An extended radar relative calibration technique for research radars
Hardin, Joseph Clinton — Pacific Northwest National Laboratory
Area of research
Weather radars need routine calibration to maintain high-quality performance. However, for radars located in remote regions with limited onsite monitoring, this calibration is challenging and sometimes impossible. To mitigate this, researchers at the U.S. Department of Energy (DOE) enhanced an existing method for radar calibration so it could be used at remote sites. This method uses the stability of radar returns from ground clutter to estimate and track radar calibration. The researchers validated the use of this extended relative calibration adjustment technique for both research radars and radars of higher frequency (C-, X-, and Ka-band).
Previous uses of this calibration technique were limited to lower radar frequencies and a single operational mode. Now researchers have expanded the technique to be used with more radar frequencies and other scan types. This extension is particularly relevant to DOE's Atmospheric Radiation Measurement (ARM) user facility, given its fleet of high-frequency radars. The extension of this calibration technique makes it easier to monitor and correct calibration drifts, both during the operational period and a posteriori with historical data sets. The open-source code that accompanies the release of this publication has already been used in multiple field campaigns for ARM, including the calibrated radar data set for the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) campaign in Argentina.
The researchers recognized a need for monitoring and tracking radar calibration, especially for ARM’s research-grade weather radars located at remote sites. Their work extends the relative calibration adjustment technique for calibration of weather radars to higher-frequency radars (including cloud radars) as well as range-height indicator scans.
The relative calibration adjustment technique uses the statistics of the ground clutter surrounding the radar as a monitoring source for the stability of calibration. At higher frequencies, the properties of clutter can be much more variable. This work introduces an extended clutter selection procedure that incorporates the temporal stability of clutter and helps to improve the technique’s operational stability for relatively higher-frequency radars. The researchers also extended this technique to use range-height scans from radars where the elevation is varied rather than the azimuth. Research radars often use range-height scans to examine the vertical structure of clouds. The researchers applied the newly extended technique, called extended relative calibration adjustment or eRCA, to four DOE ARM weather radars ranging in frequency from C- to Ka-band. Cross-comparisons of three co-located radars with frequencies C, X, and Ka at the ARM CACTI site show that the technique can determine changes in calibration with high accuracy. Using an X-band radar at the ARM Eastern North Atlantic site in the Azores, they showed how the technique can be modified to be more resilient to clutter fields that show increased variability, such as sea clutter in this case. The results show that this technique is also promising for a posteriori data calibration and monitoring.