Eddie sez:

The radar's antenna gain is, pretty much, the focus of the beam which determines how strong it will be. The gain is calibrated to produce standard display levels. Rainfall reflects the beam and can totally attenuate the radar energy. The Honeywell Primus 880 uses a Rain Echo Attenuation Compensation Technique (REACT) that can overcome this limitation. But you have to understand when to use it and what it is telling you when you do.

Everything here is from the references shown below, with a few comments in an alternate color.

Last revision:



Attenuation Compensation

[Honeywell Primus 880 Pilot's Guide, pages 5-37, 38]

  • As radar energy travels through rainfall, the raindrops reflect a portion of the energy back toward the airplane. This results in less energy being available to detect raindrops at greater ranges. This process continues throughout the depth of the storm, resulting in a phenomenon known as attenuation. The amount of attenuation increases with an increase in rainfall rate and with an increase in the range traveled through the rainfall (i.e., heavy rain over a large area results in high levels of attenuation, while light rain over a small area results in low levels of attenuation).
  • Storms with high rainfall rates can totally attenuate the radar energy making it impossible to see a second cell hidden behind the first cell. In some cases, attenuation can be so extreme that the total depth of a single cell cannot be shown. Without some form of compensation, attenuation causes a single cell to appear to weaken as the depth of the cell increases.
  • Honeywell has incorporated attenuation compensation that adjusts the receiver gain by an amount equal to the amount of attenuation. That is, the greater the amount of attenuation, the higher the receiver gain and thus, the more sensitive the receiver. Attenuation compensation continuously calibrates the display of weather targets, regardless of the amount of attenuation.
  • With attenuation compensation, weather target calibration is maintained throughout the entire range of a single cell. The cell behind a cell remains properly calibrated, making proper calibration of weather targets at long ranges possible.

Cyan REACT Field


Figure: REACT on and off, from Honeywell Primus 880 Pilot's Guide, figure 5-38.

[Honeywell Primus 880 Pilot's Guide, page 38]

  • The receiver gain is adjusted to maintain target calibration. Since there is a maximum limit to receiver gain, strong targets (high attenuation levels) cause the receiver to reach its maximum gain value in a short time/short range. Weak or no targets (low attenuation levels) cause the receiver to reach its maximum gain value in a longer time/longer range. Once the receiver reaches its maximum gain value, weather targets can no longer be calibrated. The point where red level weather target calibration is no longer possible is highlighted by changing the background field from black to cyan.
  • Any area of cyan background is an area where attenuation has caused the receiver gain to reach its maximum value, so further calibration of returns is not possible. Extreme caution is recommended in any attempt to analyze weather in these cyan areas. The radar cannot display an accurate picture of what is in these cyan areas. Cyan areas should be avoided.
  • Any target detected inside a cyan area is automatically forced to a magenta color indicating maximum severity.

Without REACT a black area behind other targets are simply unknown areas, either shadows or areas of no returns. With REACT you have the same situation but the radar tells you it has reached maximum gain and the area in cyan is indeed a shadow.

Shadowing Technique

[Honeywell Primus 880 Pilot's Guide, page 5-40] An operating technique similar to the REACT blue field is shadowing. To use the shadowing technique, tilt the antenna down until ground is being painted just in front of the storm cell(s). An area of no ground returns behind the storm cell has the appearance of a shadow behind the cell. This shadow area indicates that the storm cell has totally attenuated the radar energy and the radar cannot show any additional targets (WX or ground) behind the cell. The cell that produces a radar shadow is a very strong and dangerous cell. It should be avoided by 20 miles.

For more about this technique, refer to Radar - Techniques.

Honeywell Airborne Weather Radar Training, Rev E, 12/09/02, Honeywell Inc. Commercial Flight Systems Group, Phoenix, AZ.

Honeywell Primus 880 Pilot's Guide, Pub. No. A28-1146-102-03, Revised January 2006, Honeywell International Inc. Commercial Electronic Systems, Glendale, AZ.