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2.3.3 Antenna Location Planning Guidance
NOTE
The ability of any radar altimeter system to accurately and reliably detect and process
radar altitude signals is highly dependent upon correct transmitter (TX) antenna and
receiver (RX) antenna placement on the airframe. Stated performance specifications for
the GRA 5500 can only be achieved when the system is installed in accordance with all the
guidelines stipulated in this section and with all other instructions given in this
Installation Manual.
Careful planning for optimal antenna placement is extremely critical in any radar altimeter installation
and requires a high level of attention to detail in order to achieve optimal system performance of the
GRA 5500. The following guidelines must be followed in order to achieve acceptable performance of the
system during all stages of normal operation. Failure to follow any of these guidelines will result in
degradation of system performance.
If there is any question as to whether a planned installation will meet all the guidelines given below or may
require special consideration, contact Garmin for further support.
1. Location on Aircraft: Antennas must be mounted on a surface of the bottom of the aircraft near the
center point of pitch and roll rotation (this reduces the effect of momentary pitch and roll altitude
offsets/inaccuracies during approach and landing or while maneuvering the aircraft).
2. Angle of Antennas to Ground: Antennas must be mounted such that the surface of each antenna is
within +/- 6° of horizontal during level flight. If adapter plates are manufactured to ensure this
angle is met, the adapters must not recess the antenna surfaces within the skin of the aircraft. The
adapters must only extrude the antenna surfaces outside the skin of the aircraft.
3. Mounting Surface: The antennas are normally mounted on a continuous metallic surface. Non-
metallic surfaces or ones with discontinuities require special consideration. Contact Garmin for
more information or clarification if needed.
4. Minimum Height: Due to antenna spacing limits (described in following guideline 8.), the
minimum antenna height possible above ground while parked, taxiing, or at aircraft touchdown is
20 inches for any aircraft.
5. Antenna Clearance Cones: The locations chosen for both antennas must be free of aircraft
projections (landing gear, flaps, drainage vents, etc...) within a 120° cone from each antenna
downward to prevent locking onto false altitude returns. Consider all possible modes of aircraft
operation when establishing compliance with this guideline (e.g. – landing gear distances while in
transition, flap distances while fully extended, etc.).
6. Distance to Other Antennas: The radar altimeter antennas should be located no closer than 3 feet
to any other aircraft antenna.
7. Relative Location (Single System): The two radar altimeter antennas must be mounted either in a
straight, nose-to-tail longitudinal line with both antenna arrows pointing forward, or in a straight,
side-to-side, lateral line with both arrows pointing to one side of the aircraft or the other (they may
alternatively point toward each other or away from each other). The longitudinal installation is
preferred over the lateral installation to provide more roll than pitch angle coverage. Additionally,
the angle between the antenna surface planes must not exceed 6° and the antennas must not be
tipped toward each other at any angle. In a single system, either antenna may be selected as the
transmitter (TX) or receiver (RX) antenna.
GRA 5500 Installation Manual
Page 2-4
190-01277-00
Rev. A
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