Warning! Radar operating

Warning! Radar Operating By Jim Sparks May-June 1998 The primary purpose of weather radar is to detect storms along the flight path and give the pilot a visual indication of rainfall intensity, and with doppler radar, possible turbulence...


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"Tilt" is another selection available to the crew and is used to set the "up, down" position of the antenna beam relative to the horizon. Most systems provide a range of 15 degrees above zero to 15 degrees below zero. "Altitude compensated tilt" (ACT) is a means where the antenna tilt is automatically adjusted as the aircraft changes altitude. This capability is available in many new technology systems and requires an input from an air data computer. Another automatic function is "stabilization," which uses a vertical gyro to provide aircraft turn and bank information so the radar antenna can remain in the same azimuth and elevation relative to the ground. The pilot has the option of disconnecting this stabilization by activating a switch on the radar control panel. Differences in some antenna installations may require stabilization trim adjustments. Airframe manufacturers' maintenance manuals, as well as radar manufacturers' procedures, should always be closely observed when making any adjustment.

Receiver sensitivity needs to be properly calibrated to eliminate background noise, yet provide for reception of even the weakest reflected signal. The "gain" control is a useful tool for weather analysis and ground mapping. In the mapping mode it is possible to reduce the level of the typically very strong returns from ground targets, while in the weather mode, calibrated sensitivity can be increased to allow very week targets to be observed.

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In some later technology weather radars, it can be difficult to distinguish where the receiver/transmitter (RT) stops and the antenna begins. This type of device is referred to as a receiver transmitter antenna (RTA).

In earlier systems the antenna and the RT were connected by a radar wave guide. This is a hollow, usually rectangular metal conduit that would allow passage of the ultra high frequency (UHF) signal. The transmitted radio wave departing the RT could not escape through the walls of the wave guide, so it will then flow to the end where the antenna could radiate the signal through the air. Wave guides are sealed and frequently pressurized to prevent moisture ingress as any contamination has the potential to distort the radar signal.

One of the numerous cautions in dealing with radar systems is to avoid an open end of a wave guide while the radar is operating. Severe eye damage can result. In fact, any technician who is involved in maintenance of radar equipped aircraft should obtain a copy of Advisory Circular 20-68B "Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar," and become thoroughly familiar with its contents.

Weather radar antennas are located in some forward facing section of the aircraft. These areas may include a wing leading edge or most frequently the nose section, which incorporates a radar dome (radome). The radome is a covering whose primary purpose is to protect the radar antenna from the elements. This component has to be strong enough to withstand the aerodynamic loads of the aircraft, yet made of material that will allow free passage of the transmitted output as well as the return signal.

The construction of a radome is usually in one of two methods. The "thin wall" which is useful with low frequency systems and in areas where aerodynamic and structural loads will allow, and "sandwich" style radomes that are constructed of two or more skins separated by a nonconductive core. This can include foam-filled or hollow chambers such as with honeycomb. Radome manufacturers frequently experiment with new exotic materials, including quartz, to deliver a unit with the ultimate in electrical and structural properties.

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