1. DUAL POLARIZATION AND ZDR CALIBRATION IMPROVEMENTS 5.2(6)
March 1, 2017
Christina Horvat
NWS Radar Operations Center

2. Since Dual Polarization Deployment
We learned there are many contributing factors that can lead to radars being out of calibration and negative impacts to data quality.
Specifically, the Radar Operations Center has studied the sources of systemic biases and proposed modifications to improve the baseline calibration process.
We completed an assessment outlining five findings and recommending twenty three solutions.
The recommendations were developed into a comprehensive action plan.
Twenty three solutions were broken down into four Engineering Change Proposals (ECPs) based on the amount of time and resources required to complete the work.

3. 2017 Work In Progress
With dual polarization modification and upgrades with the digital receiver and signal processor, there exists an opportunity to vastly improve data quality.
In 2017, we continue support for differential reflectivity calibration by investigating technical sub-elements of the Cross Polarization Power Dual Polarization Calibration method, including solar and ground clutter scans.
Calibration stability issues are being explored including possible temperature effects on antenna bias and on transmit power division. Work includes CMD algorithm support, Hybrid Spectrum Width Estimator, investigation of a new spectral clutter filter (CLEAN AP), with emphasis on Staggered PRT Clutter Filtering support.
Work includes CMD algorithm support, Hybrid Spectrum Width Estimator, investigation of a new spectral clutter filter (CLEAN AP), with emphasis on Staggered PRT Clutter Filtering support.
Work also includes improving Dual Polarization variable data quality and system noise estimation techniques.

4. Software and Hardware Improvements for ZDR Calibration
ECP 715 focuses on improving ZDR calibration methods associated with RDA test software. A refinement to Suncheck Sub-test 1 was accomplished. Technical manual updates were done that streamline the process. CW test signal timing and receive linearity tests were updated. Remaining work focuses on improved solar scans for obtaining pedestal pointing corrections, absolute antenna gain and differential antenna bias.
ECP 716 is to migrate separate tools to the NEXRAD Radar Product Generator for analyzing ZDR bias from external targets (rain, snow, Bragg Scatter, sun). The ability to monitor calibration status is needed for performance measurement and future improvements. It also lays ground work for eventual migration of the bias estimation process to the RDA. The implementation of external target monitoring to the RPG is included in RPG Build 18.

5. Software and Hardware Improvements for ZDR Calibration (Continued)
ECP 718 is to improve Dual-Polarization hardware calibration and testing. The in-field calibration routine for the RF Pallet and AME equipment is cumbersome, difficult to execute and in some cases inaccurate. The calibration routine needs update and depot-level expertise needs to be developed. The task focuses on hardware in the RF Pallet, including the critical test couplers. One of the six identified projects was completed. The task completed ensured that all sites were using the correct factory measured values for the coupling factors. Future work includes improved methods for estimating the coupling factors based on engineering analysis and testing at the ROC.
ECP 4(TBD) is dependent on the outcome of the other ECPs. ECP 4 provides for RDA hosted external target ZDR calibration, to correct transmit and receive paths using external targets, cross polarization power calibration, and performance check box scans.

6. Provide ZDR Bias Adjustment Option for the Radar Product Generator
Due to current limitations of WSR-88D hardware, it is very difficult to eliminate a bias in ZDR through current calibration procedures.
Dual polarization products, such as hydrometeor classification and rainfall estimates, have suffered whenever the bias exceeds the specifications for the algorithms that create those products.
The success of dual polarization has often been measured by the improvement in quantitative precipitation estimation (QPE), the algorithm for which is highly dependent upon a well-calibrated system for reflectivity and ZDR. Even though +/- 0.2 dB is considered to be the recommended range of acceptable ZDR bias values, many WSR-88D systems have exceeded these limits for months or longer periods of time, resulting in rainfall underestimates or overestimates.
Recently, three methods of estimating the ZDR bias have been used: Bragg scatter, dry snow, and rain. These are measured routinely.
In 2018, we plan to use the ZDR bias estimates to make a correction to RPG ZDR used by algorithms such as the Hydrometeor Classification Algorithm (HCA) and dual pol QPE.
This corrected ZDR will also be visible in Level III ZDR products.

7. Reference Data and Current Status
Fleet wide ZDR data have been processed monthly since October The ROC monitors ZDR biases using external hydrometeor targets.
Methods are used to show when sites have significant absolute systematic ZDR biases (e.g. > 0.20 dB).
The methods, when compared to the hardware calibration measurements and configuration parameters, have been used successfully to indicate trends and to isolate gross potential component failures and/or human errors.
On a site-by-site basis, the ROC and the field sites are successfully cooperating to correct sites exhibiting poor calibration, and bring them back into acceptable tolerance.
At this time, the ROC estimates that between 60 and 70 percent of the sites are operating within the 0.2 dB margin.