This paper describes the transformation of the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) National Radar Facility from a single-frequency (S band) dual-polarization Doppler weather radar system to a dual-frequency (S and X bands) dual-polarization Doppler system with coaxial beams. A brief history regarding the development of dual-wavelength radars is first presented. In the past, dual-wavelength measurements were used to detect hail using the dual-wavelength ratio defined as the ratio of intrinsic (or attenuation corrected) X-band reflectivity to the S-band reflectivity. Departures of this ratio from unity were taken to indicate the presence of hail, produced by Mie scattering at the shorter wavelength by hail. Most dual-wavelength radars were developed with attempts to match beams for S and X bands, which implies that the sample volumes for the two frequencies were essentially the same. The X-band channel of the CSU-CHILL radar takes a different approach, that of making use of the already existing dual-offset-fed antenna designed to give a 1 degrees beamwidth at S band, resulting in an X-band beamwidth of approximately 0.3 degrees, with very high gain. Thus, the X band provides about a factor of 3 more resolution than the S-band component while maintaining the same sensitivity as the S-band component. Examples of cold season and warm season data from the X-band and S-band radar components are presented, demonstrating the successful transformation of the CSU-CHILL radar into a unique multifrequency, multipolarization system. The new CSU-CHILL dual-wavelength, dual-polarization weather radar will serve as an important asset for the scientific community.

This paper presents field evaluations of the procedure. We have evaluated the hv R based echo power estimator in varied meteorological conditions, in view of practical use cases. In typical operational conditions, the enhancement compensates the loss of 3 dB in signal-to-noise ratio acquired in the mode of simultaneous transmission in the H and V polarization channels ('STAR'), with respect to the mode of transmitting in the H channel, only ('H-only'). The detectability can be enhanced beyond this by the dedicated surveillance scans of enhanced detection capability.

Since 2008 the personnel of the Colorado State University CSU-CHILL National Radar Facility and the National Center for Atmospheric Research S-Pol Radar Facility have been developing, with guidance from the National Science Foundation, a plan to more closely align the two radar’s architectures so that operation, development, maintenance and management of the two facilities would become more efficient. Initially, the radar groups focused on adopting common software and hardware structures including antenna control, radar process control, signal processing, calibration, and data display capabilities. These common structures allow the two radars to be more streamlined in their operation, which easily enables shared development. Another highlight of this joint activity paradigm is the Front Range Operational Network Testbed (FRONT). The common streamlined infrastructure enables close networking of these systems forming the anchor points of an experimental infrastructure. FRONT, formed by S-Pol, CSU-CHILL and CSUPawnee S-band radars, along with nearby NEXRAD radars, brings a new observational capability to the community. An important aspect of the envisioned FRONT mission is moving S-Pol to a new site in Weld County, called the Firestone Site (near the intersection of Weld County Roads 18 and 19), in order to create a dual-Doppler, dualpolarization radar configuration. It is planned that radar data from the NEXRADs KFTG (Denver Airport) and KCYS (Cheyenne) will be integrated with CSU-CHILL, S-Pol and Pawnee (located near Nunn, CO) data thereby

The calibration of radar differential reflectivity (Zdr) to an uncertainty of 0.1 dB is desirable for accurate rainfall estimates. However, determination of the Zdr calibration figure and maintenance of the that calibration is difficult. ThreeZdr calibration techniques were investigated and compared in Hubbert et al. (2008, 2007): 1) vertical pointing, 2) crosspolar power, and 3) engineering techniques. Probably the mostly widely accepted technique is (1), using vertical pointing data in light rain. It works well since it is an “end-to-end” method that exercises the full transmit and receiver paths as they would be for meteorological measurements. Engineering techniques attempt to estimate the Zdr calibration factor by injecting test signals and using passive measurements of solar radiation. Because of the uncertainties introduced by the calibration test equipment, the uncertainty of the engineering Zdr calibration technique is 0.25 dB at best, at least for the engineering techniques investigated by Hubbert et al. (2008). This is confirmed by the experience of both the NCAR/EOL staff (using S-Pol) and CSU-CHILLL staff (using CSU-CHILL). Both use engineering calibrations for Zdr but routinely find that the final Zdr calibration number must be determined from vertical pointing (VP) data. The third method, the crosspolar power technique, uses solar scan data plus crosspolar power data to calibrate Zdr (Hubbert and Bringi 2003). This technique is similar to the VP technique in that no obtrusive calibration test equipment is used. The crosspolar power (CP) technique uses the principle of radar reciprocity that states that the two crosspolar powers, (1) transmit horizontal (H) polarization, receive vertical (V) polarization and 2) transmit vertical polarization, receive horizontal polarization), are equal, assuming that the H and V transmit powers are equal. It has been shown that the CP technique can calibrate Zdr to a similar uncertainty (< 0.1dB) as is possible with the VP technique (Hubbert et al. 2008). An advantage of the CP technique is that crosspolar power from ground clutter targets, which are

Since 2008 the personnel of the Colorado State University CSU-CHILL National Radar Facility and the National Center for Atmospheric Research S-Pol Radar Facility have been developing, with guidance from the National Science Foundation, a plan to more closely align the two radar’s architectures so that operation, development, maintenance and management of the two facilities would become more efficient. Initially, the radar groups focused on adopting common software and hardware structures including antenna control, radar process control, signal processing, calibration, and data display capabilities. These common structures allow the two radars to be more streamlined in their operation, which easily enables shared development. Another highlight of this joint activity paradigm is the Front Range Operational Network Testbed (FRONT). The common streamlined infrastructure enables close networking of these systems forming the anchor points of an experimental infrastructure. FRONT, formed by S-Pol, CSU-CHILL and CSUPawnee S-band radars, along with nearby NEXRAD radars, brings a new observational capability to the community. An important aspect of the envisioned FRONT mission is moving S-Pol to a new site in Weld County, called the Firestone Site (near the intersection of Weld County Roads 18 and 19), in order to create a dual-Doppler, dualpolarization radar configuration. It is planned that radar data from the NEXRADs KFTG (Denver Airport) and KCYS (Cheyenne) will be integrated with CSU-CHILL, S-Pol and Pawnee (located near Nunn, CO) data thereby

This paper describes how the synchronization and networking capabilities of the transmit and receive chain used at the CSU-CHILL and CSU-Pawnee radars are used to form a bistatic radar system capable of observing clear air echoes from atmospheric boundary layer. An overview of the bistatic radar geometry and resolution volume are presented, along with a discussion of the methods used to achieve timing coherence. Some preliminary results from clear-air observations are included.

The CHILL radar was originally designed as dual polarization radar. It was put into operation in late 1971. The original concept was a dual frequency X-band and S-band system. In 1972, the idea of using it as polarization radar was suggested by Drs. Tom Seliga and Viswanathan Bringi. After several years, the X-band radar was discarded and the system became a dual polarization radar instead. This discussion outlines the changes that were preformed on the system to make it a good polarization system. The radar was dual polarization capable from the beginning with a switch which changed the polarization of the transmit signal. A short list of the changes that were made in the radar and there success and problems are presented. The changes started with an electrical operated switch (original design) which took a second to switch but no special processing; a modified processor and the same switch changing to a 4 second cycle; a ferrite switch with 2 microseconds switching time and changing the processor to one with more channels; two channel separate transmitter and receiver; new antenna; and finally a second new antenna. Most of these upgrades also involved signal processing updates.

The Virtual CHILL (VCHILL) system makes it possible to transfer the educational and research experience of the Colorado State University dual polarization radar to remote locations over the Internet. The VCHILL operation includes remote control of radar and display of radar images, as well as the ability to locally process high-bandwidth radar data transferred over data networks. The low-bandwidth VCHILL operation allows the distant users to access the archived and real-time data estimated at the radar site and simultaneously display them on their local systems. A parallel receiver was developed exclusively for the high-bandwidth VCHILL. End-system architectures were designed to accommodate the demands of the high-bandwidth VCHILL operations in real time. A graphic user interface was also developed with the objective of easy installation and usage at various end-user institutions. The VCHILL not only expands the education experience provided by the radar system, but also stimulates the development of innovative research applications for atmospheric remote sensing. The VCHILL is being used by several universities for research and education.

The Colorado State University (CSU)-CHILL radar system is a National Facility operated under a cooperative agreement with National Science Foundation to support education and research in remote sensing of the atmosphere. The CSU-CHILL radar is an S-band fully polarimetric Doppler radar with polarization agility and diversity (Brunkow et al. (2000)). A fully polarimetric radar computes the full covariance matrix of each radar resolution volume. The full covariance matrix provides a complete set of measurements on the observation volume. The radar system consists of a unique two transmitter design that can be used to transmit either horizontal and vertical polarization state on a switched mode, or a combined slant linear, or elliptical polarization state. The polarization states of the receiver are always fixed at horizontal/vertical independent of the transmit mode. The CSU-CHILL radar has been continuously upgraded to enhance its functionality and to provide the research communities with high quality data with advanced capabilities (Brunkow (1999)). The data generated from the CSU-CHILL radar system have been being used by many universities and research communities over the United States for educational and research activities. In 1997, CSU launched an initiative to enable the realtime operation of the CSU-CHILL radar over the Internet. The goal of this project named as Virtual CHILL (VCHILL) is to extend the educational and research experiences of CSU-CHILL via the network infrastructure and enhanced computing capability to remote locations (Chandrasekar et al. (2001)). The concept of the VCHILL has been implemented at two levels, namely, low-bandwidth VCHILL and high-bandwidth VCHILL, depending on the required bandwidth of the network for the data transmission as shown in Fig. 1. The low-bandwidth VCHILL transfers either the archived or the real-time radar parameters, which corresponds to a subset of the covariance matrix, to the remote sites for display and further end-user applications. A more advanced low-bandwidth application is an active mode, where radar control is transferred to a remote location enabling the remote users to control the radar operation over the Internet. The data transmission in these modes requires a bandwidth of the order of fraction of one

A procedure for calibration of the radar covariance matrix for the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) radar and S-Band Dual-Polarization Doppler Radar (S-Pol) systems is described. Two relative magnitudes and three offset phases are determined that allow for the calibrated covariance matrix to be constructed. Precise calibration of Z(dr) is accomplished with use of only sun calibration measurements and crosspolar power measurements from precipitation. No assumptions about the precipitation medium are made. It is also shown how to determine the co-to-cross phase offsets for the CSU-CHILL radar from precipitation data. A novel method for calculating linear depolarization ratio (LDR) that is effective in low signal-to-noise-ratio regions and that requires no knowledge of the background noise temperature is given. This technique utilizes the cross-to-cross covariances. CSU-CHILL data from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) are used to illustrate the LDR estimator and the Z(dr) calibration technique.

A number of studies have demonstrated improvement in rainfall estimation with polarimetric techniques compared to traditional horizontal reflectivity (ZH) – rainrate (R) methods. However, the degree of improvement appears to vary with storm type and/or geographic location, and this topic continues to be an area of active research (e.g., Brandes et al. 2001; Ryzhkov and Zrnic’, 1995). Following the devastating Fort Collins flash flood in 1997, an algorithm was developed at Colorado State University (CSU) to estimate rainfall from CSU-CHILL dual-polarization radar data (Carey and Rutledge, 1998; Petersen et al. 1999). The flash flood event was unusual in it’s drop size distribution characteristics and the polarimetric algorithm provided significantly improved rain accumulation estimates compared to the National Weather Service (NWS) NEXRAD Z-R technique. Over the last several years, the algorithm has undergone several iterations in order to make improvements in the estimation procedure. During the summer of 2002, a UCAR-COMET grant provided an opportunity to apply the polarimetric algorithm in a real-time environment and quantitatively evaluate the performance in comparison to the standard NEXRAD Z-R technique on a variety of rainfall events in northeast Colorado. Maps of warm season accumulated rainfall are now generated routinely and are available at the CSU-CHILL web site (http://chill.colostate.edu/).

he design, architecture, and implementation for the high-throughput data transmission and high-performance computing,which are applicable for various real-time radar signal transmission applications over the data network, are presented. With a client-server model, the multiple processes and threads on the end systems operate simultaneously and collaborately to meet the real-time requirement. The design covers the Digitized Radar Signal (DRS) data acquisition and data transmission on the DRS server end as well as DRS data receiving, radar signal parameter computation and parameter transmission on the DRS receiver end. Generic packet and data structures for transmission and inter-process data sharing are constructed. The architecture was successfully implemented on Sun/Solaris workstations with dual 750 MHz UltraSPARC-III processors containing Gigabit Ethernet card. The comparison in transmission throughput over gigabit link between with computation and without computation clearly shows the importance of the signal processing capability on the end-to-end performance. Profiling analysis on the DRS receiver process shows the work-loaded functions and provides guides for improving computing capabilities.

Colorado State University (CSU) operates the CSU CHILL radar at Greeley, Colorado as a NSF national facility. The VCHILL (Virtual-CHILL) project is aimed at developing and implementing protocols for providing the raw radar data to researchers in real-time over the internet. The Next Generation Internet has features, which could be used to transfer raw digitized radar signals (DRS) generated by the radar. A network transfer application using TCP to transfer DRS was developed. This network transfer application has been successfully tested to provide throughput of 90 Mbps over a 100 Mbps link and 300 Mbps over gigabit link. A software digital signal-processing module was developed. A network transfer application with User Datagram protocol as the underlying protocol was also developed. The performance of the software digital signal processing unit and the 'Reliable-DRS' version of the application to transfer the data across the network meet the requirements in most areas.

The CSU-CHILL radar facility has embarked on an initiative to enable the real time operation of the radar over the Internet called VCHILL or Virtual CHILL. The concept of operation over the Internet can be implemented at several levels. The simplest one is to make the routine CHILL images available on the Internet as soon as the scans are completed. This type of data dissemination has been available with CSU-CHILL over 5 years. However such images are clearly not sufficient for aircraft coordination or actively conducting coordinated scans with rapid updates. The VCHILL initiative has a more ambitious goal of providing the same quality of service (QoS) at remote locations as at the radar site. In addition, the goal of the VCHILL initiative is to provide the educational experience of polarimetric radar at a remote location, without compromising on features of an on-site radar console. This paper describes some progresses and plans of the VCHILL initiative

The subject of this paper is the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) National Radar Facility's S-band polarimetric research radar. Key features of this system include polarization agility (provided by the dual-transmitter, dual-receiver design), a recently updated signal processor, and a low (-34 dB, two way) integrated cross-polar ratio (ICPR2) antenna system. After reviewing the technical description of the radar, the authors present a new differential reflectivity (Z(DR)) calibration technique and data examples collected in different polarization modes.Although the CSU-CHILL radar is transportable, it can also be operated in a dual-Doppler configuration with the CSU-Pawnee radar, an 11-cm Doppler radar system situated 48 km north of the CSU-CHILL Greeley field site. Used together, these radars provide three-dimensional kinematic and hydrometeor information in precipitating cloud systems.