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Alberta Hail Project Weather Radars
Alberta Hail Project Weather Radars
BACKGROUND
A weather radar facility, is located at the Red Deer Industrial Airport (Latitude 52 ° 18' N, Longitude 113 ° 88' W) approximately 150 km south of the city of Edmonton, Alberta. This site served as the centre for severe storm research and assisted in weather modification research during the Alberta Hail Studies Project (1956 - 1985). The facility is well suited from a hydrological perspective covering both foothills and plains basins. It consists of a unique polarization-diversity S-band (10 cm) weather radar, a standard C-band (5 cm) weather radar, and an X-band (3cm) radar used to track aircraft through a transponder system. The Radar Facility has been, used to conduct research into precipitation mechanisms, severe storm development, hail suppression, hydrology and microwave propagation.
RADAR DESCRIPTIONS
1. S-band polarization-diversity radar The S-band polarization-diversity radar, installed in 1967, operates at 2.88 GHz and has a parabolic reflector antenna with a 6.67 m diameter dish that produces a 1.15° beamwidth in both azimuth and elevation. The radar sweeps out a helical volume scan rotating at 48° s-1 (7.5 s per revolution) and rising 1° in elevation for every 360° in azimuth, up to a maximum elevation of 8 or 20° depending on the proximity of the storms to the radar. The radar records data with an approzimate azimuthal resolution of 1°, for 147 range gates, from 3 km to a distance of 157 km from the radar, with a range resolution of 1.05 km per range gate.
The radar can transmit any polarization, but has usually transmitted left-hand circular (LHC) polarization with 450 kW peak power. The receiver circuitry digitally records four measurements from the LHC and RHC components from each range bin. These are the RHC co-polar signal power, the LHC cross-polar signal power, and the correlation and phase between the LHC and RHC signals. The characteristics of the S-band radar are summarized in Table 1.
Table 1: Characteristics of the ARC S-band Polarization Diversity Radar
Characteristic Description _____________________________________________________________________________ Frequency 2.88 GHz Polarization LHC Beamwidth 1.15 degrees Diameter (antenna reflector) 6.67 m Gain 43.2 dB Peak Power 450 kW Pulse width 1.05 us PRF 478 Hz Range gate size 1.05 km Number of gates 147 Time averaging 21 ms per bin Independent samples * 7-14 Measurements i) RHC (co-polar) power ii) LHC (cross-polar) power iii) correlation of LHC and RHC signals iv) phase between LHC and RHC signals _____________________________________________________________________________
* The number of independent samples depend upon several parameters. Some are determined by the radar hardware, e.g. the number of pulse volumes averaged and the time each bin is sampled. Others depend on the scattering geometry and rain decorrelation time. That is why a range of 7 to 14 independent samples is quoted. (English et al, 1991)
English, M. B. Kochtubajda, F.D. Barlow, A.R. Holt, and R. McGuinness, 1991: Radar measurement of rainfall by differential propagation phase. Atmosphere-Ocean , 29, 357-380.
2. C-band radar
The C-band radar operates at 5.635 GHz and has a parabolic reflector antenna that produces a 1.5° circular beamwidth. The radar antenna azimuth rotates at 8 rpm. A step scan antenna elevation program performs from 0 to 9 degrees elevation in 1.5 min or 0 to 21 degrees in 3 minutes, depending on the proximity of the storms to the radar. The radar records data with an approzimate azimuthal resolution of 1°, for 147 range gates, from 3 km to a distance of 157 km from the radar, with a range resolution of 1.05 km per range gate.
The characteristics of the C-band radar are summarized in Table 2.
Table 2: Characteristics of the ARC C-band Radar
Characteristic Description ______________________________________________________________________________
Frequency 5.64 GHz Polarization horizontal Beamwidth 1.5 degrees Gain 41 dB Peak Power 250 kW Pulse width 2 us PRF 480 Hz Range gate size 1.05 km Number of gates 147 ______________________________________________________________________________
3. Radar Calibration (ref: Al-Jumily, 1989)
The major S-band calibration is carried out by injecting a known RF signal into the lower waveguide of the antenna towards the pedestal. With the antenna polarization control set to linear polarization, the signal is split equally into both receiver ports (port 1 and port 2). The injected signal and the corresponding digital data values are stored in the computer. A computer program is available to extract and display these calibration data graphically. The known signal levels are plotted against the corresponding raw data values. The major calibration is performed at least once a day when the radar is operational. The calibration data are required to convert the collected raw data values of port 1 and port 2 to power levels (Barge et al, 1976).
The calibration of the antenna and the phase discriminator is carried out by pointing the antenna at a remote transmitter, or monitor, situated on top of a 27.4 m tower located 400 m from the radar. This monitor can be set to transmit RHC or LHC polarizatrions as well as linear polarization of any orientation.
In antenna calibration, the polarization control device is used to direct a signal transmitted by the monitor to the appropriate port. By setting the antenna to transmit LHC while the monitor is transmitting LHC, the signal is directed to port 1 so that the amount of signal which leaks into port 2 can be measured. The difference between signals received by port 1 and port 2 is called the isolation. By setting the antenna to transmit LHC and the monitor to transmit linear polarization, equal signals should be received by the two ports. By inserting appropriate attenuation, the relative signal to noise ratio can be measured.
To calibrate the phase discriminator outputs, two methods are used. In the first method (phase shifter method), the antenna is set to transmit LHC polarization while the monitor transmits linear vertical polarization. The RF phase shifter is then rotated. This changes the phase of one of the received components with respect to the other. At each step, the outputs of the phase ports, port 3 and port 4, are obtained. In general, the phase shifter is rotated 20° steps until a complete revoulution is obtained. The raw data values of port 3 and port 4, together with the phase shifter setting, are then stored in the computer. This procedure is performed on a daily basis. Figure 1 shows an example of a set of phase shifter calibration curves for 7 July 1984. The phase shifter setting is plotted along the abscissa and the corresponding raw data values of port 3 and port 4 are plotted along the ordinate.
The second method (monitor method) involves setting the phase shifter to 0° and rotating the monitor while it is transmitting linear polarization. The monitor orientation is then recorded, along with the outputs of port 3 and port 4. This method is carried out at irregular intervals over each summer of operation. It provided information on the correspondence between the phase shift and the monitor orientation which is related to the orientation of a dipole scatterer. Figure 2 shows an example of a set of the monitor orientation. The curves represent one complete rotation in a clockwise fashion as viewed from the radar site. Since linear vertical polarization is transmitted when the monitor is pointing up or down, the curves repeat themselves twice during one complete rotation of the monitor.
References:
Al-Jumily, K.J.L., 1989: Remote identification of rain and hail with a polarization diversity radar. PhD Thesis. University of Alberta, 177pp.
Barge, B.L., R.G. Humphries and M.R. Johnson, 1976: Alberta hail radar-computer consideration. ASD Report 76-1, Alberta Research Council, 45-67.
4. X-band radar
The X-band radar is an aircraft tracking radar with a frequency adjustable receiver. The radar pulse interrogates aircraft with transponders. The transponders reply with coded pulses at the radar receive frequency, which is slightly different from the transmit frequency. In this mode of operation, only aircraft equipped with transponders can be detected by the radar, and weather echoes are not observed. Because of the large vertical beam width, no elevation drive is required. This type of antenna is known as a pillbox antenna. The characteristics of the X-band radar are summarized in Table 3.
Table 3: Characteristics of the ARC X-band Radar
Characteristic Description ______________________________________________________________________________ Transmited Frequency 9.475 GHz Receive Frequency 9.410 GHz Polarization vertical Beamwidth horizontal 1 degree Beamwidth vertical 16.5 degrees Peak Power 100 kW Pulse width 2 us PRF 478 Hz Antenna rotation rate 14 rpm maximum _____________________________________________________________________________
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