Warning! Radar Operating
By Jim Sparks
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. This enables the crew to navigate around potentially hazardous areas. Another capability is to "ground map." In this mode, the flight crew receives a topographical picture which can be of assistance for navigation and position reference. In some cases, weather radar systems will detect aircraft in their path; however, these systems are not intended as collision avoidance.
A typical radar system includes a flight deck display, receiver transmitter, and an antenna. The cockpit indicator is referred to as a plan position indicator (PPI) as it indicates both distance and direction to the target. This can be a stand alone cathode ray tube (CRT), or in aircraft using electronic flight instruments (EFIS), a multi-function display (MFD) can be used to paint the radar image.
Control panels for weather radar contain a function switch, which in addition to selecting the unit "OFF," can be positioned to "standby" (STBY or SBY). This allows warm up with the antenna not scanning and the transmitter inhibited. Most recently developed systems require about 90 seconds to become operational, while older units require several minutes. Some installations have what is known as a "forced standby." This is a situation where the radar will automatically stop transmitting and the antenna will stop sweeping. One common means of introducing this forced stand by is to have the aircraft in a "weight on wheels" configuration or by activating the approach mode in a flight guidance system.
It's important to investigate possibilities of forced standby prior to performing maintenance. Jacking some aircraft with avionic equipment powered up may result in activation of the radar unit. With Electronic Flight Instruments (EFIS), separate radar controls may be available to both pilot and copilot. In this case, both will need to be selected to STBY or OFF. When in doubt, pull all radar circuit breakers prior to turning on electrical power.
After the warm-up is complete, selection to the "weather" (WX) mode causes the transmitter portion of the receiver transmitter (RT) to deliver high power pulses to the antenna. Between two transmissions, the RT serves as a receiver processor and configures the return signal into a format for useful display to the crew. Most of today's weather radars are "X"-Band and operate on a frequency of around 9345 Mhz, and power outputs of 8 to 10 kilowatts. This enables a viewing range of up to 300 nautical miles. The length of the pulses may vary depending on the requirements of the system, with one to two microseconds being nominal values. Pulse repetition frequency (PRF) will vary depending on the range being monitored. For long distances, the rate must be slow enough to allow the return to be processed before the next transmission. This is determined by the range selector position on the radar controller panel.
The antenna is supplied by the transmitter and radiates the electromagnetic energy into the air. It then becomes the receptor of the echoes from the targets. These devices come in sizes from 10 to 24 inches and the larger the diameter the narrower the beam width.
Radar antenna platforms are rather complex as they have to rotate and pitch to accurately scan the area in front of the aircraft.
"Scan" or "sweep" control will determine the total area in front of the aircraft to be observed. In most systems, the rotation of the antenna will encompass a 120 degree arc. This "azimuth" is displayed in either 15 or 30 degree increments. When an area of weather is detected and the crew wishes to observe it in more detail, the scan can be reduced to a 60 degree sweep. In most cases when the area observed is reduced, the "looks per minute" will increase. Normally, in an 120-degree arc there are 12 looks per minute. With a 60- degree sector being observed, the scan rate goes to 24 looks per minute. This provides the crew with the most up to date and accurate view of the target.
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