90Detailed OperationAngle of IncidenceThe angle at which the radar beam strikes thetarget is called the Angle of Incidence. Incident angle(“A”) is illustrated below. This directly affects thedetectable range, the area of illumination, and theintensity of the displayed target returns. A large inci-dent angle gives the radar system a smaller detectablerange and lower display intensity due to minimizedreflection of the radar energy.A smaller incident angle gives the radar a largerdetectable range of operation and the target displaywill show a higher intensity. Since more radar energyis reflected back to the antenna with a low incidentangle, the resulting detectable range is increased formountainous terrain.Operating DISTANCEThe following information establishes a minimumsafe distance from the antenna for personnel near anoperating airborne weather radar. The minimum safedistance is based upon the FCC’s exposure limit at9.3 to 9.5 GHz for general population/uncontrolledenvironments which is 1 mW/cm2. See AdvisoryCircular 20-68B for more information on safe distancedetermination.MAXIMUM PERMISSIBLE EXPOSURE LEVEL (MPEL)The zone in which the radiation level exceeds theUS Government standard of 1 mW/cm2, is the semi-circular area of at least 10.83 ft from the 12” antennaas indicated in the illustration below. All personnelmust remain outside of this zone. With a scanningor rotating beam, the averaged power density at theMPEL boundary is significantly reduced.Angle of IncidenceRadar
