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Mountains also provide strong return signals to the antenna, but also block the areas behind. However, over mountainous terrain, the radar beam can be reflected back and forth in the mountain passes or off canyon walls, using up all or most of the radar energy. In this case, no return signal is received from this area, causing the display to show a dark spot which could indicate a pass where no pass exists.
A
ngle of Incidence
The angle at which the radar beam strikes the target is called the Angle of Incidence. The figure illustrates the incident angle (‘A’). This directly affects the detectable range, the area of illumination, and the intensity of the displayed target returns. A large incident angle gives the radar system a smaller detectable range and lower display intensity due to minimized reflection of the radar energy.
Figure 6-39 Angle of Incidence
A smaller incident angle gives the radar a larger detectable range of operation and the target display shows a higher intensity. Since more radar energy is reflected back to the antenna with a low incident angle, the resulting detectable range is increased for mountainous terrain.
316 Garmin G1000 Pilot’s Guide for the Cessna Citation Mustang 190-00494-02 Rev. B
Hazard Avoidance voidance voidance
SYSTEM
OVERVIEW
FLIGHT
AUDIO PANEL EICAS INSTRUMENTS
& CNS
FLIGHT
MANAGEMENT
HAZARD
ADDITIONAL AFCS AVOIDANCE
INDEX APPENDICES FEATURES
Safe Operatingperating Distanceistance
The following information establishes a minimum safe distance from the antenna for personnel near operating weather radar. The minimum safe distance is based on the FCC’s exposure limit at 9.3 to 9.5 GHz for general population/uncontrolled environments, which is 1 mW/cm2. See Advisory Circular 20-68B for more information on safe distance determination.
Maximum Permissible Exposure Level (MPEL)
The zone in which the radiation level exceeds the US Government standard of 1 mW/cm2 is the semicircular area of at least 11 feet from the 12-inch antenna. All personnel must remain outside of this zone. With a scanning or rotating beam, the averaged power density at the MPEL boundary is significantly reduced.
MPELBoundary11’ for 12” antenna
Figure 6-40 MPEL Boundary
BasicBasic
Antenna Tiltilt Setup
The following discussion is a simple method for setting up the weather radar antenna tilt for most situations. It is not to be considered an all encompassing setup that works in all situations, but this method does provide good overall parameters for the monitoring of threats. Ultimately, it is desired to have the antenna tilted so that the bottom of the radar beam is four degrees below parallel with the ground. The following example explains one way of achieving this.
With the aircraft flying level, adjust the antenna tilt so ground returns are displayed at a distance that equals the aircraft’s current altitude (AGL) divided by 1,000. For example, if the aircraft is at 14,000 feet, adjust the tilt so the front edge of ground returns are displayed at 14 nautical miles. Note this antenna tilt angle setting. Now, raise the antenna tilt 6 degrees above this setting. The bottom of the radar beam is now angled down 4º from parallel with the ground.
190-00494-02 Rev. B Garmin G1000 Pilot’s Guide for the Cessna Citation Mustang 317
Hazard Avoidance voidance voidance
SYSTEM
OVERVIEW
FLIGHT
INSTRUMENTS EICAS AUDIO PANEL
& CNS
FLIGHT
MANAGEMENT
HAZARD
AVOIDANCE AFCS ADDITIONAL
FEATURES APPENDICES INDEX
Practicalractical Applicationpplication Using the Basic Tiltilt Setup
With the antenna tilt set as previously described, any displayed target return should be scrutinized when flying at altitudes between 2,000 and 30,000 feet AGL. If the displayed target advances on the screen to within 5 nautical miles of the aircraft, avoid it. This may be either weather or ground returns that are 2,000 feet or less below the aircraft. Raising the antenna tilt 4 degrees can help separate ground returns from weather returns in relatively flat terrain. This aligns the bottom of the radar beam parallel with the ground. Return the antenna tilt to the previous setting after a few sweeps.
If the aircraft is above 29,000 feet, be cautious of any target return that gets to within 30 nautical miles. This is likely a thunderstorm that has a top high enough that the aircraft cannot fly over it safely.
If the aircraft altitude is 15,000 feet or lower, setting the displayed range to 60 miles may be more helpful. Closely monitor anything that enters the display.
Also, after setting up the antenna tilt angle as described previously, ground returns can be monitored for possible threats. The relationship between antenna tilt angle, altitude, and distance is one degree of tilt equals 100 feet of altitude for every one nautical mile.
　
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