Radar myths and misconceptions

Flying Safety, May, 2002 by Dave Gwinn

An abundance of energy will be lost above, below and around the storm.

Only a portion will strike the storm and the return will be a portion-of-a-portion, a-percentage-of-the-percentage that impacted the storm. In other words, the weather target will be underestimated, displayed on radar as less than its true hazard. Our Cessna 421 discovered that. Much closer and with a narrower beam, all of the energy will strike the target, and the return will be more accurate.

What's The Limit?

At what distance can the beam be filled by the target? For our typical 10-inch antenna, range efficiency is about 80 miles, a distance at which one has an 80,000-foot beam diameter. Anything viewed beyond 80 miles must be very dense to bounce back such a weak signal with any display of color.

Whatever is displayed should be assumed to be one to two levels more severe than what the pilot sees. It will appear to increase in intensity as the aircraft closes the distance and more energy is returned to the antenna.

Practically speaking, 80 miles is an adequate distance to make safe decisions. Even for the Learjet moving at eight miles per minute, the pilot has ten minutes of reasonably accurately-displayed weather avoidance time. When a display from a greater distance is properly pegged as "more severe than it looks," then the pilot has bolstered radar's physical limitations with his or her own knowledge.

Even in an MD-80 in airline operations, 80 miles is adequate for safety-of-flight purposes. The airliner's big 30-inch dish will paint accurately out to 160 miles, but all that really does is provide long-range planning and the economic diversion options so important to modern airline operations. Just remember, if you have a 10-inch antenna, 80 miles is your best range efficiency.

Leading Edge Antennas

How good is the leading edge antenna commonly seen in the wings of Bonanzas and other high performance singles? It's a 16-degree beam. It's made into a small antenna by lopping off the top and bottom to fit inside the wing. When you forfeit size, you forfeit focus.

If you use the leading edge antenna, radar calibrated accuracy--that is, a cell that will fill the beam and be accurately Displayed--may be possible at around 20 miles. My philosophy for the leading edge antenna has always been this: If you see anything, of any shape, at any distance and of any color, plan on going around it. You will rarely get the beam-filling target for which the radar receiver is calibrated. One manufacturer will sell you its radar, but won't sell or install a leading edge antenna. They haven't enough faith in it to accept any responsibility for its use.

As far as range efficiency goes, sferics devices--the Stormscope and Insight Strike Finder--are far ahead of radar with a leading-edge antenna. For one-third the price of a minimally efficient radar installation they are, in my view, a fat better choice. And the Strike Finder, with its outstanding digital circuits, light weight, low cost and lucid display, has outdistanced the WX-l000, top-of-the-line Stormscope.