Radar myths and misconceptions

Flying Safety, May, 2002 by Dave Gwinn

Its limitations are many. It's up to the pilot to fill in the gaps with knowledge and experience.

The Cessna 421 pilots were absolutely puzzled by the weather diversion discussion on Center's frequency. Only a large area of benign light rain awaited them at 120 miles, according to their onboard radar. Within 15 miles they were stunned by a solid red palisade of horrid weather, apparent both on radar and as a visual threat.

A competent DC-9 Captain began to work his way through heavy rain in an energetic and unexpected convective weather system. Apparently looking at the radar display, he remarked to his copilot, "There's a thin spot! Turn left now." Within minutes, the DC-9 exploded on a country road in Georgia and all lives were lost. The windshields had been shattered and the engines gutted in intense rain and hail.

The Captain of a regional airline's Metroliner responded: "It doesn't look bad on our radar. We'll continue for the time being." Within 30 minutes, the turboprop slammed into the ground in a 5000- foot-per-minute rate of descent, pushed downward by penetration into a Level 6 thunderstorm.

In each instance, the pilot had flawed expectations of radar's capabilities. One relied on radar in formation rendered questionable by long-range access; the other two were lured into trouble by the seductive effects of heavy rain at close range. The on-board radar, even though it was performing perfectly up to its design capabilities, did not provide the safety margin that was expected of it. Why? Because the pilots either didn't know or had forgotten that radar is not the bulletproof weather avoidance tool we imagine it to be.

Picky, Picky

Understanding the limitations of radar permits a pilot to make an educated assessment of its "panacea" value in weather avoidance. Its maladies are many: range, geography, altitude, and limited penetration. And it's picky; there's an ideal raindrop diameter which radar detects. It may ignore those smaller or be fully consumed by those much larger. Therefore, the pilot's radar education, expectations and accumulated experience must be brought into play when radar is used as an avoidance tool.

That understanding begins with this simple fact: Radar is a water detector. Radar energy interacts electromagnetically with water in a process commonly called "reflectivity." But that's not really what happens. As radar energy travels through waterdrops (in cloud form or as rain) it interacts best with raindrop diameters about .1 to .2 the size of the wavelength. With airborne radar (X-band) the frequency is about 9400 MHz and the wavelength is 3.2 centimeters or about 1.2 inches.

When raindrops are illuminated by radar energy, the molecules within the droplets are energized or "dipoled". Everything in nature seeks equilibrium. A charged raindrop begins to discharge (actually called "scattering"), to deplete itself of energy, isotropically or omni-directionally. It's those few vectors of energy "back scattered" (returned) to the radar receiver that are detected and displayed.

Waterdrop diameters below these target sizes (such as fog or shallow cloud formations) will be ignored by radar. When droplet cross-sections approach the size of the wavelength in heavy rain, the radar's energy is simply absorbed: it vanishes into the ether. Frozen water (cirrus clouds, small hail, snow) is crystalline; the molecules are no longer free to charge and perform the electronic tap-dance which results in radar displays, thus the pilot will see no return at all.

Antennas and Range Woes

Most radars have variable ranges set by a knob on the control panel, but most pilots don't realize how sharply limited radar's range really is.

Radar efficiency is a function of the frequency and the size of the antenna installed. Airborne antennas vary in size. Ten inches is standard in most general aviation aircraft, such as light to medium twins, King Airs, Learjets or any plane lacking the radome to accommodate anything larger.

The pods you see suspended below wings usually enclose a 10-inch antenna. The smaller the antenna, the wider the radar beam and the more its energy is dispersed with distance. A 10-inch antenna produces a 10-degree focus. In contrast, an airline installation, with a 30-inch antenna, yields a 3-degree beam. (Editor note: Sec Figure for radar antenna sizes on some USAF aircraft.)

Figure 1

Whether it's VOR radials or airborne radar; all electromagnetic energy obeys the same postulate: I degree equals 6000 feet of diffusion at 60 nautical miles. (VOR axiom: 1 degree equals 6000 feet or 1 nautical mile left-to-right at 60 miles distance from the station.) The typical 10-inch antenna/10-degree beam (actually a cone, not a beam) has a 60,000-foot diameter at 60 miles. At 100 miles, the 10-inch antenna produces a cone of 100,000 feet in diameter. That's big! (At this same distance, the airliner has a 30,000-foot cone.)

The receiver calibrates returned signals in decibels of reflectivity (dBz). For the sake of accuracy, all of the energy is assumed to impact the weather target. The return reflectivity is weighed as a percentage or proportion of the total transmitted power. Returned energy depicts (by color) the waterdrop density of the precipitation within the cone. You can visualize that at 100 miles and with a cone of 100,000 feet diameter, no thunderstorm will "fill the beam."