Airborne WX Radar: “Stretching” and
“Smearing” the Truth
By Erik Eliel
Erik Eliel, the acknowledged expert on airborne weather radar use and training (www.rtiradar.com), has taught at the last three Twin Commander Universities. His radar training tips appear periodically in Twin Commander Aircraft’s Flight Levels update and monthly eLetter.
In the last issue of Flight Levels update (Summer 2011, www.flightlevelsonline.com), I left off with a question about a “thin radial line” that a pilot had observed on his radar display. I said it was actually a “great capability” of the radar. The same question has come up several times since from other pilots who also thought their radar was malfunctioning. That is not the case.
1. This radar system has a relatively large antenna (narrow beam) but long pulse width, so displayed objects smear much more in range—the elongated red returns at the 11 o’clock position—than they will in width
When we talk about radar resolution, we deal with two basic dimensions: right to left (horizontal), and front to back (range). Clearly, those dimensions are important to pilots because, ultimately, they define what is displayed. You and I make judgment calls about what it is we think we’re seeing on the display, whether or not we need to avoid it, and if so, by how far. At the practical level, what you need to know is that the radar system adds “dimension” to objects it detects. This is sometimes referred to as “smearing” or “stretching.” To understand how much it smears or stretches, horizontal and range resolution must be discussed separately.
Horizontally (left to right, right to left), the amount an object is smeared is primarily a function of beam width, which is determined by antenna size and the displayed object’s distance from the antenna. The wider the beam, the more it smears the object.
2. Thin purple area at 11 o’clock and just under 80 miles is reflected radar energy from a city. This is how a radar system with a relatively small antenna (wide beam) but short pulse width (high power) will “smear” distant objects horizontally much more so than in range.
The Twin Commander is equipped with a 12-inch antenna, giving it an approximate beam width of 8 degrees. The Boeing 737 is equipped with a 28-inch antenna, giving it an approximate beam width of 3.5 degrees. No need to get too specific on how much it smears, just know that a smaller antenna with a wider beam is going to smear displayed objects more than a larger antenna with a smaller beam. Also, as a displayed object’s distance from the antenna increases, the wider the beam becomes and the more it will smear.
Range smearing is a function of pulse width—the time the transmitter is on, which varies by design. High-powered radar systems pump out very short pulses (2 to 4 microseconds). Low-powered radar systems may put out pulses in excess of 28 microseconds.
For those who have attended the Twin Commander University-sponsored radar program, you may recall that in one-millionth of a second, radar energy travels 984 feet. So, to put the above numbers into context, a 2-microsecond pulse translates to about 1968 feet, a 4-microsecond pulse is about 3936 feet, and the 28-micro-second pulse would be a train of radar energy in excess of 27,000 feet. The longer the pulse the more dimension it adds, and thus the more smearing that occurs on the backside of a target (approximately half the pulse width).
With that, take a look at the first image accompanying this article. It shows a (purple) “thin radial line” at 11 o’clock near the outer perimeter of the display—about 80 miles away. The aircraft has a 12-inch antenna (8-degree beam) so the approximate beam width at that distance is about 64,000 feet (about 10 miles wide). Therefore, it will smear/stretch the target a fair amount horizontally.
The radar in the aircraft uses a 4-microsecond (about 3936 feet) pulse width. On the 80-mile scale, that width is virtually undetectable. This is how a radar system with great range resolution paints a city. Fortunately, a thunderstorm depicted on this radar display would look very different.
Contrast that to the second image from a radar on an aircraft with a 28-inch antenna (3.5-degree beam). At 80 miles, the beam width is approximately 28,000 feet (about 4.5 miles wide) so it will have reasonable horizontal resolution. But it is a low-powered radar and the pulse width is very long, meaning it will smear a lot on the backside of a target. The photo was taken on a clear weather day, and the round red returns at 11 o’clock (270 degrees) that look a lot like thunderstorms are actually cities.
Unfortunately, this can be the source of confusion during a day when thunderstorms are in fact present. So, even though an aircraft with the 28-inch antenna would have better horizontal resolution than one with a 12-inch antenna at a given distance, a radar with high power and a short pulse-width (similar to what some Twin Commanders are equipped with) provides superior range resolution. This gives pilots a significant advantage in determining what it is they’re actually seeing, if it needs to be avoided, and by how much.
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