The Importance of Keeping
Network Radars Set to a Common Standard
Mrinal Balaji, Baron Chief Radar Scientist
The importance of calibration in weather radars can hardly be understated. An uncalibrated or poorly
calibrated radar generates degraded base data, limiting the usefulness of the radar and contributing to
erroneous weather forecasts. When major storms are tracked across a network of radars, the combination of
data required from several radars makes it crucial that the entire network is calibrated to a commonly known
standard.
In addition, since the base data is used to generate products for rainfall estimation, hail detection, rain/snow
discrimination and identification of biological scatterers, large errors in the derived products can be caused by
small errors in base data.
Naturally, accurate calibration is always required for a new radar, and the goal is to ensure a radar is providing
a precise indication of its measurements when it is installed. Factors such as temperature drift and environment
cause radar calibration to vary with time, so after the initial calibration, it is important not to assume the radar
remains calibrated for its lifetime.
NEXRAD WSR-88D
site in Nome, Alaska, USA
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Calibration Overview
Instead, a perfect calibration procedure should detect
these changes and periodically adjust the radar calibration
accordingly. One must note that even if a periodic calibration reveals that no adjustment is required, this cannot
be determined unless a calibration is performed. Baron
calibrations not only provide accurate results to within the
specified tolerances, but are performed automatically in
real-time without the operator’s intervention.
Comprehensive automated and accurate techniques to
keep the weather radars calibrated at all times are part of
Baron’s radar solutions. Through its partnership with L-3
STRATIS, it was chosen to provide the technical design,
development, production, testing and deployment of
the dual-polarization upgrade kit for the 171 US National
Weather Service (NWS) NEXRAD WSR-88Ds, owned and
operated by NWS, the Federal Aviation Administration
(FAA) and the Department of Defense (DOD).
The upgrade to the network is steadily demonstrating
significant improvements in precipitation estimation and
severe weather forecasting, as predicted by the scientific
community.
The implemented polarimetric WSR-88D design incorporates calibration procedures that are run during the time
it takes to retrace the antenna from the last elevation of
a completed volume coverage pattern (VCP) to the first
elevation of the next VCP.
As a case study here, there is a focus on calibration
data collected after every VCP from 10 polarimetric
WSR-88D sites. Data was collected during the time
2012-05-15T00:00:00Z / 2012-05-29T00 :00:00Z to illustrate
stability and repeatability of the calibrations performed
over the analysis period of 14 days. (See image below)
Polarimetric WSR-88D sites used to illustrate calibration stability in the network
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Figure 1
Calibration Procedures Description
The most basic of the calibrations monitors the noise floor in both the horizontally and vertically (H&V) polarized receiver
channels. Figure 1 presents the results of the calibration along the horizontal receiver channel at the10 sites, with the
values exhibiting diurnal behavior due to temperature changes. If a single value derived during installation was used
during all times, these variations would go unnoticed, resulting in incorrect signal-to-noise ratio estimation, harming the
quality of the base data. Inaccurate base data would result in wrong hydrometeor classification, ultimately rendering the
radar ineffective.
Figure 2
Yet another calibration monitors the degree of noise added by the components in the receiver chain and is performed
on both the H&V receive channels. Figure 2 presents the results of the calibration along the H channel at the 10 sites. A
notable feature of the radars is that their noise figure in the receive path from the back of the antenna to the signal
processor is better than 2.5dB on all sites. By performing the calibration periodically, it would be easy to detect any major
deviation between consecutive measurements, which would indicate problems along the receiver path.
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Figure 3
Baron measures the system dBZ0, which is the measured reflectivity by the radar at 1km for a target with a signal-to-noise
ratio of 0dB along the H&V channels. In order to accomplish this, the dynamic ranges and noise levels of the H&V receiver
channels, and the transmit powers along the H&V polarizations, are determined in real time by the Baron calibrations. The
dynamic range of a receiver is defined as the difference between the tiniest and largest signals that can be sensed by the
radar. Figure 3 illustrates the results of the calibration along the H channel at the 10 sites.
Figure 4
Each exhibit extremely repeatable measurements over time, with an accuracy of better than 0.5dB. Studies performed by
the Radar Operations Center, which maintains all the NEXRAD WSR-88D radars, show that the radars are well calibrated
to within 0.5dB of each other. (Figure 4)
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Achieving accurate differential reflectivity measurements
The dual-polarimetric capability introduced within the NEXRAD WSR-88D network helps determine the size and shape of
hydrometeors through a polarimetric variable termed differential reflectivity (ZDR). Differential reflectivity is the ratio of the
H&V power returns. To determine the true size and shape of a hydrometeor, the radar system-induced power bias along
the H&V polarizations needs to be determined and removed from the measurements made in real time by the radar.
Traditionally, an approach used within the radar community involved pointing the antenna in zenith during light stratiform
rain, with intrinsic differential reflectivity assumed to be 0dB. Any deviation from it is attributed to the system-induced ZDR
bias. However, this approach suffers from quite a few shortfalls: other scattering mechanisms can be present as well as
direct scatter from the precipitation in the main beam of the radar; and the determined values can be a function of
azimuth angle, having sinusoidal-like variations with peak-to-peak excursions of several tenths of a dB. Additionally, this
technique is unable to remove the effect of wet radomes, and is very dependent on the occurrence of precipitation over
the radar.
Polarimetric Upgrade
For the polarimetric upgrade of the WSR-88Ds, this clearly
posed a problem, since any calibration is required to be
suitably analytic so that it does not unintentionally create a
new problem to be solved. The calibration method’s
dependence on a unique weather condition makes it
unappealing from an operational standpoint, especially
when a hardware component needs to be replaced.
Yet another stringent requirement is determining the
system-induced ZDR to within an accuracy of 0.1dB, since
imprecise ZDR would cause incorrect hydrometeor
classification – ultimately defeating the very purpose of
having a polarimetric radar.
To combat this problem, Baron’s internationally patented
calibration scheme determines the system-induced ZDR
through a mixture of internal test equipment, derived path
difference measurements, and solar scanning procedures.
The calibration scheme monitors the difference in
transmit powers along the two polarizations, difference
in gain along the H&V receiver paths, and the antenna/
radome bias in real time. Periodically updated during every
VCP, it has been proven to determine the system ZDR to
within an accuracy of 0.1dB. The upper panels of Figure 4
illustrate the results of the calibration at the 10 sites, while
the lower panels exhibit histograms of the calibration
values demonstrating the repeatability of the
measurements.
KVNX Baron Calibration
Equipment
This material was originally published in the Meteorological Technology International magazine.
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