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DO-253D Change 1
levels specified in the ICAO Annex 10 as indicated in Table K-1 in Appendix K of this
MOPS for either horizontal or vertical polarization reception. Furthermore, both classes
of receiver avoid burnout at the maximum field strengths indicated in Table K-2 in
Appendix K of this MOPS for horizontal or vertical polarization reception. In addition to
meeting sensitivity, signal saturation, and burnout level, receivers in both receiver classes
must also meet all the other requirements such as adjacent channel rejection and FM
immunity performance. These and other performance requirements associated with Class
B receivers are adjusted from the Class A receiver requirements in accordance with the
manufacturer specified Class B parameter values of X, Y, and Z (see Section 18.104.22.168.2).
Class A Receiver Equipment Class
Class A receivers must meet the requirements as specified in Section 2.2 of this MOPS,
except for those requirements specifically identified as only applying to Class B receivers.
Note that a Class A receiver has performance requirements that are identical to a Class B
receiver when the Class B parameters of X, Y, and Z (as described in Section 22.214.171.124.2
As is described in Appendix K, Class A receiver requirements have been specified for
installation on aircraft with an Aircraft Implementation Loss for horizontal polarization in
the range of 2 to 15 dB producing a range of desired VDB signal power at Receiver Input
(dBm) for horizontal polarization of -87 dBm to -1 dBm. The range of AIL accommodated
by Class A receivers for vertical polarization is 2 dB to 11 dB producing a range of signal
power for vertical polarization of -87 dBm to -5 dBm.
Class A receivers accommodate an antenna gain variation in the horizontal plane of up to
7 dB for the installed horizontal or vertical antenna polarization. Note that some aircraft
installations may no longer be compatible with Class A receivers if the maximum antenna
gain variation in the horizontal plane exceeds the 7 dB limit.
Class A was defined to allow equipment designed to previous versions of this standard to
remain compliant to this standard and be easily identified as such. The increase in the
minimum AIL from –6 dB (i.e., a -6 dB loss is equivalent to a +6 dB gain) as indicated in
previous versions of RTCA/DO-253 to the new minimum AIL of 2 dB as specified in this
revision of RTCA/DO-253 (Rev. D) is meant to reflect actual aircraft antenna performance.
Note: In versions of RTCA/DO-253 through revision C, the terminology of Aircraft
Implementation Factor (AIF) was used rather than Aircraft Implementation Loss.
These terms differ only in sign. AIF is in terms of gain and AIL is in terms of
attenuation loss (e.g., AIF of -2 dB = AIL of +2 dB).
Class B Receiver Equipment Class
Class B receivers must meet the requirements as specified in Section 2.2 of this MOPS,
except for those requirements specifically identified as only applying to Class A receivers.
Class B receiver requirements have been specified for installation on aircraft with a
manufacturer specified range of Aircraft Implementation Loss and a manufacturer
specified maximum antenna gain variation in the horizontal plane that does not exceed 15
Class B receivers are intended to accommodate installation on aircraft with a range of AIL
and/or maximum antenna gain variation in the horizontal plane incompatible with Class A
receivers. The range of Aircraft Implementation Loss supported by Class B receivers is
specified by the manufacturer indirectly by two parameters (X and Y) that define the
minimum and maximum receiver input power levels. The maximum antenna gain variation
in the horizontal plane is specified by the manufacturer by one parameter (Z).
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