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.

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Abstract—The Atacama Large Millimeter/submillimeter Array(ALMA) Band 1 receiver covers the frequency band between 35-50 GHz. Development of prototype receivers, including the key components and subsystems has been completed and two sets of prototype receivers were fully tested. We provide an overview of the ALMA Band 1 science goals, and its requirements and design for use on the ALMA. The receiver development status is also be discussed and the infrastructure, integration, evaluation of fully-assembled band 1 receiver system is covered. Index Terms—ALMA, Band-1, Receiver.

I. INTRODUCTION he Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) is leading the construction of the

Band 1 (35-50 GHz) receiver system to the ALMA project, in collaboration with the National Research Council of Canada Herzberg Astronomy and Astrophysics (NRC Herzberg), the National Radio Astronomy Observatory (NRAO) in USA, the University of Chile (UCh) in Chile, and the National Astronomical Observatory of Japan (NAOJ). ALMA Band 1 will be able to study a very broad range of astrophysical environments, from nearby stars to the re-ionization edge of the Universe. The two main scientific cases of ALMA Band 1 are also two of the ALMA Level One Science Goals: 1) the study of grains in protoplanetary disks to sizes as large as ~1 cm; and 2) the detection of the CO (3--2) line from nearby to high-redshift galaxies (6 < z <10 ). But, the scientific case for ALMA Band 1 is much more extended: studies of the cold molecular ISM, through the observation of many molecular lines close to their fundamental rotational transitions, which will allow the study of star formation processes in nearby galaxies; the study of the Sunyaev-Zel'dovich effect to probe the physics of galaxy clusters; the study of protoplanetary disks; detection of spinning dust; study of chemical differentiation in cloud cores; solar observation of flares in the Sun; or study of magnetic fields through the use of the Zeeman effect. Band 1 will allow ALMA to bridge the gap between the mm/sub-mm and cm radio astronomy and, given the large ratio of the available bandwidth to frequency, it will be able to study a large range of energy regimes. The plan is to complete and deliver all 73 receiver units by the end of 2019. Not only are the technical requirements for the

receivers far more stringent than any existing receiver system at this frequency band, but also the development and delivery schedule is challenging. To meet this requirement, the integration/test infrastructure and human resources must be planned to sustain the requirements over the next several years. Industrial involvement is also one of the important elements in the project planning. This document present an overview of the Band 1 receiver project, the status of receiver development, and the progress of establishing the infrastructure and the projected project time-line.

II. DESIGN The ALMA Band 1 Receiver will operate in the 35-50

GHz frequency band. The coverage is desirable for 50 – 52 GHz range, which shall be considered as best effort basis. This frequency range is corresponds to 36 % bandwidth with a center frequency of 42.5 GHz. The receiver comprises of three subsystems: optics, cold cartridge and warm cartridge. The technology used for Band 1 is dual-polarization, SSB heterodyne receiver covering the specified frequency rang with IF band of 4-12 GHz.

Unlike other ALMA cartridge bands between Band-3 and Band-10 where superconductor-insulator-superconductor (SIS) mixers were used to achieve state-of-art sensitivity for signal detection. Band 1 is using the technology of the hetero-junction field effect transistor (HFET) cryogenically cooled amplifiers which providing high sensitivity, positive signal gain, and higher operating temperature when compared with SIS mixer. In Band 1 cartridge design, despite the unavoidable optics loss and insertion loss of the orthomode transducer, the sensitivity of the receiver is mainly determined by the noise and gain performance of the cryogenic low noise amplifiers. The input signal is collected by a corrugated aluminum feedhorn at the 15K stage. The two orthogonal polarizations (0 and 1) are then split using an Orthomode Transducer (OMT)[1]. The signals are then amplified with cryogenic low noise amplifiers fixed to the 15K stage[2]. Since these amplifiers are optimized for low noise, bandwidth and gain rather than the input RF impedance match, the use of Q Band isolators at cold cartridge 300K plate section prevents signal reflections between the cryogenic amplifier and room temperature amplifier unit.

The Atacama Large Millimeter/submillimeter Array (ALMA) Band-1 Receiver

Yau De(Ted) Huang , Oscar Morata, Patrick Michel Koch , Ciska Kemper , Yuh-Jing Hwang , Chau-Ching Chiong , Eddie Huang, Bill Liu, Shou-Hsien Weng, Chin-Ting Ho, Po-Han Chiang,

Hsiao-Ling Wu, Chih-Cheng Chang, Shou-Ting Jian, Chien-Feng Lee, Yi-Wei Lee, Satoru Iguchi, Shin'ichiro Asayama , Daisuke Iono , Alvaro Gonzalez , John Effland, Kamaljeet Saini, Marian

Pospieszalski, Doug Henke, Keith Yeung, Ricardo Finger , Valeria Tapia , Nicolas Reyes

T

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Room temperature down-converter is used in the Band 1 system. To minimize the impact of mixer noise to the system, a commercial available Q-band amplifier is used to further amplify the signal. The upper sideband configuration is realized by using high-pass filter[3] after the room temperature Q-band amplifier. The commercial available mixer is used to achieve the less maintenance and high reliability requirement. The IF signals are then brought out to the commercial available IF amplifier, in order to bring the IF power level to the ALMA specifications.

III. RECEIVER PERFORMANCE The Table 1 summarizes the key specifications for Band 1, based on the requirements set by the ALMA Scientific Advisory Committee[4][5]. In this section we summaries the performance that achieved for the current Band 1 receiver cartridges performance was measured in a single cartridge test cryostat.

A. Noise temperature and IF power performance The receiver noise temperature was measured by a

standard technique using 300K and 80K black body radiators presented in front of the receiver. The receiver noise temperature, integrated over the 4-12 GHz IF band is presented in Fig 2 and Fig. 3 as a function of LO frequency. The measured receiver noise temperature of the Band 1 is below 30K full-band for Pol-0 and Pol-1, except some resonant spike reach 31K.

Table-1 Band 1 CARTRIDGE MAIN PERFORMANCE REQUIREMENTS

Parameter Specification

RF port frequency range 35.0-50.0GHz/50.0-52.0(Best Effort)

LO Port Frequency rang 31.0-40.0 GHz

IF Port Frequency range 4.0 –12.0 GHz

SSB Noise Temperature < 25 K over 80% of band < 32 K over entire band

Image band suppression and sideband mismatch

10 dB over 90% of IF frequency range. > 7 dB over entire IF frequency range.

Large signal gain compression

5% caused by the different RF load temperatures of 77K and 373K

The total power within the IF band

>-32dBm to -22dBm

IF power variations 4dB p-p over any 2GHz window 7dB p-p full band

Parameter Specification

large signal gain compression

<5% @load exchange between 77 K and 373K

Amplitude stability: Allan variance

4.0 x 10-7 for timescales in the range of 0.05 s T 100 s 3.0 × 10-6 for T = 300 seconds.

Signal path phase stability

22fs over 300s

Aperture efficiency >80%

The current cartridge maximum receiver IF output power variations within any 2 GHz wide window is shown in Fig. 4. The current measurement result indicates the system performance is not compliant with peak-to-peak variation 4 dB for 2GHz window. This is an important receiver parameter, because limited dynamic range of the backend. As a result, the ripple budget for the 2 GHz window is indicating the dominant components are the cold LNA, the mis-match relevant to the cold LNA and warm Q-Band LNA. The IF chain including the IF amplifier is not the dominant component in ripple budget by both analysis and measurement result. Further elaborate model by collecting more sample data is needed with pre-production cartridges. This would allow us to confirm the 2GHz window specification performance.

Figure 2: Measured receiver noise temperature for Pol 0

Figure 3: Measured receiver noise temperature for Pol 1.

Figure 1: Block Diagram of the Band 1 cartridge assembly

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Figure 4: IF power variation within 2 GHz window

B. Image band suppression and Gain compression The image band suppression characterization method is

injecting a weak RF signal synthesized by the signal generator at RF signal chain. Keeping the input signal power in the same level and swap the frequency between USB and LSB. The measured result is mostly large than 25dB suppression, except the case of LO = 38GHz and IF = 4GHz, where the corresponded lower sideband RF frequency is 34GHz and the image-band suppression is 10dB that still compliant with specification.

The Gain compression is injecting an external signal in the input port of RF. The IF output power from the IF output channels are measured. The test is sweeping the RF input power at different LO frequency. The gain compression measurement result indicates the Band 1 receiver is compliant with 5% specification.

C. Amplitude stability and Phase stability Test results on Band 1amplitude stability was below 2.0 x 10-7 for timescales in the range of 0.05 s T 300 s. That is compliant with the specification. Fig 5. Shows the amplitude stability data at 38GHz LO frequency for Polarization 1. The phase stability for all the LO frequencies was measured, the results are all below 10fs, which are well fit to the specification

Figure 5: Band 1 Amplitude Stability: 38 GHz LO at Pol 1

D. Receiver Optics performance The ALMA band 1 optics layout is shown Fig. 6. The optics of the ALMA band 1 receiver optics design is considering the beam should be tilted 2.48° with respect to the cryostat, in order to point to the sub-reflector. Then, the horn and the lens should be tilted toward the sub-reflector in the same angle. Moreover, in order to minimize the truncation in the rings supporting the 15-K infrared filter of the cryostat, the horn has been placed 5 mm away from it. This is as close as possible without risking collision during cool-down. Therefore, the lens is located at 80 mm from the cryostat top plate on a holder structure. Since the lens is used as vacuum window, the assembly is composed of several components, a holder that provides the appropriate distance, a support ring that uniformly distributes the load across the lens in order to avoid vacuum leaks and O-rings as vacuum seals. In summary, the final optical system consists of a compact spline-profile corrugated horn machined from a single block of aluminum, a low-loss HDPE one-zone bi-hyperbolic lens and the holder of the lens including vacuum interfaces. Because of the IR filters truncation, reflection and coupling effects impact the final efficiencies. The aperture efficiency has decreased 1.7% in average with respect to the simulations, showing 4 frequency points below 80%, which is out of specifications.

Figure 6: Cross-section of the baseline optical design. 1: Spline profile horn antenna, 2: 15 K infrared filter, 3: 110 K infrared filter, 4: lens holder, 5: One-zone modified Fresnel lens.

Figure 7: Full system efficiencies for polarizations 0 and 1.

Amplitude Allan Variance of Band 1, LO=38GHz

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

0.01 0.1 1 10 100 1000T(sec)

Alla

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aria

nce

( 2(

T) )

Allan Spec.(0.05~100 sec) Spec.(300 sec)

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IV. BAND 1 RECEIVER PRODUCTION Development and Production ALMA Band 1 Receiver is considering the standard commercially obtainable components. If not available, then consider the custom made components. In Production phase, NRAO and NRC Herzberg will extend their contribute from components development to supplying the production components: 1st LO(NRAO), Cold Low Noise Amplifier (NRAO) and OMT (NRC Herzberg) in the same manner of RAL (ALMA cartridge body). The UCh is focusing on supplying the optics components incudes horn and lens. Band 1 receiver production will be carried out by (Aeronautical System Research Division) ASRD, former EA FEIC team, in cooperation with ASIAA.

A. Facility and Staff Band 1 cartridge integration and testing converted the East Asia Front-End integration center(EA FEIC) facility as band 1 integration and testing lab. The test system was convert and modified for Band 1 production unit integration and testing during the development phase (2013-2015) in conjunction with the assembly and testing of the prototype receivers, and the test procedures were also refined in the same period. Fig 8 shows the Band 1 production process and relationship. Trained persons are necessary for both tasks in the mass-production phase. The availability of those qualified staff could introduce an impact on the Band 1 cartridge delivery schedule. To mitigate this risk, we are establishing a training program to ensure the new staff can pick up the knowledge. We also develop a cross-training techniques where multiple team members have the same skill sets.

Figure 8: Production process

B. Production Schedule The first year of the production phase in year 2016 is dedicated to setting up second complete receiver test line that allows for a higher production rate of approximately two receivers per month starting from mid. of 2017. The goal is completed the #73 receiver delivery to ALMA Operation Support Facility (OSF) by end of 2019.

V. CONCLUSION We have demonstrated the complete assembled and

fully characterized ALMA band 1 receiver. The receiver shows good sensitivity and wide Bandwidth coverage without degradation performance. Because of the existing component of IR filters causing the truncation and coupling effects impact the final efficiencies. We are still able to demonstrate a careful design of the optics that the aperture efficiency has been reached at 80% with exception of 4 frequency points.

Meeting the planned schedule requires that cartridges are built as planned, using the best available components at the time of assembly. The facility is converted the EA FEIC facility and experience for Band 1 production integration and testing. This would allow the experienced staffs continue their expertise contribute to the Band 1 integration and testing. The established PA/QA process will continue and promote the safety and product assurance activities for Band 1 production.

REFERENCES

[1] Doug Henke and Stéphane Claude,” Minimizing RF Performance Spikes in a Cryogenic Orthomode Transducer (OMT)” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 62, NO. 4, APRIL 2014

[2] E. W. Bryerton, M. Morgan, M. W. Pospieszalski , “Ultra Low Noise Cryogenic Amplifiers for Radio Astronomy ,” 2013 IEEE Radio and Wireless Symposium (RWS), pp. 358 - 360, Jan. 2013.

[3] Y.-S. Lin, Y.-S. Hsieh, Y.-J. Hwang and C.-C. Chiong, “Q-band bandpass filter designs in heterodyne receiver for radio astronomy,” IEEE Asia-Pacific Conf. Circuits and Systems (APCCAS 2008) , Macau, China, Nov. 2008.

[4] Wild, W. & Payne, J., “ALMA Specification: Specifications for the ALMA Front End Assembly”, 2001

[5] C. T. Cunningham, G.-H. Tan, H. Rudolf, K. Saini, “Front-End Sub-System for the 12m Antenna Array Technical Specifications,” Atacama Large Millimeter Array Internal Technical Document, ALMA-40.00.00.00-001-A-SPE, April 17, 2007.

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