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DO-253D Change 1
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GPS Antenna and Installation Requirements
Verify that the antenna is compatible with the particular type of GPS/LAAS equipment
installed (e.g., that the antenna incorporates or does not incorporate a low-noise amplifier
as expected by the GPS/LAAS equipment). Verify that adequate isolation is provided
between the GPS/LAAS antenna and any transmitting antenna(s) installed on the aircraft
(or between multiple GPS antennas). Ensure that antenna installation minimizes the effects
of blockage due to the wings, tail, etc. during aircraft maneuvering. Verify that the GPS
antenna is not installed just aft of the top of the windscreen (i.e., avoid case-to-antenna
coupling) or in a location which may result in ice accumulation on the antenna.
Antenna installation in close proximity to traffic alert and collision avoidance system
(TCAS), satellite communication (SATCOM), and other transmitting antennas
(particularly "L" band) should be carefully evaluated for potential mutual interference.
VDB Aircraft Implementation Loss (AIL)
The total implementation loss is the algebraic sum (in decibels) of the negative value of
the antenna gain (referenced to an isotropic radiator and measured into a 50 Ohm load) and
the attenuation (as a positive number) between the antenna and the VDB receiver. The
attenuation includes line dissipation, any external power splitter losses if an antenna output
port feeds more than one receiver, and impedance mismatch losses. The variation of the
impedance mismatch losses with frequency must be limited to 6 dB. This places additional
constraints on the dissipation of the connection or the VSWR. The implementation loss
should be verified over the frequency range 108.000 to 117.975 MHz for signals received
from any direction in the horizontal plane.
1. The aircraft implementation loss is based on the VDB link budget in Appendix K.
2. The impedance mismatch losses variation limitation supports the conversion of the
D/U requirements from receiver input to isotropic antenna output as described in
3. The antenna gain, which is a component contributing to the AIL, may be variable
depending on the arrival angle of the VDB signal relative to the aircraft’s VDB
receiving antenna. Therefore, the AIL must be evaluated in a manner that accounts
for variations in antenna gain in the horizontal plane. Refer to Section 18.104.22.168 for
assumptions on the effect of variations in antenna gain versus elevation for the
desired VDB signal.
4. Refer to Appendix K.6 for a complete description of the installed performance
related to the aircraft implementation loss that addresses both desired and
undesired signals arriving from any direction.
Class A Receivers
A VDB Class A receiver shall [LAAS-262] only be installed on aircraft that have aircraft
implementation loss greater than AILmin = 2 dB and less than AILma x = 15 dB with a
maximum of 6 dB impedance mismatch loss variation when using a horizontally polarized
antenna that has a maximum antenna gain variation in the horizontal plane of 7 dB, —or—
have aircraft implementation loss greater than AILmin = 2 dB and less than AILmax = 11 dB
with a maximum of 6 dB impedance mismatch loss variation when using a vertically
polarized antenna that has a maximum antenna gain variation in the horizontal plane of 7
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