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
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
1
Using Antennas with Pattern Synthesis
K.A. Lukin, V.V. Kudriashov, P.L. Vyplavin, Sergii Lukin and V.P. Palamarchuk Laboratory for Nonlinear Dynamics of Electronic Systems (LNDES)
O. Ya. Usikov Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine IRE NASU, 12 Ak. Proskura Str., Kharkiv, 61085, Ukraine
e-mail: lukin.konstantin@gmail.com
Abstract— The paper is devoted to experimental investigation of coherent radiometric imaging in azimuth-range plane. The Ka-band radar system suggested is a version of bistatic noise waveform SAR (Synthetic Aperture Radar) using two antennas with pattern synthesis and additional receive channel used as a reference signal. Indoor radiometric imaging has been validated using absorbers as the targets. These experiments enabled to estimate both range and angular resolution capabilities of a new radiometric system that was achieved due to application antennas with pattern synthesis and SAR signal processing. This estimate may be done within the range span limited by bistatic radiometer baseline.
Keywords—Millimeterwave radiometric imaging; bistatic radiometer; antenna with pattern synthesis; thermal electromagnetic radiation
I. INTRODUCTION
Radiometric monitoring allows obtaining information about thermal electromagnetic radiation of objects in the chosen frequency band. Imaging of distant targets using their thermal electromagnetic radiation may be implemented with the help of bistatic radiometric systems. These systems are based upon measurement of time difference of arrival (TDOA) of the radiation received by two or more antennas [1-3]. This approach has been developed in long wave radio-astronomy to improve angular resolution of radio-telescopes [4, 5].
Imaging in millimeter waveband is often done using multichannel reception via antenna array and mechanical or frequency scanning along the axis perpendicular to the array plane [6]. Further development of this approach consists in implementing of 2D aperture synthesis which enables radiometric coherent imaging in the plane perpendicular to the antenna beam. Such images contain not only amplitude information but also information concerning relative phase [7].
There is a possibility of coherent radiometric imaging using combination of information gathered by a bistatic radiometer response and angular position of narrow beam of the antenna with pattern synthesis [8, 9] at one (or both) of the receivers. Each pixel of such image can be obtained as focusing of equal difference of paths of the received signal areas via the pattern of the antenna with beam synthesizing. Besides, the factors limiting usage of this approach and possibility of obtaining of
2D resolution using combination of bistatic radiometer data and principles of aperture synthesis have not been investigated yet. This paper is devoted to investigation of coherent radiometric imaging via proving the concept of 2D imaging by bistatic radiometer based on two antennas with beam synthesizing. Experimental investigations have been carried out using a bistatic ground based Ka-band radiometer based upon Ground Based noise waveform SAR [8, 9].
II. RADIOMETRIC IMAGING IN AZIMUTH-RANGE PLANE
Proposed approach to coherent radiometric imaging consists in using bistatic radiometer and moving of the antennas for realizing aperture synthesis. This scheme enables obtaining range resolution of objects at a distance comparable with bistatic baseline. This is possible due to the fact that shifting of one antenna of the interferometric radiometer with respect to a stable target varies phase relations between received signals. For different positions of the target these phase relations will depend on antenna positions in different way. This fact can be used for building a reference function (a function of antenna position) and performing co-variation of radiometer returns with it. This gives a chance to improve the resolution and to some limit to generate 2D images. The paper is devoted to investigation of possibility and constraints of generation of 2D images using this approach at short ranges.
For realization of spatial scanning we use antennas with pattern synthesis [8, 9]. Phase centers of antennas are moved along a real aperture in stepped like manner and signals are being recorded only when the antennas are stationary. This enables using common in SAR principle of splitting of the processing into two parts. The first step of coherent radiometric imaging is to perform cross-correlation between signals received in two channels. This will give information concerning mutual path length difference between signals. The second stage consists in co-variation of those signals obtained in different positions of antennas with synthesized beam with corresponding reference function.
TDOA relative delay a (x, y)τ of the signals propagated
from a scatterer with coordinates (x, y) to the antennas of the
bistatic passive radar at position a equals to
( )a 1,a 2,a(x, y) l (x, y) l (x, y) cτ = − , where c is velocity of
1007
light; 1,(2)al (x, y) are paths lengths between a scatterer with
coordinates (x, y) and the receive antennas [3]. Note that difference of paths lengths can’t exceed the distance between antennas Bl .
Suppose, we receive a signal from the scatterer at the input of antenna of bistatic passive radar reference channel. We denote the recorded signal as 1,aS (t) . The second signal is
delayed with respect to the first one by a (x, y)Δτ . So the
signal at the second antenna can be denoted as 2,a aS (t )− τ . Cross-correlation of these signals can be estimated as:
* a 1,a 2,a a
T 0 τ = ⋅ + τ ⋅ − τ τ , (1)
where: B Bl l [ ; ]
c c τ∈ − is the mutual delay of the signals; T is
the integration time; symbol * denotes complex conjunction. When τ coincides with a (x, y)τ correlation gives a peak
enabling to detect the scatterer and related mutual delay of the signals.
Compression of the signals over the path length difference consists in estimation of cross-correlation Ra(τ, T) for the required mutual TDOA values and for all antenna positions.
Movement of phase center of antenna with pattern synthesis [1, 2] causes change in path of the radio waves in waveguide. This change has to be taken into account during signal processing. Estimation of phase shift is easier to perform in frequency domain by adding related phase shifts to every frequency components of the signals. Using known relation for the main wave of rectangular waveguide this phase shift can be estimated using the following relation:
( ) ( ) 22 a i a i2 l 2p −− λ = π εμλ − , (2)
where: i il c f= is the wavelength for the given frequency if
in free space; al is the waveguide length corresponding to the
antenna position a ; ε , μ are the relative electric and
magnet permittivity; and p is the width of wide wall of the
waveguide. Cross-correlation (1) estimate has been performed in
frequency domain:
j ( )R ( ) S ( ) S ( )ei
ω
ω τ − ωτ = ω ω (3),
where: ωi =2 π fi is circular frequency of the received waveform spectrum limited by receivers’ band pass ωi∈[ωmin; ωmax].
The second step in radiometric image generation uses the results of the correlation processing (3). It consists in estimation of expected variation of the received signals as a function of antenna position using theoretical response for the
current point. Then the received signals are compared with this reference function via co-variation. After using common simplification consisting in neglecting the amplitude factor in the reference function, the relation for generation of coherent radiometric image ( ),I x y in time domain will get the
following form:
N j x, y I(x, y) R x, y e
a 1
spectrum; aN is the number of antenna positions in the
performed scan. In this way, the radiometric coherent image formation
consists in matched filtration of the signals received by interferometric radiometer with moving antenna and further averaging of the results obtained with taking into account the phase differences between the signals received by two antennas of bistatic interferometric radiometer (4).
III. KA-BAND BISTATIC RADIOMETER BASED ON ANTENNAS
WITH PATTERN SYNTHESIS
Experimental evaluation of the possibility to obtain range resolution on coherent radiometric images has been implemented using a unique equipment – namely, Ka-band Ground Based Noise Waveform SAR (GB NW SAR) [10,11]. The equipment (fig. 1) uses the new type of antennas – antennas with pattern synthesis [8, 9].
A. Antennas with pattern synthesis This is a special type of antennas phase center of which is
moved along a real aperture (fig. 2). As a real aperture antenna, a waveguide with a non-radiating half-wavelength
longitudinal slot in its wider wall is used. The wall is covered with a metallic tape having a half-wavelength transverse resonant slot. In order to enhance its efficiency we placed a sliding plunger tied to the tape at the proper distance from the radiating slot. The tape moves along precise guides.
Fig. 1. Picture of Ka-band ground based bistatic Noise Waveform SAR modified for operation in the radiometric mode.
1008
Position of the slot is controlled by angle sensor. In the work phase centers of antennas are moved in stepped like manner and signals are sampled when the antennas are not moving.
B. Radiometric imaging experiments For the coherent radiometric imaging experiment the
equipment was placed inside a laboratory room. The phase delay of signal passes along antennas and receivers has been taken into account by calibration measurements using noise waveform generator placed in the scene. Correlation processing of received signals has been performed in spectral domain using Fourier transform. Sidelobes level can be improved applying weighting function to spectrums of received. Coherent radiometric imaging has been done taking into account the time difference of arrival of the emitted signals each receiver antenna (5). Step-like moving of the tape scanners’ phase centers and SAR focusing modified for radiometric mode enabled to generate coherent radiometric image. The image contains amplitude and phase information along coordinates (range and cross range direction).
The radiometer operates in Ka-band (36-36.5 GHz).
Receiver has 94-97 dB gain and noise figure of4.8 dB within the working frequency band. Antennas with pattern synthesis used in the system provide spatial movement of their phase centers along synthesized aperture of 0.7 m. Stepped-like motors provide synchronous movement of the antennas and their stability during the measurement. Scanning consists in reception of signals with different positions of the antenna phase center on an equally spaced grid. The received signals are amplified, down converted and fed into the ADC with 1 GS/s sampling rate. The acquired data are processed using described above algorithm.
In order to test the ability of the radiometer to detect the
objects in the receiver's intrinsic noise background and the difference in thermal radiation of the objects, as well as to estimate radiometer sensitivity, the related experiments have been carried out.
In order to prove the concept of range-azimuth radiometric
imaging an indoor experiments was carried out with a noise generator served as a source of strong noise signal in Ka-band. Bistatic baseline was about 3 m. The indoor field of view was 6x9.5 m. Moving the phase center of one of the antennas with synthesized beam (fig. 1) and applying proposed processing (3,4) enables focusing the target’s position. Figure 2 shows the obtained image of the single bright target obtained in radiometric regime. -3 dB threshold was applied to the image in order to show the resolution of the radiometer.
The proposed approach enables to use two antennas with
synthesized beams (fig. 1) to obtain images with more precise focusing and better range-azimuth resolution. In order to test range and azimuth resolution in the proposed approach we have performed set of experiments with two targets.
Fig. 2 Range – azimuth coherent radiometric image obtained by proposed
approach and bistatic passive radar with synthesized beam antenna
Noise waveform generator’s signal was split into two parts and fed into two Ka-band horn antennas placed at different positions. One of the signal copies was delayed in order to guarantee that signals in the antennas are not correlated. Figure 3 illustrates resolution capability in both azimuth and range where the Ka-band radiation sources have been placed at different range and cross-range coordinates. Threshold applied to the image shown equals -20dB.
Fig. 3 Range – azimuth coherent radiometric images of a noise generator signal obtained by the proposed approach with one synthesized beam antenna
For proof of concept for indoor coherent radiometric
imaging two absorbers were placed between two windows (fig. 4a). The room inside temperature was about 293 K. To perform Ka-band radiometric imaging based on antennas with synthesized beam the equipment was placed in front of a window (fig. 4.a). Two 0.9x1.35 m absorbers were placed at the range of 2.7 m with distance interval of about 0.5m to check the range resolution. Two independent measurements
1009
were carried out. In the first measurement antenna positions number was 31 and in the second it was 51 to check the range resolution. Antenna step for both measurements was 2.95 mm. Antenna positions and objects are shown with semitransparent boxes in figure 4.b,c along with radiometric images obtained. The signal to noise ratio of the images was rather low.
a
b
c
Fig. 4 Range–azimuth coherent radiometric images of pure radiometric scenario obtained using proposed approach with two synthesized beam
antennas
Nevertheless, stronger signal is coming from the absorbers which have higher radiometric temperature. Their presence is repetitive from measurement to measurement. Using longer aperture enables to generate narrower synthetic beams to improve range resolution - comparing images fig. 4.b and 4.c
one can see that absorbers are distinguished separately in range when the synthetic aperture length is increased.
IV. CONCLUSIONS
In the work, a principle for range-azimuth radiometric imaging using bistatic passive radar based on Ka-band antennas with pattern synthesis has been presented. Unlike others, this approach enables range estimation and radiometric images generation at short ranges in azimuth-range plane. This approach has been experimentally validated using Ka-band ground based noise waveform SAR operating in radiometric mode. It has been shown that the proposed approach enables resolution of the targets in both azimuth and range using radiometric regime of the measurement and spatial scanning with antennas with synthesized apertures. Positions of responses in radiometric images are in a good agreement with the positions of the test targets in the scene. Besides, the first time range-azimuth radiometric image of thermal radiation was obtained with antennas with pattern synthesis [1,2]. Operation in both regimes has shown a good agreement with theoretical expectations.
REFERENCES [1] Almazov V.B. Methods for passive radar. – Kh.: «VIRTA»,
1974.- 85 pp
[2] Edelsohn at al. Interferometric radiometer. US patent No. 4.990.925, 1991, 14 pp
[3] Shilo et. al. Microwave radiometric system «ZIR» for the customs service // Technology and design in the electric equipment, 2003, 3, pp. 11-13
[4] Interferometric radiometer, by C.R. Edelsohn and C.A Wiley. 1991, Feb. 5. 4990925.
[5] A.R. Thompson, J.M. Moran, and G.W. Swenson, Jr, Interferometry and synthesis in radio astronomy, 2nd edition. New York: John Wiley and Sons, 2001.
[6] S.A. Shilo, V.A. Komyak, “Millimeter band scanning multi-beam radiometer,” in Proc. MSMW, 1998, Kharkov, Ukraine, pp. 529-531.
[7] J. Dong, R. Shi, K. Chen, Q. Li, W. Lei, “An external calibration method for compensating for the mutual coupling effect in large aperture synthesis radiometers,” International Journal of Antennas and Propagation, vol. 2011, 8 pp., January 2011.
[8] K.A. Lukin, “Scanning Synthetic Radiation Pattern Antennas,” Radioelectronics and communications systems, vol. 53, No. 4, pp.219- 224 April 2010.
[9] K.A. Lukin, “Sliding Antennas for Noise Waveform SAR,” Applied Radio Electronics, vol. 4, No. 1, pp. 103-106, April 2005
[10] K.A. Lukin et. al., ”Ka–Band Bistatic Ground-Based Noise Waveform SAR for Short-Range Applications,” IET Radar, Sonar & Navigation, vol. 2, no. 4, pp. 233-243, August 2008.
[11] D. Tarchi, K Lukin, J. Fortuny-Guach, A. Mogyla, P. Vyplavin, A. Sieber, “SAR imaging with noise radar,” IEEE Transactions on Aerospace and Electronic systems, vol. 46, no. 3, pp. 1214-1225, July 2010.
[12] K.A. Lukin, “Noise Radar Technology, ”Journal of Telecommunications and Radio Engineering, vol. 55, no. 12, pp. 8-16, December 2001 [Radiophysics and electronics, vol. 4, No 3, pp. 105-111, 1999]
1010