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DO-253D Change 1
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The values of X, Y, and Z are specified in dB and must be consistent with meeting all the
requirements specified in Section 2.2 of this MOPS for Class B receivers. The values of
X and Y can be independently either positive, negative, or zero while the value of Z is
The range of the AIL accommodated by Class B receivers for horizontal polarization is (2
Y) dB ≤ AIL ≤ (15 + X) dB, such that the range of desired VDB signal power at Receiver
Input (dBm) for horizontal polarization is (-87 – X) dBm to (-1 + Y) dBm. The range of
AIL accommodated by Class B receivers for vertical polarization is (2 – Y) dB ≤ AIL ≤
(11 + X) dB, such that the range of signal power for vertical polarization is (-87 – X) dBm
A positive value of X increases the AILmax and decreases the minimum desired signal level
that must be received (increases receiver sensitivity). A positive value of Y decreases the
AILmin and increases the maximum desired signal level that must be received and the
burnout level required.
The manufacturer of Class B equipment shall [LAAS-407], for each Class B receiver
model, declare values X, Y, and Z (in dB).
The declared maximum antenna gain variation in the horizontal plane Z shall [LAAS-408]
be less than or equal to 15 dB (Zmax).
The Zma x constraint is required to determine the VDB transmitter siting criteria which are
established in terms of field strength; the conversion of VDB equipment requirements from
receiver input power level to signal field strength is described in Appendix K.5.
Transmitter siting guidance is outside the scope of the MOPS and is provided in ICAO
The antenna gain in the horizontal plane and its variation cannot completely consume the
Aircraft Implementation Loss (AIL) specified in Section 3.10. More specifically, the
maximum antenna gain variation in the horizontal plane plus antenna/cabling/receiver
impedance mismatch loss variation of each installation must be below the difference
between the maximum and minimum AIL allowed for the receiver class by a sufficient
amount to support the total matched interconnection losses.
The VDB receiver subsystem shall [LAAS-035] be capable of tuning frequencies in the
range of 108.000 MHz to 117.975 MHz in increments of 25 kHz.
Note: In accordance with ICAO Annex 10, operational frequency assignments will be
assigned on 25 kHz centers in the range from 108.025 MHz to 117.950 MHz
The VDB receiver subsystem shall [LAAS-036] accept input of either a VDB frequency or
a LAAS channel number. If it accepts input of the LAAS channel number (N), it shall
[LAAS-037] convert it to a VDB frequency (F) using the equation given in Section 2.3.5.
Selection of a LAAS channel number shall [LAAS-038] not result in the VDB receiver
subsystem tuning to frequencies outside of the range 108.000 to 117.975 MHz.
The VDB receiver subsystem shall [LAAS-039] output the LAAS messages from the new
frequency within 3 seconds of receipt of the command to change to that frequency.
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