Dual-polarization weather radar

Is your station prepared?
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In the decades since World War II radar operators in search of enemy aircraft inadvertently discovered precipitation returns on their scopes, weather radars have used horizontal scanning techniques to identify the location and intensity of rainfall in the atmosphere. Now, with meteorology's move into dual-polarization radar technology — which employs both horizontally and vertically polarized signals to detect weather targets — things are changing.

Dual-polarization has been an emerging weather capability in the broadcast industry for a few years now, with several stations across the U.S. already employing advanced dual-polarimetric radar systems. As the U.S. National Weather Service's entire NEXRAD radar fleet is upgraded to dual-pol by the end of 2013, however, this new technology is quickly becoming a relevant matter to every broadcaster in the country. So what's dual-pol all about?

Radar meteorology

When it comes to conventional weather detection, this much is universal: Radars operate by transmitting horizontally polarized RF energy pulses, which are reflected back by precipitation (anything from raindrops and hailstones to sleet and snow) suspended in the atmosphere. The drops and particles that make up precipitation are known meteorologically as hydrometeors. Using the properties of the returned signal as a basis for calculation, conventional Doppler radar systems produce two data products of extreme value: reflectivity and velocity, which depict the patterns and intensity of precipitation and wind, respectively.

In dual-polarimetric applications, the radar transmits simultaneously in two polarizations, horizontal and vertical, allowing the radar's signal processor to make direct measurements of the size, shape and moisture content of hydrometeors, as shown in Figure 1. (Frozen hydrometeors have less water content than rain.) Since the radar processor can now measure the shape of any hydrometeors the signal has encountered, it can calculate the type of precipitation your viewers may expect. In this way, the guesswork needed for the meteorologist to accurately distinguish between, for example, hail and heavy rain, or snow and freezing rain, is eliminated. Dual-polarimetric radars can more accurately determine instantaneous rainfall rates, too, so flooding can be more accurately forecasted. And, there's lot more to come from dual-pol advances in the future.

Choosing dual-polarization

Television stations have two options when it comes to securing a dual-polarimetric live radar of their own: implement a new installation, or upgrade legacy radar hardware with the new capability. From a budgetary standpoint, maximizing current hardware is always the preferred method, but it's important to note that due to increased signal processing demand and other requirements, not every radar currently in use will be up to the task of handling dual-pol operations.

Peak transmitting power is an area of serious consideration. To achieve simultaneous horizontal and vertical transmission, a dual-pol radar's RF pulses are physically split between the two polarizations, effectively halving transmission strength per channel.

The risk in using lower powered radars for dual-polarimetric applications is attenuation, caused when the transmitted beam is scattered and absorbed as it travels through storm cells. The weaker the signal as it reaches a target, the lower the resolution of the returned measurements. It's really this simple: Brute force equals better punching power and higher data resolution, and that's important when it comes to dual-pol radar.

In many instances, the most immediately visible modification to a radar is the addition of an RF palette behind the antenna, mounted to one of the pedestal's swing arms. The palette contains a waveguide splitter that divides the RF signal emanating from the radar's klystron or coaxial magnetron transmitter into separate channels. As the newly separated pulses travel through the remaining waveguide, one channel is physically converted into a vertical polarity before the signal is dispatched through the feed horn.

On the other side, the opposite swing arm hosts, in this example, an antenna-mounted electronics module — a climate-controlled box housing receiver components, as well as low noise amplifiers (LNAs) that convert the received transmission into an electrical signal the Doppler signal processor can decipher. This configuration also allows the radar to be more easily calibrated by directly injecting a test signal into an LNA without the attenuation or loss associated with directional couplers. The solid-state climate control system maintains an interior module temperature of 20˚C to 30˚C throughout the year to ensure reliability and signal stability.

Inside the radar shelter on the ground, additional modifications are made to the radar equipment cabinet; for example, any new transmitting equipment would be installed during the upgrade. To support the increased processing requirements, a higher-spec Doppler processor will typically be retrofitted to the legacy hardware stack. The processor contains a suite of components, including built-in test equipment (BITE).

The BITE is a multi-purpose and essential modification. In addition to performing continuous system checks, it allows radar technicians to, with a station's permission, remotely dial into the radar system. Troubleshooting, calibration and other system checks can then be performed without waiting for the vendor to arrive on-site, or taking the radar system offline.

The NEXRAD factor

Dual-polarization is not functionality limited to privately owned radars. In fact, the NEXRAD radar network for the National Weather Service will soon be upgraded as well. As the upgrade program moves into nationwide deployment, NEXRAD will become even more relevant to your station as dual-polarimetric data products become available. To find out when your local NEXRAD site will receive the dual-pol upgrade, log on to http://www.roc.noaa.gov/WSR88D/DualPol/Default.aspx, and click the “Deploy Schedule” link.

Conclusion

The future of broadcast weather radar is in dual-polarization, but as with any transition, there are sure to be challenges. One of these is preparing the broadcast meteorology community for the influx of new data it will soon have, whether through a local NEXRAD upgrade, or a privately owned weather radar. Knowledge is power, so encourage your station's weather team to attend seminars on the topic. Both the National Weather Service and at least one vendor have begun their own education efforts, ensuring broadcast meteorologists have the tools to understand dual-polarimetric data products, and how the data can be used to their stations' advantage.

By anticipating the role dual-polarimetric radar will soon play in your market, your station can provide even better local radar information, and harness the potential of this new technology.

Michael Richardson is marketing communications manager, Baron Services.