Garmin G500 Pilots Guide - Page 193
Radar Signal Attenuation
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Foreword NM Sec 1 System Sec 2 PFD Sec 3 MFD Sec 4 Hazard Avoidance Features Sec 5 Additional Sec 6 Annun. & Alerts Figure 4-56 Radar Beam in Relation to the Curvature of the Earth 4.8.1.3 Radar Signal Attenuation The phenomena of weather attenuation needs to be kept in mind whenever operating the weather radar. When the radar signal is transmitted, it is progressively absorbed and scattered, making the signal weaker. This weakening, or attenuation, is caused by two primary sources, distance and precipitation. Attenuation because of distance is due to the fact that the amount of radar energy at a distance from the antenna is inversely proportional to the square of the distance. The reflected radar energy from a target 40 miles away that fills the radar beam will be one fourth the energy reflected from an equivalent target 20 miles away. This would appear to the operator that the storm is gaining intensity as the aircraft gets closer. Internal circuitry within the GWX 68 system compensates for much of this distance attenuation. Attenuation due to precipitation is not as predictable as distance attenuation. It is also more intense. As the radar signal passes through moisture, a portion of the radar energy is reflected back to the antenna. However, much of the energy is absorbed. If precipitation is very heavy, or covers a large area, the signal may not reach completely through the area of precipitation. The weather radar system cannot distinguish between an attenuated signal and area of no precipitation. If the signal has been fully attenuated, the radar will display a "radar shadow." This appears as an end to the precipitation when, in fact, the heavy rain may extend much further. A cell containing heavy precipitation may block another cell located behind the first, preventing it from being displayed on the radar. Never fly into these shadowed areas and never assume that all of the heavy precipitation is being displayed unless another cell or a ground target can be seen beyond the heavy cell. The WATCH™ feature of the GWX 68 Weather Radar system can help in identifying these shadowed areas. Areas in question will appear as "shadowed" or gray area on the radar display. Proper use of the antenna tilt control can also help detect radar shadows. 190-01102-02 Rev. B Garmin G500 Pilot's Guide 4-65 Sec 7 Symbols Sec 8 Glossary Appendix A Appendix B Index