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budget calculations, one typically might consider the worst
case satellite antenna orientation, and a statistical estimate of
the effects of AES antenna orientation,
A1.3.4 PATH LOSS
The path loss due to space is a function only of the frequency
and range. The path loss is easily calculated by:
where:
r = the range from the AES to the satelk in metres.
h = the wavelength in metres.
In general, the range to the satellite, r, is a function of the
geographical position of the AES. Conveniently, the range to
the satellite is simply a function of the observed elevation to
the satellite and is given by:
r = 62 + (R+# - 2R ( ~ + hC)O R fl M.71
where:
R = the Earth's mean radius z 6 378 km
h = the geosynchronous altitude = 35 786 km (from
earth surface at the subsatellite point).
r 7
where:
8 = the elevation angle of the satellite relative
to level flight.
With the satellite directly overhead, the path loss at 1 545
MHz is 187.3 dB. At 5" elevation, the path loss is 188.5 dB.
Therefore, the path loss for ahcraft operating with a 5"
elevation angle to the satellite is 1.2 dB greater than an aircraft
with the satellite directly overhead. In practice, the path loss
is calculated for a specific elevation angle to the satellite.
Raindrops cause attenuation to radio waves by both absorption
and scatter. The magnitude of attenuation is a function of
frequency, average droplet size, aircraft latitude, elevation
angle and rainfall rate. The relationships among these factors
are well established through years of research and experimental
measurement, making it possible to predict performance
with confidence.
In general, attenuation due to rainfall is not significant at the
L-band frequencies used for the AMSS service links.
However, the feeder links for the AMSS services will be at
much higher frequencies where the rain attenuation could be
very significant. Feeder link design must take into account the
expected rainfall for the location of the ground earth station,
particularly as regards link availability.
The effect of rain attenuation on the feeder link in the forward
direction can be compensated for by GES power control. The
GES power is increased such that the signal maintains the
Annex 10 - Aeronuutiml Tehcommu&uti6ns
required level when received at the satellite. One consequence
of this increase In power can be to increase the intermodulation
products originating at the GES. The link must be
designed so that this additional interference will not degrade
the over-all achieved carrier-tenoise performance below the
required level.
In the retum direction, rain attenuation will lower the canier
power to thermal noise ratio for the feeder link. Again, this
additional interference must not degrade the over-all achieved
carrier-to-noise performance below the required level.
There is no spec if^ allotment in the required rllargin to
account for the effects of rain fading. It is the responsibility of
the satellite system designer to ensure that the satellite and
GES design is such that the over-all link carrier-to-noise ratio
caa be maintained under the expected raid co~lditionsid the
stated coverage areas.
- END -
ICAO TECHNICAL PUBLICATIONS
The following summary gives the status, and also
describes in general terns the contents of the various
series of technical publications issued by the International
Civil Aviation Organization. It does not include
specialized publications that do not fall specifically
within one of the series, such as the Aeronautical
Chart Catalogue or the Meteorological Tables for
International Air Navigation.
International Standards and Recommended Practices
are adopted by the Council in accordance with
Articles 54, 37 and 90 of the Convention on International
Civil Aviation and are designated, for
convenience, as Annexes m the Convention. The
uniform application by Contracting States of the specifications
contained in the International Standards is
recognized as necessary for the safety or regularity of
international air navigation while the uniform application
of the specifications in the Recommended
Practices is regarded as desirable in the interest of
safety, regularity or efficiency of international air
navigation. Knowledge of any differences between the
national regulations or practices of a State and those
established by an International Standard is essential to
the safety or regularity of international air navigation. In
the event of non-compliance with an International
Standard, a State has, in fact, an obligation, under
　
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