Weather Radar designed for probing of the airspace ahead of aircraft and inspection of the underlying earth's surface (Fig. 87) .
Fig. 87. Weather Radar
Modern Weather Radar can detect cloud structures at a distance of 200 km and area of storm activity and prevent entry of aircraft into them. To solve navigation tasks Weather Radar can work in mode of browse of surface relief of the land. Modern Weather Radar also able to detect wind shear, which poses a real danger to aircraft and can lead to disaster.
Basic principle of weather radar
Principle of operation of Weather Radar based on the principle of action of primary radar.
Principle of operation of Weather Radar illustrates Fig. 88. Weather Radar transmitter generates radio probing signal at a frequency 9345 ± 15 MHц, which through antenna switch is supplied to the antenna system. Antenna system designed for radiation and receiving of probing radar signals. Weather Radar antenna is mounted on the nose part of aircraft under the fairing, made of radioparent material.
Fig. 88. Weather Radar principle of operation
Antenna system forms a narrow directional diagram in the horizontal plane and wide in the vertical. Diagram of direction of antenna moves so that the radar beam passed all the probing zone.
In older types of Weather Radar such as antenna system used parabolic antenna that mechanically revolved around its axis in a particular sector. Modern Weather Radar used antenna array. Slotted antenna array can create a diagram direction of desired shape (Fig. 89).
Fig. 89. Antenna system of weather radar
Antenna system of Weather Radar emits radar signals propagating in space. Probing radio signals reemitted from air re objects droplets of moisture in the air or the earth's surface and return in the opposite direction to the antenna system of Weather Radar. Antenna array scans left to right from the direction of flight in range ± 90°, scanning speed is at least 15 times per minute. In the range of scanning weather radar produces many separate radio beams (to 1024), each beam is broken during the reception on many placed one by one points (the default - 256 points maximum - 512) for each point measured level of the reflected signal indicates the presence and density of clouds and turbulence.
Through the antenna switch recieved by antenna signal is passed to a receiver that handles call-out. Evaluator estimates by travel time of the probe radio signal to the air object and back distanceIn addition, during the probing signal reradiation from objects changing certain parameters of the signal. Evaluator with different algorithms fixing change of probing signal parameters and their values evaluates physical properties of objects, from which he was reradiated. If at board of aircraft available electronic display system for output of meteorological information using its indicators. For this computer unit contains a standard digital output for transferring images in digital form (for example by ARINC 708). If there is no system of electronic indication, to the meteo radar indicator includes its own (Fig. 90).
Fig. 90. Picture on radar indicator
Evaluator handles the information and gives measurement results in system of display. The information scaled depending on the set on the remote control range of distance. On the screen of indicator levels of reflecting signal represents as points of different colors. With the increase level of the reflected signal colors are placed so: black, green, yellow, red. In the case of turbulence measurements provided brown, for strong turbulence - purple. As a result, points create on the screen sketch of meteo formation in the direction of flight. An example of an image on the indicator of cloudiness straight ahead of flight is shown in Fig. 90. Darker (red) zone corresponds to more dangerous meteoformations. In the conditions of maneuvering Weather Radar stabilizes the beam of antenna in a horizontal position. It uses the information about the angles of roll and pitch from INS.
To reduce the time of inspection pilot with remote control can reduce the area of view to ± 45 °. You can also set the antenna tilt angle in limits ± 15 ° from the horizontal axis of aircraft. This prevents interference and increase the clarity, consider the vertical structure of clouds and when antenna is tilted down to the ground - using Weather Radar for inspection earth relief in accordance with the needs of navigation. At high altitudes above 12,000 m Weather Radar allows to identify the storm area and shape of the city at a distance of 550 km.