaxy, the solar system and the nearby to high-redshift universe.

Keywords—ALMA, Optics, Dielectric Lens, Feed Horn.

I. INTRODUCTION

The National Radio Astronomy Observatory (NRAO) has developed a prototype of an ALMA Band 2 cartridge. The receiver covers the 67-90 GHz band, with dual linear polarization. The specification for the optics calls for a total aperture efficiency of greater than 80% and polarization efficiency of at least 99.5% at the subreflector. The subreflector subtends a half-angle of 3.6° from the secondary focus. When fully equipped, ALMA will have 10 receiver bands covering the frequency range of 35-950 GHz. The two lowest frequency bands will have HEMT receivers and the rest SIS based receivers. The receiver cartridges are distributed around the central axis of a cryostat. The 80% aperture efficiency translates to a receiver beam nominal illumination taper of -12 dB at the edge of the subreflector.

II. LIMITS AND CONSTRAINTS

The window for the Band 2 cartridge is located at a radius of 255 mm from the center of the cryostat and the receiver beam makes an angle of 2.48° with the telescope axis. The beam angle tolerance is ±5 mrad. The receiver beam passes through a pair of infra-red (IR) filters. The 15 K shield on the cartridge has an opening of 50 mm diameter and a 1.5mm thick Gor-Tex film serves as the 15 K filter. A clamp that holds the filter in place has an aperture diameter of 25 mm. A 10.6 mm thick and 60 mm diameter PTFE filter with matching grooves on both sides serves as the 110 K filter.

The aperture at the 110 K stage is 61 mm in diameter. The two IR filters are tilted with respect to their mounting plates and are not orthogonal to the beam, thus reducing reflections back into the feed horn. The nominal antenna focal plane is aligned with the cryostat top plate which is at 300 K. The exit aperture for Band 2 on the plate is 92 mm in diameter. The distance between the 300 K window and the 15 K filter is 83 mm. The beam waist of the antenna at the Cassegrain focus is 25.5 mm for the given illumination taper at 67 GHz. The specified noise budget for the band requires that the feed horn be cooled to 15 K. Cooling the focusing element in the optics to either 15 K or 110 K also, is desirable in order to reduce noise. The robotic arm of the Amplitude Calibration Device (ADC) moves in a plane above the cryostat plate. Any optical element of the cartridge should be below this plane.

III. OPTICS DESIGN

In order to obtain frequency independent illumination taper of -12 dB at the subreflector, the feed horn will have a diameter of 106 mm and length of 785 mm. The beam waist at 67 GHz is 23.7 mm and placing the feed aperture 5mm below the 15K filter, the maximum power that is intercepted at the 92 mm aperture is 94.53%, resulting in beam truncation. Cooling this feed to 15 K will result in excessive thermal loading. Hence, it is desirable to start with a smaller beam waist at the feed horn and transform to the required telescope waist by use of reflective/refractive optics.

Reflective optics with a curved mirror on top of the feed horn to refocus the beam and a flat mirror to redirect the beam towards the subreflector presents the best option as the loss can be kept low compared to the use of a dielectric lens. The flat mirror will have to be located outside the 255 mm radius, for lack of space, resulting in additional scan loss. The extra offset of the beam results in an angle of incidence at the subreflector greater than 2.48° with additional contribution to crosspolarization. The curved mirror also will interfere with the movement of the robotic arm of the ADC. Hence, a refractive optics option is selected.

The waist at the Cassegrain focus of the ALMA antenna is 21.7 mm at 78 GHz for an edge taper of -10.8 dB, using Gaussian beam approximation. The beam waist of the designed optics should match the waist at the Cassegrain focus. Since the feed horn needs to be cooled to 15 K, it

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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should be located below the 15 K window. If the lens is cooled to 15 K or 110 K, the dielectric constant of the material is likely to be different from the nominal value and also when mounted independently, the lens could move relative to the feed horn. This warrants the lens to be positioned at or outside the 300 K window. Taking into account the distance constraints, location of the IR filters, and the maximum size of the feed horn and the lens, three designs of lenses have been arrived at to work with a smaller feed horn. The focal lengths of the lenses and distances from the feed horn are different by a small amount. The power intercepted by the lens is proportional to the radius of the lens. For all three designs, the lens radius is 46 mm and the maximum aperture efficiency achievable at 67 GHz is 84%. It was decided that the lens will be mounted at the 300 K window serving as a vacuum barrier at the same time. The position of the feed horn relative to the 15 K window is different for the three lenses.

IV. FEED HORN

A linear taper corrugated horn with input diameter of 2.06 mm, aperture diameter of 15 mm and length of 86.75 mm has been designed. The spherical phase error at the aperture of the horn is less than 0.1 at the lowest frequency of operation. The waist radius at 78 GHz is 4.71 mm. The beam has a taper varying between -10 dB and -22 dB at 20° in the 67-90 GHz range. However, in combination with any of the three lenses the beamwidth is nearly constant. There are 76 corrugations with a pitch of 0.86 mm, rendering about four corrugations per wavelength at the highest frequency of operation. The depth of the corrugations changes continuously from 0.46 to 0.23 .

The feed horn was fabricated by first machining a mandrel out of aluminum. A circular-to-square transition that is required to mate to the square input of an orthomode transducer (OMT) was part of the mandrel. Copper was electroformed on the mandrel, cut to size, and the aluminum mandrel was dissolved in acid. A UG-387/U flange was soldered and the feed was finished with a thin layer of gold.

Reflection coefficient of the feed by itself and then with the lens at the appropriate distance was measured. Reflection coefficient is better than -22 dB in both cases. Copolar far-field patterns of the feed were measured in the principal planes and crosspolar patterns in the diagonal (D) planes in an anechoic chamber range in Green Bank, WV. Measured and simulated patterns are shown in Figure 1 at 78 GHz. The measured patterns agree well with theory and there is good match of the patterns in the two planes. Cross-polar patterns measured in the D-plane are shown in Figure 2. Patterns above 82 GHz are shown and crosspolarization is better than -25 dB.

Fig. 1. Measured (Me) and simulated patterns (Th) of the feed horn in E- and H-planes.

Fig. 2. Measured copolarization (78 GHz) and crosspolarization (82-90 GHz) of the feed horn.

V. DIELECTRIC LENS

A bi-hyperbolic lens effectively matches the beam waist of the feed to the beam waist at the Cassegrain focus at all frequencies. As mentioned earlier, three designs of lenses have been arrived at, each with different focal lengths. The absorptive loss in the dielectric lens is proportional to the thickness and also depends on the material properties. In an attempt to reduce the thickness, a Fresnel lens with a single zone at a radius of 42.5 mm was designed [2]. A detailed analysis of Fresnel lens was carried out while varying parameters such as focal length, thickness, location of the zone etc., optimizing aperture efficiency. The parameters of the four lenses are shown in Table 1.

TABLE 1. DIELECTRIC LENS PARAMETERS

Lens # Focal length

mm

Distance from feed mm

Thickness mm

1 86 86.76 26.60 2 87.2 89 26.34 3 92 92 25.36 4 86 78 23.28

Different materials such as high-density polyethylene (HDPE), quartz, fused silica, polytetrafluoroethylene (PTFE), were considered for the lens material. HDPE was chosen because of its low dielectric constant, low loss tangent, mechanical properties and ease of machining. Since the lens

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forms the vacuum barrier, a thickness of 5 mm was chosen for the flange. The thickness at the center shown in Table 1 includes the thickness of the flange. Antireflection (AR) structure was required to compensate for the difference in relative permittivity between HDPE and air. This would minimize reflection loss and standing waves between the lens and adjacent surfaces. The AR structure consists of circular corrugations on both sides of the lens. The pitch and width of the corrugations are 2 mm and 1 mm, respectively. The depth of the corrugations takes into account the angle of incidence on the lens surface [3] and the average depth is 0.77 mm. The lens was machined on a numerically controlled mill. The top surface of the lens has the hyperbolic curve while the bottom of the groove is perpendicular to the normal at the center of the lens. This facilitates using a tool with a flat surface.

Before installation in the cryostat, far-field patterns of the feed horn/lens combination were measured in the anechoic chamber range. The IR filters were not included in these measurements. Each of the four lenses was measured, placing the lens at the appropriate distance from the feed horn aperture. Copolar patterns were measured in the E-, H-, and D-planes while crosspolar patterns were measured only in the D-planes. Figure 3 shows the patterns at 78 GHz for lens 2. The average taper at 3.6º is -12.7 dB and the worst crosspolarized lobes are -22 dB below the peak of the beam at 90 GHz. Crosspolarization is worse by about 3 dB with the addition of the lens, compared to that of the feed horn. The patterns measured with other lenses have the expected edge taper values.

60

50

40

30

20

10

0

15 10 5 0 5 10 15

Gain(dB)

Angle (degrees)

E H D Xp

Fig. 3. Measured copolar patterns in E-, H- and D-planes and crosspolarization (Xp) in the D-plane of the feed horn+lens at 78 GHz.

VI. CRYOSTAT MEASUREMENTS

The feed was mounted on an A-bracket of the cartridge assembly as shown in Figure 4. The lens was mounted on the window of the Band 2 cutout. For each lens, a spacer of appropriate thickness was introduced between the OMT and the feed horn, so that the aperture of the horn was at the right distance from the lens. For lens 2 shown in Table 1, the feed horn aperture is 5 mm below the 15 K filter. The A-bracket has the 2.48º tilt built in so that the beam from the feed horn points towards the center of subreflector. A special bracket that holds the lens tilts the lens at the same angle, keeping the feed horn beam orthogonal to the lens. The optics was

Fig. 4. Cartridge assembly of Band 2

characterized by the Nearfield Systems Inc. (NSI) scanner at the Integration Center in Charlottesville, VA. The 2D-scanner probes the near field above the lens and Fourier transformation gives the far-field pattern. Polarization 0 is along the radial line from the center of the window to the center of the cryostat and polarization 1 is orthogonal to this. Measurements were made at room temperature and also after cool down. Figure 5 shows the efficiency calculated from the far-field patterns of a cold cartridge for Polarization 0 for lens 2. Aperture efficiency is greater than 80% over the entire band. However, polarization efficiency is below 99.5% at a few frequencies.

In order to diagnose the lower polarization efficiency, crosspolarization measurements were carried out in the anechoic chamber. Measurements were done introducing the IR filters and apertures representing the openings in the cryostat at various stages, one at time. The filters were mounted at the appropriate angle with respect to the beam axis. Polarization efficiency was calculated using only two diagonal cuts, where the peaks of the crosspolarised lobes occur. Hence, the efficiency will be lower than that calculated with many cuts, as done in the NSI software. Figure 6 shows efficiency with and without the filters. It is clearly seen that the filters cause deterioration of the crosspolarization and a major contributor is the 15 K filter.

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Fig 5. Efficiency of Band 2 beam on the secondary computed from 2-D patterns measured in the Integration Center.

Fig. 6. Polarization efficiency of feed horn & lens with and without the IR filters.

VII. CONCLUSION

The aperture efficiency values calculated for lens 1 and 3 were found to be lower than that of lens 2. The zoned lens (4) has a tight fit in the 92 mm aperture at the 300 K plate. Once installed, there is no freedom to adjust its position to repoint

the beam. Except for the loss, which is about 10% lower for this lens, there is no other advantage. The contribution to system temperature using any of the other 3 lenses is about 8 K. Receiver temperature measurements are in progress. NRAO is looking at other options for manufacturing the feed horn so that it is less expensive and has quick turnaround. A major portion of a second horn was machined out of aluminum at the NRAO Green Bank machine shop. The section with the circular-to-square transition could be electroformed and attached to the machined section. NRAO has successfully completed the design, construction, and characterizing performance of the optics for ALMA Band 2.

Acknowledgment

The authors thank M. Hedrick, H. Sipe, and P. Doolittle of the Green Bank machine shop who were responsible for the machining of the feed horn and lens and C. Beaudet at Green Bank for assisting in the measurements. The authors also acknowledge the contributions of J. Buchanan, K. Crady, J. Effland, J. Meadows, G. Morris, G. Petencin, and K. Saini of the Central Development Laboratory.

References[1] A. Wootten and A. R. Thomson, “The Atacama Large Millimeter/

Submillimeter Array,” Proceedings of the IEEE, vol. 97, Issue 8, pp. 1463-1471, August 2009.

[2] A. Gonzalez, “ALMA Band 2 Optical Design,” private communication, April 2015.

[3] R. E. Collin and F. J. Zucker, Antenna Theory Part 1, McGraw-Hill Book Company, 1969, pp. 650-653.

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