PRIMUSr 880 Digital Weather Radar System
ALTITUDE COMPENSATED TILT (ACT)
The PRIMUSâ 880 Digital Weather Radar has an ACT feature that can be selected by pulling out the tilt control knob. This feature is annunciated on the radar display by adding an A suffix to the tilt readout. While in ACT or manual tilt the digital tilt readout always shows the actual (true) tilt of the antenna.
In ACT, the antenna tilt is automatically adjusted with regard to the selected range and the aircraft’s barometric attitude. ACT adjusts the tilt to show a few ground targets at the edge of the display. In ACT, the ideal setting can be adjusted ± 2°to accommodate terrain height or pilot preferences.
NOTE: Since ACT uses air data computer barometric altitude to adjust the tilt, operating near high altitude airports or even high terrain can result in a lower than desired tilt angle. In such cases, use of the manual tilt is recommended.
To calculate the tilt angle, the weather radar uses the air data computer’s barometric altitude with reference to an assumed ground level of 2000 feet above sea level. This assumed ground level is a factor during low altitude flight, especially when flying in mountainous areas. The ground targets that are usually at the edge of the display tend to migrate to the middle of the display. This also happens when longer ranges (200 NM to 300 NM) are selected and the altitude is such that the earth’s curvature is a factor.
In ACT the range control can be used to sweep the beam along the ground to look for storms at various ranges, as shown in figure 5- 18.
ACT is best suited for high altitude operation while in the weather surveillance mode; i.e., aircraft is in cruise and there is no weather within 100 NM. The operator can then use the range control to frequently sweep the beam down to avoid overflying any fast developing storms.
At lower altitudes, manual tilt should be used to frequently sweep above and below the flight level to avoid flying under or over storms, as shown in figure 5- 18. Manual tilt should also be used exclusively when analyzing weather.
NOTE: The radar system does not have enough information to be able to tilt the beam into the wet, lower portions of cells by itself. The operator must manage tilt dynamically or manually to locate and analyze weather. ACT simply adjusts the beam to the earth’s surface at the selected maximum range. Also, it assumes that the surface is at 2000 feet above sea level.
Radar Facts | A28- 1146- 102- 00 |
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