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PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00
Radar Facts
5-16
ALTITUDE COMPENSATED TILT (ACT)
The PRIMUS
â
880 Digital Weather Radar has an ACT feature that can
be selected by pulling out the tilt control knob. This feature is
annunciated onthe radar display byadding an Asuffix tothe 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 terraincan result in alower thandesired tiltangle. Insuch
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 of2000 feet above sea level. This assumedground 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 tiltshould be used to frequently sweepabove
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.