P12R.6 NCAR S-POL SECOND FREQUENCY (KA-BAND) RADAR Gordon Farquharson,∗ Frank Pratte, Milan Pipersky, Don Ferraro, Alan Phinney, Eric Loew, Robert Rilling, Scott Ellis, and Jothiram Vivekanandan National Center for Atmospheric Research, Boulder, Colorado 1. INTRODUCTION The National Center for Atmospheric Research (NCAR) has recently extended the observational capability of the S-band dual-polarimetric weather radar system (S-Pol, Keeler et al. (2000)) by adding a Ka-band (35 GHz) po- larimetric Doppler radar (Vivekanandan et al., 2004). The Transmitter Ka-band radar employs a dual channel receiver and can be configured for either HH and HV, or HH and VV polari- metric measurements. The Ka-band and S-band antenna beams are matched and aligned, and timing signals for both systems are generated from the Global Position Sys- tem (GPS) ensuring that a common resolution volume is sampled by both systems. This dual-wavelength capabil- ity provides the potential for retrieving water vapor profiles Radar Processor (Ellis et al., 2005) and liquid water content in Rayleigh scattering conditions (Vivekanandan et al., 1999), im- proving remote sensing of various precipitation types, and studies on cloud microphysics. 2. RADAR DESCRIPTION Receiver The K -band radar is housed in three enclosures which a Figure 1: K -band radar attached to the S-band dish. The are mounted to the S-Pol S-band dish and pedestal struc- a transmitter, receiver, and processor enclosures are vis- ture (Figure 1); these include the transmitter enclosure, ible. The K -band antenna is mounted to the receiver the receiver enclosure, and the radar processor enclo- a enclosure and is facing away from the viewer in the pho- sure. The system does not rely on the S-Pol pedestal tograph. infrastructure, and requires only an AC power feed. This allows the Ka-band radar to be run independently from S- Pol for development and testing. The system is weather Tallguide loss is around 3 dB/100 ft compared to around proof, and has been operated successfully during heavy 17.5 dB/100 ft for WG-28, and the loss through the rain. waveguide assembly is around 3.5 dB. Figure 2 shows an block diagram the K -band system. a The receiver enclosure is mounted on the side of Dashed lines in the figure represent each of the enclo- the S-band dish. It contains a dual-channel receiver sures. In the configuration shown, the transmitted power front-end (H and V polarizations), electronics to down- is split, and both H and V polarizations are transmitted si- convert received signals to an intermediate frequency (IF, multaneously. The power divider in the transmit path can 132.5 MHz), and the K -band antenna. The receiver uses be replaced by waveguide that directs all of the transmit- a a super heterodyne structure to keep the receiver center ted power to one of the antenna ports thereby reconfigur- frequency tuned to the magnetron frequency. The first lo- ing the radar to transmit a single polarization and receive cal oscillator is tunable from 32 to 35 GHz, but is usually on both. set to a fixed frequency for a particular magnetron. This The transmitter was built by Applied Systems Engi- down-converts the received signal to a range between neering and uses a Litton L-4046A magnetron which is 1532 to 1932 MHz. The second local oscillator is tunable specified to produce a 125 kW peak power pulse with from 1400 to 1800 MHz, and is constantly changed such a pulse length up to 1 µs. The transmitter enclosure that the IF signal is centered within 1 MHz of 132 MHz. is mounted to the S-Pol antenna counter weight (Fig- The last down-conversion stage is implemented on the ure 1). A combination of WG-28 and low loss Tallguide digitizing cards in the radar processor, and down-converts (TG-40) from Antennas for Communications is used to the IF to 25 MHz and samples the result. This local os- connect the transmitter enclosure to the receiver enclo- cillator is tunable from 95 to 120 MHz in 100 kHz steps. sure on which the K -band antenna is mounted. The a Thus the center frequency of the sampled signal is always ∗Corresponding author address: Gordon Farquharson, within 100 kHz of the receiver center frequency. NCAR/EOL, 1850 Table Mesa Dr., Boulder, CO 80303; e-mail: The IF signals are sent to the processor which is [email protected] housed directly behind the S-band dish. The radar pro- Tallguide Transmitter Control and Communications Parabolic Transmitter Enclosure reflector antenna 802.11b Frequency conversion Wireless link LO tunable from 32.0 to to data archiver 35.0 GHz Frequency conversion LO tunable from 1.4 to 1.8 GHz Tx Sample H−channel Data Acquisition and Processor Calibration V−channel signal synthesizer Receiver Enclosure Radar Processor Enclosure Figure 2: Ka-band block diagram. Dashed lines represent the three enclosures housing the Ka-band radar. The antenna is mounted to the receiver enclosure. cessor uses Binet Sampler cards originally designed for a bistatic network of radars. Timing signals generated by Table 1: Ka-band Radar Operating Parameters these cards are synchronized with GPS to ensure that Transmit Frequency 34.7–35.0 GHz the resolution of the Ka-band radar volumes are matched Peak Transmit Power 15 kW with the S-band resolution volumes. These cards band Pulse Repetition Frequency 500 Hz limit the 25 MHz IF signals and sample the waveform at a Pulse Length 0.8 µs 20 MHz sampling rate. Digital IF filtering and demodula- Antenna Parabolic Reflector tion is performed in hardware. The radar processor com- Antenna Diameter 0.7 m putes moments from the raw data and transmits these Antenna Gain 45 dB to the S-Pol data archiver over an 802.11b wireless link. Antenna Beamwidth 0.93 degrees This connection is also used to access the graphical user Minimum Detectable Signal −25 dBZ interface for control and real-time display. @ 10 km A separate channel in the receiver is used to sample the transmitted pulse. These samples are used to deter- both radars derive system timing from GPS to ensure that mine the phase and frequency of the transmitted pulse. the resolution volumes are illuminated at the same time The phase is used to adjust the phase of the received and samples are recorded simultaneously. echos to a common phase reference such that Doppler Table 1 lists nominal system parameters for the K - velocity and differential phase can be computed. The fre- a band system. These numbers are based on the single quency is used to tune the local oscillator frequencies to polarization transmit configuration for the radar. adjust the receiver center frequency for changes in the magnetron frequency. The receiver also contains a cali- 3. POLARIMETRIC CAPABILITIES bration signal which is injected into the front-end of both The Ka-band system can be reconfigured for either trans- channels to track differential gain changes. This signal mit H and receive both H and V polarizations (HH/HV), occupies three range gates near the end of the range or simultaneous transmit and receive both H and V polar- swath recorded by the radar. izations (HH/VV) modes. Polarimetric variables available The Ka-band antenna is built by Seavey Engineering. using the HH/HV scheme are LDR, and using the HH/VV The antenna consists of a 28 inch solid parabolic reflec- scheme are ZDR, jρhvj, and φDP. tor with a 9 inch focal length, and a dual polarized feed 4. EXAMPLE DATA assembly. Both co- and cross-polarization sidelobes are below −26 dB, and the polarization isolation was mea- The system was fielded three times in 2004: Winter Ic- sured at below 31 dB. An important requirement for dual- ing Storms Project 2004 (WISP), North American Mon- wavelength measurements is that both radars sample the soon Experiment (NAME), and Rain in Cumulus over the same resolution volume at the same time. The S- and Ocean (RICO). Figure 3 shows radar images of reflectiv- Ka-band radar beams are aligned using solar scans, and ity and Doppler velocity from the S- and Ka-band radars collected during the RICO project. The S-band Doppler caused large ZDR values in regions of large gradients in image (Figure 3(b)) shows that the wind is from the east. reflectivity. In addition, ghost echos of existing features, Both the S-and Ka-band radar reflectivity images (Fig- displaced in azimuth, appear in the data. The problem ures 3a,b) show scattering from clouds and rain in the has been narrowed down to code in the radar processor, upper half of the plots. In this region, S-band reflectivity and will be fixed before the next deployment of the Ka- values range from 5 to 35 dBZ, and Ka-band reflectivity band system. values range from −10 to 20 dBZ. In most other regions, 6. SUMMARY Bragg scattering is evident in the S-band image, but the The addition of a Ka-band radar strengthens S-Pol as a Ka-band sees virtually no reflections. Because of this, unique remote sensing tool available to the atmospheric Most of the Ka-band velocity image (Figure 3d) is noisy, research community by allowing researchers to conduct but in regions of good signal to noise ratio, the Ka-band dual-wavelength studies. The system is readily trans- Doppler velocity is folded. It should be noted that Ka-band velocities include a component from the movement of the portable, and comprises a small fraction of the operat- ing cost to field programs involving S-Pol.