ROC RADAR ENGINEERING
ROC Radar Engineering provides radar consulting services to the
NEXRAD program. The radar section provides the following services
to WSR-88D system users:
- Direct analysis of radar problem reports and solution development.
- Management of radar test programs for approved engineering changes.
- Coordinate reviews of all radar-related ECPs, TCTOs, test plans, and technical manual changes.
- Functional design management for the transmitter, receiver, antenna, pedestal and all associated microwave components.
- Engineering support to field operations for all radar-related calls.
Click here for NEXRAD Technical Information.
Differential Reflectivity Calibration
Differential Reflectivity (ZDR), which is the ratio of the power received in the horizontal and vertical
channels of a polarimetric radar, is a new variable available with the recent upgrade to the WSR-88D network. Since ZDR is a measure
of the shape of the hydrometeors, it can yield precise estimates of the types of particles as well as the water content, leading to
accurate rainfall rate measurements. However, biases due to the radar hardware affect the usefulness of ZDR for these purposes.
The component of the ZDR measurement contributed by the radar system must be measured accurately through the calibration process.
The current requirement is that the radar’s bias must be measured to within an uncertainty of +/- 0.1 dB.
This is necessary to achieve maximum benefit from the polarimetric radar when producing rain rate estimates. The WSR-88D system
employs a number of calibration processes designed to support this goal. The three main components of ZDR calibration involve
measurements of the following quantities: (1) transmitter power in both channels, (2) differences in gains and losses of the receiver
channels, and (3) the bias of the antenna assembly between the two channels. The first two measurements are done automatically using
Built-in Test Equipment (BITE), however the test equipment must be carefully calibrated to account for the bias induced by the
equipment. These biases are obtained through off line tests performed by electronic technicians and involve disconnecting and
reconnecting test cables. The third parameter, the antenna bias, is measured using scans of the sun, which provides a near ideal
target for polarimetric measurements.
While the system was carefully designed and is expected to support the requirement in theory, a number of
implementation issues have been noted since the network upgrade was completed in the Spring of 2013. A team of engineers and
scientists at the ROC and at the National Severe Storms Laboratory have identified a number of improvements to the process that are
necessary to achieve the performance goals. The team also implemented a number of independent calibration state monitor tools that are
based on external targets such as precipitation and Bragg signal backscatter. The ROC’s Radar Engineering Team is currently leading
efforts to implement the recommended changes.
Dual Polarization Data Quality
The dual polarization upgrade provided three new radar estimated variable for the forecast community.
These are Differential Reflectivity (ZDR), Differential Phase (PHIDP) and Cross Correlation (RHO). The initial deployment versions
employ basic signal processing for producing the estimates. Clutter identification and removal techniques are also provided, and work
acceptably for an initial deployment. However, a number of improvements in both the estimation process the artifact removal are
possible. In the example of ground clutter contamination, researchers have concluded that the polarimetric variables are more
susceptible to biases induced by clutter than the original radar moments such as reflectivity were. In some cases, the correlation
coefficient can be unacceptable biased when the clutter is as much as 13 dB or more below the weather signal power. Current methods
of identifying clutter and removing do not perform down to this level.
The ROC Radar Engineering Team is leading a project to investigate better methods of identifying
contamination and removing it. The project also will develop improved tools for quantifying the dual polarization estimators and
artifact removal methods. Key to this project is support from the National Center for Atmospheric Research (NCAR). The team at NCAR
developed the currently used clutter identification methods and they will be supporting the ROC in developing and implementing improved
methods which will yield higher quality results. NCAR operates a research grade S-band polarimetric radar which can be transported to
any location. Currently it is positioned such that it can obtain data from common volumes also scanned by the Denver Colorado WSR-88D.
The ability to compare estimates derived from an operational WSR-88D and a research radar provides a unique capability and will lead to
greatly improved methods of calibration, variable estimation, and artifact removal. The ROC maintains a technical support contract with
NCAR and continues to collaborate with them.
Range-Velocity Ambiguity Mitigation
The so called “Doppler Dilemma” where a weather radar can obtain long range data or high velocity data,
but not both easily, is a regular issue with modern Doppler radars. Reducing unambiguous estimates in velocity leads to increasing
ambiguity, or overlaid echoes, in power measurements at longer ranges and vice-versa. The WSR-88D uses a number of signal processing
methods to reduce these ambiguities, and the most recently implemented method is based on changing the phase of the transmitted signal
for each pulse. This allows the signal processor to “sort out” the returns and reduce ambiguities in range.
Another method for addressing ambiguities is to change the rate at which the transmitter pulses are
generated and sent. One method, Staggered Pulse Repetition Time (SPRT) changes the timing between every pulse, with spaced pairs of
pulses being separated by different delays. The signal processor can then use this feature to sort out ambiguous velocity estimates
while maintaining good long range power estimate performance.
The RCO Radar Team is leading efforts to implement SPRT. This project includes a new method of
identifying and filtering clutter as well.
Radar Calibration
The ROC Radar Team continues to support developments in basic radar calibration, whether it is for
reflectivity (power) calibration or other methods such as Initial System Differential Phase (ISDP). ROC Radar Team Engineers are
supporting international calibration workshops, which are useful for learning new methods and for improving the quality of radar data
around the world. Some of the newest techniques the ROC team is implementing have been proposed by research teams from other
countries. Participating in workshops and monitoring progress in the international community continue to be cost effective ways to
introduce improved capabilities into the WSR-88D network.
World Meteorological Organization
The Radar Engineering Team provides support to the World Meteorological Organization (WMO) by representing
the US on expert teams. These teams cover projects in operational remote sensing and surface based observations. By participating in
these international groups of experts, ROC engineers are able to learn about how other countries address fundamental issues with radar
performance. Most of the recent benefits have been in the area of dual polarization calibration and artifact removal. ROC engineers
are also helping the WMO plan for new standards in international radar data exchange.
Multi-function Phased Array Radar (MPAR)
ROC Radar Team engineers fully support the MPAR project by participating in working group meetings and
by developing requirements. ROC engineers served as team members and subject matter experts on a project to develop NOAA/NWS radar
functional requirements that will be used in these future programs.
