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Get Smart

Why you want this weather radar

By J. Mac McClellan.

The onboard weather radar is the only piece of avionics that requires a reasonably high level of pilot skill and understanding to be used effectively. But thanks to the ongoing miracles of digital electronics, “smart” radars eliminate nearly all the guesswork when using the radar to avoid dangerous weather.

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I compare using conventional weather radar to searching a totally dark room with only a flashlight. We can learn about the contents of the room only where the flashlight beam points. And even then, something may be hiding behind an object the beam illuminates. A person skilled in using the flashlight will discover more in the dark room than the untrained and inexperienced. Still, there is much that may escape the detection of the narrow beam of light.

But smart radars automatically scan the large parcel of air ahead searching for precipitation, and even for turbulence, in the atmosphere the airplane is about to fly through. The pilot need not fiddle with the tilt angle of the radar, or even adjust the gain, to understand the weather picture ahead. And wondering if you’re looking at a radar return from a line of hills, or even tall buildings, or a significant area of precipitation is a thing of the past because the smart radar knows the difference and suppresses the ground returns.

The most important technology in smart radars is the use of solid-state transmitters in place of the vacuum tube-type magnetron. For decades weather radars needed to blast out a massive pulse of energy in the hope that enough signal would bounce back to the antenna to “paint” a target. A magnetron can produce megawatts of energy. In fact, it’s a magnetron that generates the heat in a microwave oven.

But the frequency of the pulse produced by a magnetron is not stable. That means the receiver of a radar using a magnetron must be listening for a wide range of frequencies to detect the return “echo” of the transmitted energy. The wide spectrum receiver also detects a bunch of noise from other sources. Weather radars can separate the noise from the desired “echo,” but detail is lost.

Smart radars use a solid-state digital electronic device to create the transmitted signal. This signal is very stable, so the radar receiver is listening for a returned “echo” over a narrow frequency range. That narrow range eliminates most of the ever-present electronic noise and makes it possible for the radar to paint a very detailed picture of the precipitation that is reflecting the transmitted signal.

A very important side benefit of the stable solid-state transmitter is the ability to measure the Doppler shift of the returned energy. As you probably know, sound and radio frequencies appear to change when the source is in motion. The usual explanation is how we hear a changing tone from a moving train horn even though the horn is emitting a constant frequency.

Smart radars look for a Doppler shift in the frequency of the returned energy. If there is a frequency shift the radar can know—and show us—that the precipitation that reflected that shifting signal is moving at a different velocity than the overall precipitation. Rapidly moving rain drops mean something very important to pilots: turbulence. The only phenomenon that can cause some precip to move at different speeds or in different directions is powerful moving air currents, and that is the very definition of turbulence.

The more recent technology that makes smart radars really smart is the ability to rapidly scan at different angles, and then assemble the results of those scans into a radar picture of the weather ahead. This is not new to ground based radars. The Next Generation Weather Radar (Nexrad) ground stations also scan the sky at a huge range of angles, but it can take those ground stations more than five minutes to complete and assemble a single scan. That won’t do any good when you’re flying toward a possible thunderstorm at more than 400 knots. The new smart radars complete their scans of the area of interest so quickly that detail is not lost.

All three leading makers of avionics for turbine aircraft, Collins, Garmin, and Honeywell, now make smart radars. Collins was early to the technology and trademarked MultiScan for its radar. Garmin and Honeywell refer to their systems as “volumetric scanning,” meaning they scan ahead at multiple angles to evaluate a volume of the atmosphere. But all three use essentially the same multi-scan techniques.

Detection of turbulence is still limited in range, and by the necessity of some precipitation to reflect a radar pulse. Depending on the radar, and usually the size of antenna that will fit in your airplane, turbulence detection is limited to 40, or at most 60 nautical miles. The reason is the amount of returned energy for the radar to measure a Doppler shift is attenuated over distance. But when making weather avoidance decisions, even 20 nm warning of turbulence in an area of precipitation is very useful.

Smart radars also employ electronic analysis of a return to predict if a threatening cell is behind a closer storm. Heavy precip indicative of a thunderstorm attenuates—that is, scatters and weakens a radar-transmitted pulse—so the signal may not penetrate a nearby cell, thus “hiding” significant weather. But some energy does return, and the smart radars show us that there is an area of concern being blocked by a closer cell. It may not be able to show details of the distant storm, but it alerts us to not expect to be in the clear when past the closer weather.

Depending on the manufacturer, a smart radar may be able to analyze the character of a return to predict the presence of lightning, hail, or wind shear, and show that on the radar display. Some systems use a terrain database to help eliminate ground returns. Knowing that there are hills or buildings or a shoreline area ahead helps the radar suppress returns from those objects. Some of the very top line smart radars even contain historical data unique to your location to predict the likelihood of severe weather at that place and that time of the year. Because the smart radars automatically scan vertically as well as horizontally, they measure the top and bottom of cells. Some can predict that, based on vertical characteristics, there is probably turbulence above a cell even though the
air is clear.

I’ve been flying with Collins MultiScan for a little more than four years and it has been a terrific experience and improvement over the very good radars I used before. I always keep the radar in automatic mode and displayed on the PFD. In the old days the magnetron wore out over time so many pilots didn’t power up the radar when flying in clear skies. But the solid-state electronics in the smart radar do not wear in any normal sense, so I always keep it on. The avionics system is also smart enough to power down the radar on touchdown and not activate it until in the air on takeoff.

There are at least two differences you will probably notice at first when transitioning to smart radar. One is that on takeoff the radar typically needs a few sweeps to identify and eliminate ground returns, so shortly after liftoff you may see a few seconds of high-level targets on the radar, which then disappear. Also, as with Nexrad or any other radar, anomalous propagation cannot be eliminated by smart radars. Random atmospheric conditions can cause the radar to show a low-level target when no precip is present, but the brains of the smart radar can analyze these anomalous propagation returns and eliminate them quickly.

The other area that may cause concern at first is that the smart radars show pertinent weather for your altitude, not necessarily all weather. Since most of us also fly with satellite-delivered Nexrad radar, you will often see strong returns on Nexrad but nothing on your smart radar. That means you are flying well above the weather shown by Nexrad, and it’s not a concern.

Many turbine airplane makers have included a smart radar as standard equipment on new airplanes. But you can also retrofit one of the units into your existing avionics suite. Smart radars aren’t cheap, but the weather avoidance information they deliver is worth a lot. And I’m sure you’ll discover that you were making many unnecessary deviations based on your Nexrad view and lack of confidence in conventional radar. Once you fly with a smart radar, I guarantee you won’t want to fly without it.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.

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