Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
DO-253D Change 1
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to the output of an isotropic receiving antenna while the term receiver power refers to the
power at the receiver input. The computation of isotropic antenna output power from the
field strength is a standard formula that is frequency dependent.
This appendix converts the D/U from receiver input (D/Urx) to the isotropic antenna output
(D/Uant); therefore there is no frequency dependency in the conversion. Since the frequency
dependency is small, the antenna output D/U is assumed to be a good enough
approximation of the signal-in-space D/U and both terms are employed interchangeably
D/USIS ≈ D/Uant.
For reference, the relationship between field strength E in V/m and power P in watts at the
output of an isotropic antenna is given by E = (4πf/c)√(30P) where f is the frequency in Hz
and c is the speed of light in m/s. This frequency dependence results in a 0.8 dB increase
in power at 108 MHz with respect to 118 MHz for a constant field strength. In the link
budget tables, this effect is included in the free space loss term that related the transmit
EIRP to the received power at the output of the isotropic antenna; the free space loss is 0.8
dB higher at 118 MHz with respect to 108 MHz. The VDB receiver minimum and
maximum desired signal requirements account for this frequency dependency.
All affected receiver requirements are adjusted by the X and Y parameters of Class B
receivers in such a way that the conversion to the power from an isotropic receiving antenna
produces the same result as for Class A receivers. However this standard specifies a Zma x
of 15 dB for Class B receivers instead of the 7 dB fixed Z value for Class A; this affects
the conversion of requirements that are sensitive to the relative direction of two signals.
The additional antenna gain variation in the horizontal plane allowed for Class B receivers
is meant to accommodate existing localizer antenna installations that did not need to be
omnidirectional to receive the ILS signal whereas the VDB signal can come from any
The description of the requirement conversion will use the Zma x of Class B values since this
is the constraining case to ensure compatibility of the RF environment with all VDB
receivers. Class A results are easily derived by setting the Z parameter to 7 dB. The values
of the X and Y parameters do not affect the conversion since the receiver requirements
were written expressly to make the conversion independent of these two parameters. The
conversion also depends on signal polarization; the results for both horizontal and vertical
polarization are provided.
Conversion of Receiver Requirements Specified in Absolute Power (dBm)
Signal Power at the Desired VDB Frequency
The conversion from power at the receiver input to antenna power is performed by adding
the minimum AIL for a maximum signal power requirement and adding the maximum AIL
for a minimum signal power requirement:
1) For the message failure rate requirements of Section 2.2.5, the maximum desired signal
antenna power is Smax + AILmin = (-1 + Y) dBm + (2 - Y) dB = +1 dBm for horizontal
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