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DO-253D Change 1
Class B Receivers
A VDB Class B receiver shall [LAAS-429] only be installed on aircraft that have aircraft
implementation loss greater than AILmin = 2 - Y dB and less than AILmax = 15 + X 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 Z
dB, —or— have aircraft implementation loss greater than AILmin = 2 -Y dB and less than
AILmax = 11 + X 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 Z dB.
Note: The parameters of X, Y, and Z are specified by the manufacturer of the Class B
receiver. These parameters are as defined in Section 2.2 .1.2.2 of this MOPS.
GNSS Electromagnetic Compatibility
Verify electromagnetic compatibility between the GPS/LAAS equipment and other aircraft
equipment. Particular attention should be given to other "L" band equipment, such as
TCAS or SATCOM equipment; VHF transmissions on the frequencies listed below; high
frequency (HF) communications systems; and other transmitting equipment (e.g., ACARS,
AFIS, Flightfone, etc.).
Verification of adequate isolation from the interference of VHF communication
transceivers is required. These tests shall [LAAS-263] be conducted on the completed
GPS/LAAS installation by tuning each VHF transmitter to the frequencies listed below and
transmitting for a period of 20 seconds. Degradation of individually received satellite
signals below a point where navigation is no longer possible is not acceptable and will
require that additional isolation measures (e.g., low pass or notch filters installed at the
output of the VHF transmitter) be included in the aircraft installation. The following VHF
frequencies shall [LAAS-264] be evaluated:
121.150 MHz 131.250 MHz
121.175 MHz 131.275 MHz
121.200 MHz 131.300 MHz
VDB Electromagnetic Compatibility
Electromagnetic compatibility between VDB equipment and other equipment shall [LAAS-
265] be verified by a combination of compliance with design requirements, bench testing
and installed performance testing.
The VDB antenna should operate such that the maximum received power from any on-
board transmitter does not exceed the desensitization levels of the VDB receiver specified
in Section 22.214.171.124.
Note: Due to proximity in frequency, special care must be taken to reject transmission
from any on-board VHF transmitter communication systems operating in the band
118.000 to 136.975 MHz. For horizontally polarized VDB antenna operating in
the presence of a +40 dBm (EIRP) vertically polarized VHF communications
transmission, at least 53 dB of isolation is required to protect the VDB receiver
from desensitization. This isolation maybe achieved through a combination of 15
dB of cross-polarization rejection and 38 dB of antenna coupling. For vertically
polarized VDB antenna operating in the presence of other vertically polarized
transmissions, the full 53 dB of isolation must be achieved by the antenna coupling.
The GPS/LAAS avionics equipment interfaces should be designed such that when properly
installed with other adequately designed equipment:
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