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.
[Honeywell Primus 880 Pilot's Guide, pages 5-37, 38]
Figure: REACT on and off, from Honeywell Primus 880 Pilot's Guide, figure 5-38.
[Honeywell Primus 880 Pilot's Guide, page 38]
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.
[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.
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