|CHAPTER 5. Information Systems
Onboard Weather Radar Systems
Onboard weather radar uses an adjustable aircraft mounted radar antenna to detect, in real time, weather phenomena near the aircraft. The coverage of an onboard weather radar system is similar to a flashlight beam, as illustrated in Figure 5-12. You should always remember that the radar displays only areas of water or moisture (rain, sleet, snow, and hail). Radar does not display turbulence or lightning.
Although the tilt of the radar antenna can be adjusted upward and downward, the weather phenomena that the weather radar can detect are limited in both direction and range. The radar system in Figure 5-12 fails to detect the two cells that lie below and beyond the radar beam.
As illustrated in Figure 5-12, you must be careful not to assume that the only cells in the area are the ones shown on the radar display. The two additional cells in Figure 5-12 are present, but not detected by the onboard weather radar system.
When a cell is detected by an onboard weather radar system, that cell often absorbs or reflects all of the radio signals sent out by the radar system. This phenomenon, called attenuation, prevents the radar from detecting any additional cells that might lie behind the first cell. Figure 5-13 illustrates radar attenuation, in which one cell “shadows” another cell.
A simple color-coding scheme, as shown in Figure 5-14, is used to represent the intensity of radar echoes detected by an onboard weather radar system.
Ground Weather Surveillance Radar
Ground weather surveillance integrates weather information from many ground radar stations. The weather information collected from many sources is then used to create a composite picture that covers large volumes of airspace. These composite radar images can then be transmitted to aircraft equipped with weather data receivers.
Except in those areas for which no ground radar coverage is available, the range of ground weather surveillance radar systems is essentially unlimited. Ground radars have the luxury of large antennas; big, heavy power supplies; and powerful transmitters—without the constraints of aerodynamic drag, power, weight, and equipment volume restrictions and concerns.
Unlike onboard weather radar systems, weather data received from a ground weather surveillance radar system is not realtime information. The process of collecting, composing, transmitting, and receiving weather information naturally takes time. Therefore, the radar data reflect recent rather than current weather conditions.
The color-coding scheme used by one ground weather surveillance radar system (NEXRAD) is shown in Figure 5-15. Note that this color-coding scheme is slightly more sophisticated than that for the onboard system in Figure 5-13. It is capable of distinguishing rain, snow, and mixtures of the two.
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