Datalink weather information has revolutionized the way many of us think about weather and deal with in-flight weather decisions. With datalink information such as METARs and TAFs can be called up for studied review — there's no struggling with trying to write as fast as a flight-watch briefer speaks (though flight watch is still the official source for late-breaking weather updates and the sole source of such information for the thousands of us who aren't lucky enough to have datalink service). Airmets and convective sigmets are broadcast to cockpit multifunction displays in near-real time. And, best of all, ground-based Doppler weather radar imagery gives views of precipitation and storm contours that simply weren't available before.
It's these radar images that are the big draws in most datalink buying decisions. With datalink, the attenuation problems common to radar imagery from small-diameter, light general aviation airborne weather radar antennas are avoided. Radar energy from an antenna dish 12 inches or less in diameter is too weak to penetrate areas of heavy precipitation. The result is an image that can fail to paint the worst parts of a thunderstorm, and lure the pilot into flying toward benign-looking "radar shadows" that look like they're free of precipitation, but which can contain the heaviest precipitation.
With datalink radar there is no attenuation. That's because of the high power and huge antenna size of the nation's network of 158 WSR-88D Nexrad Doppler weather radars. Their beams can penetrate the densest returns and have the ability to accurately portray a storm's coverage and precipitation contours. What's more, WSR-88Ds depict 16 different color shading levels, each corresponding to a precipitation level.
Those 158 radar sites have overlapping coverage. Datalink providers make a nationwide mosaic comprised of the totality of the coverage, and it's this processed mosaic that's sent to the cockpit. It lets pilots circumnavigate storm cells nationwide with nearly complete radar coverage. Want to see if there are thunderstorms at your destination 500 miles away? Just pan your multifunction display's view to the destination and the radar view appears. Is this better than the dinky radars on small GA airplanes? You bet it is.
As a weather avoidance tool, is it better than lightning detection? Again, the answer is an emphatic yes. Back in the 1970s, when lightning detection was cutting edge, real-time thunderstorm information was hard to come by in the cockpits of small general aviation airplanes. Lightning detection equipment (e.g., Stormscopes and Strikefinders) was the first affordable means of giving us an idea of a storm's relative strength and direction. But there are limitations. For example, strong storms often show up on lightning detection displays as being closer than they really are; weak ones can paint farther away. Also, plots of lightning strikes don't do a very good job of showing the contours of storm cells — just their electrical discharges.
Datalink radar has eclipsed lightning detection because its storm information can be very accurate and useful for strategic avoidance of precipitation and thunderstorm cells. But as datalink gear is installed in more GA cockpits, it's time to take a closer look at the intricacies of the radar imagery that it presents. There are limitations, and all pilots should understand them.
Time delays. Nexrad radar antennas scan the sky at up to 14 different tilt angles, with 0.5 degrees, 1.5 degrees, 2.4 degrees, and 3.4 degrees being the most common. The scan rates can be increased or decreased. In clear-air mode, the antenna makes five scans in 10 minutes. When precipitation is in the area, the radar can be boosted to precipitation mode, which makes nine scans in six minutes. Some radars have a storm mode used when thunderstorms are present, which makes 14 scans in five minutes.
After the radar makes its scans it can take five minutes or so to process the imagery and create the national mosaic. Then the mosaic is given a time stamp, and it's sent to the datalink service providers, which in turn send it to the cockpit. So there's one big limitation right there — a time delay of five to 10 minutes or so from the time the Nexrads create an image to the time you see it on your multifunction display.
Five minutes can be a long time when you're flying in convective conditions and cells are quickly building or decaying. That's reason enough to use datalink radar imagery for gross avoidance only. Try to use it as a tactical tool for close-range weaving around storm cells and you may get a nasty surprise: The next image update may show you boxed in by strong returns.
Time delays of up to 20 minutes or more can occur in request-reply methods of datalink transmission. This requires that the pilot send in a request for weather data, then wait while the request is processed and a reply generated. When storms are active and many pilots are making requests, a lengthy queue can form. If you're tenth in line, any images you get may well be stale by the time they pop up on your display screen. Satellite-broadcast datalink providers continuously broadcast weather information, and users can quickly access it by simply punching a few buttons. With these systems, images are seldom older than four or five minutes and often no more than two minutes old. Even so, a few minutes can be critical if you're trying to make an evasive move to better conditions.
Base reflectivity versus composite reflectivity. It's very important to know that two types of radar imagery are generated by Nexrad. One is called a base reflectivity display; the other is a composite reflectivity display. There are short-range and long-range views of each display.
Base reflectivity images come from scans of the lowest Nexrad antenna tilt angles — usually the 0.5-degree tilt angle. This scan only shows precipitation in the lowest layers of the atmosphere, since the radar beam at that angle reaches approximately 10,000 feet at a range of 90 nautical miles from the radar site, and 16,000 feet at 143 nm from the radar site — the maximum range for base reflectivity. Radar echoes beyond 143 nm probably won't show up on base reflectivity scans from a single site, so that imagery may be derived from either the long-range view (out to 248 nm from the site) or the overlap from neighboring Nexrads.
Composite reflectivity images are made up of the highest-level radar returns from all antenna scan angles. This gives you a look at much more of the vertical cross section of the atmosphere, and it is useful for examining developing thunderstorms. In their formative stages, strong thunderstorms begin by drawing water droplets and ice particles into the midlevels of the atmosphere. While updrafts prevail, all this moisture is retained at altitude — say, 20,000 to 30,000 feet — until its weight can no longer be supported by vertical currents. Then rainfall and hail at the surface begin.
Bottom line: Base reflectivity scans will miss any precipitation echoes aloft, while composite reflectivity scans will show higher precipitation levels at a wider range of altitudes. A comparison of these two views can be made on the Aviation Digital Data Service (ADDS) Web site ( http://adds.aviationweather.noaa.gov). Click on the Radar button at the top of the page. When the radar site map appears, click on a Nexrad site, then click on the menu at the left of the same page to select base or composite reflectivity views. Of course, it helps if storm cells or precipitation is occurring nearby!
Provider preferences. Which radar imagery would you rather have in your cockpit? WSI InFlight offers base reflectivity images, and so does Honeywell Bendix/King's Wingman data-link service. XM WX Satellite Weather issues composite reflectivity via its connection with Wx-Worx Inc., an affiliate of Baron Services, Inc.
Both WSI and Honeywell Bendix/King argue that base reflectivity serves the bulk of general aviation pilots well. "Composite reflectivity does offer more information," said a WSI spokesman, "but there's only a marginal improvement of information with composite.... Also, base reflectivity is a tool that's familiar, easy to interpret, and a common language. Both The Weather Channel and Intellicast, for example, use base reflectivity for their radar imagery, and the old radar summary charts were from base reflectivity scans. Composite is more of a niche product." WSI also said that a certain amount of compositing at higher altitudes occurs with base imagery anyway, since the overlapping imagery of even low-antenna-angle scans can give very good views of precipitation patterns at altitudes as high as 15,000 feet. WSI went on to mention the high speed offered by base reflectivity scans (a function of making only one antenna sweep, versus the multilayered scans of composite reflectivity images). WSI also said it uses a quality-control algorithm that eliminates spurious, false returns called anomalous propagation (AP). AP is created when temperature inversions and other abnormal conditions cause false ground returns distant from the radar antenna site.
"Most of our customers are general aviation pilots who fly at lower altitudes — below 18,000 feet," said Tom Kraft, Honeywell's datalink product manager. "So we wanted to tune our product to where customers spend the most time.
"We looked at offering both base and composite reflectivity views, but there are other products that customers want more — like temporary flight restrictions, winds aloft, and other weather products. If we did both base and composite, it would take up too much bandwidth and limit our offerings and transmission speed.
"Besides," said Kraft, "the difference between base and composite is not enormous. You can use either one just fine. And we always emphasize that datalink radar imagery should be used as a strategic, not tactical, aid, so if you steer well clear of either view you can do a very good job of avoiding precipitation and thunderstorms."
Baron Services' Bob Baron takes a different view. "We want to be scanning for the worst-case scenario...a composite image may show the conditions above or below your altitude, but at least you'll know where the worst storm potential is.
"Ninety percent of new aircraft will have XM WX Satellite Weather, in avionics by Avidyne, Garmin, and Rockwell Collins, so I think the market likes our offering," Baron said.
Pilots versus meteorologists. In the final analysis, the choice of reflectivity products is up to the pilot. Most GA pilots, as has already been emphasized, will be served well by base reflectivity. But those who fly higher or want more of a 3-D view of a convective (or potentially convective) situation will want the composite view. Meteorologists who specialize in convective weather favor composite imagery because they can watch infant storms as they gather moisture and power at altitude, then propagate downward through the atmosphere.
Base-reflectivity advocates assert that if you're flying in the 20,000-to-30,000-foot range, you're no doubt flying a turbine-powered airplane with an airborne weather radar powerful enough to be of help in tactical weather avoidance. So datalink weather radar — base or composite — may be of less value.
Then again, more information is always better. When you consider that thunderstorms move three to five nautical miles in eight minutes, then a stale datalink image won't be much help if you're using your own radar to maneuver around convective cells.
But what if your radar is scanning precipitation so heavy that it swallows up your radar beams and gives you an attenuated view that blocks the worst cells? That's when datalink radar can give you an unimpaired view of what's around the corner. That view is worth its weight in gold, so having radar redundancy makes a great deal of sense if you fly hard IFR often. Add lightning detection and you've got a highly capable arsenal of storm avoidance equipment.
We're very fortunate to have this new technology, and we should give thanks that datalink weather has come of age. Just remember the limitations and know that radar rules only when you steer well clear of precipitation echoes. Better yet, make sure you have at least 20 nm of visual separation from convective clouds.
E-mail the author at tom.horne@aopa.org.
Links to additional information about weather radar may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).
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