Installation Manual

Prerequisites
3-6 Chapter 3: Installation 98-152675-A
Cobham Public
antenna offset angles be measured to an accuracy of 0.5° and programmed into the CSDU,
refer to [6].
You can determine the roll and pitch offset of the antenna by subtracting the aircraft
attitude angles from the antenna attitude angles (relative to the same local level). The
aircraft attitude angles may be read from the aircraft avionics or measured with an
electronic level on an appropriate reference surface. The antenna pitch attitude may be
measured with an electronic level placed on the crown of the aircraft, on the longitudinal
symmetry axis of the mounting holes on the fuselage. The roll attitude may be measured by
placing suitable equal thickness spacers on 2 laterally adjacent fuselage mounting holes to
support the electronic level during roll angle measurement.
These angles define the installation misalignment of the antenna and must be stored in the
CSDU to be used as part of its beam steering calculations. The CSDU installation software
will guide the installer through the process of entering the information (refer to [6]).
Lightning strike
The LGA-5005 is rated for RTCA DO-160 Section 23, category 2A and 2A. To protect the
antenna in the event of a lightning strike ensure that the grounding considerations in
paragraph 3.4.3 are adhered to.
RF blockage
The vertical and horizontal stabilizers (particularly a T-tail) may cause blockage of the RF
beam. Placing the antenna further forward decreases the effect.
Co-location with other antennas
The LGA-5005 has passed the recommended RTCA/DO-160G Radiated Emission tests.
However, transmission from the antenna may still affect the operation of other RF systems
in particular cases. It is the responsibility of the Installer to evaluate this before deciding on
the mounting position of the LGA-5005. This principle generally applies to the installation
of any radio system on the aircraft.
Iridium and GPS/GLONASS systems are most likely to be affected by direct high power
transmission. The separation requirement depends on the equipment installed. Interference
between two Inmarsat SATCOM systems on the same aircraft normally does not present a
problem. In this case a separation distance of at least 0.5 m is adequate to avoid affecting
the radiation patterns of the antennas. Closer spacing may be possible; in this case it is
advisable to contact Cobham for guidance.
Interference-free operation between an Inmarsat-based SATCOM system and other L-band
(e.g. Iridium) SATCOM antennas cannot be guaranteed due to lack of guard bands between
frequency allocations. If dissimilar SATCOM systems are installed, the separation distance
should be as far as possible.
Simultaneous operation of dissimilar SATCOM systems may be possible most of the time,
but successful communications will depend on frequency allocations made by both satellite
networks as well the antenna's beam direction. A simple once-off test is therefore not
sufficient to guarantee satisfactory operation under all circumstances. A thorough
theoretical analysis is recommended. In particular, Iridium receivers cannot be guaranteed
to operate reliably while an Inmarsat system is transmitting. This is true even with large
separation between the antennas, as the aircraft may not be large enough to achieve a
sufficient isolation between the antennas to guarantee operation.