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delays, and in digital systems intersymbol interference can
result,
A number of investigators have researched the effects of multipath
fading on aeronautical satellite communications. The
statistical nature of rnultipath fading for aeronautical channels
is therefore well understood. The amplitude of fading is known
to have a Rician distribution. Furthermore, the carrier-tomultipath
ratio is known to be a function of the elevation
angle to the satellite, and can be expected to be less than
10 dB for elevation angles below 10 degrees.
The carrier-to-noise ratio for a channel is affected by multipath
and the particular farm of modulation and coding. It is appropriate
to include the effects of multipath in setting the carrierto-
noise requirement rather than including it in a separate
margin; a conservative approach is to treat the multipath
energy as equivalent to additive Gaussian noise, and then, in
a coded system, to add additional margin for imperfect
interleaving.
Ionospheric scintillation is a phenomenon involving the effects
of the sun and the earth's magnetic field that produces random
variations in electromagnetic waves traversing the ionosphere.
The phenomenon is manifested in satellite-earth station RF
links as "scintillation fading"; positive and negative (loss)
changes in the amplitude of the received signal that can be
significant at the Lband frequencies used for the satellite-to-
AES link, Values as high as 27 dB have been observed for
short periods of time during severe scintillation events;
however, the expected value is substantially lower. Phase
shifting is also associated with scintillation fading, the effects
of which can further degrade RF link performance.
As satellite RF link power margins are normafly small for
economic reasons, a loss value due to scintillation fading as
low as 0.3 dB could be significant. Scintillation loss is highly
correlated with the position and local time of the aircraft, thus
is of major concern to certain routes and times of flight.
Scintillation events also exhibit a seasonal influence, peaking
during the vernal and autumnal equinoxes. Significant scintillation
loss can be expected for aircraft located near the
geomagnetic equator (between 15 degrees latitude North and
South) at aircraft local time between 2130 and 0230 hours, and
for aircraft located in polar regions (latitudes greater than 265
degrees although coverage by geosynchronous satellites is
effectively limited of latitudes to 80 degrees or less) at any
time of day. Available data indicate that scintillation fading is
about twice as intense in the equatorial region as compared
with the polar regions. Fm a stationary earth station, about
1 per cent of equatorial region fades exceed 20 dB, and stay
above 15 dB for several seconds. Eastward motions of the
Attachment A to Part I Annex 10 - AeroMutiwl TeCecommunications
ionosphere at rates of SO to 200 metresfsecond are typically
seen, implying correlation distances of 10 to 100 metres. It
would be possible for an eastbound aircraft's velocity to
become "synchronized", resulting in substantially longer fading
periods.
Fading in the polar regions is less intense, (about 10 dB for a
stationary earth station) as compared with the equatorial
region. Also, the velocity of the polar ionosphere is typically
higher and more variable, in the range of 100 to 1 000
metresfsecond.
Data regarding the scintillation effects on earth stations in
motion - in particular, on the signal-in-space used by AMSS
- is currently limited. Futther, the probability of an aircraft
experiencing significant effects of scintillation is highly
sensitive to its rwte and timing of its flight. Consequently, the
effects of scintillation fading have not been accounted for
herein,
The transmission loss between two antennas due to imperfect
circular polarization can be calculated by:
LPOL = 10 LOG
(R:+I) (R: +1) 1 [A.51
(R,R,+I)~c os2 (0) + (R,+R~)' sin2 (0)
where it is assumed that the antennas have the same sense
(e.g. righthand circular) and where:
R, = the voltage axial ratio (AR) of the ith antenna.
8 = the angle between the major axes of the two
elliptically polarized waves that would be radiated,
one from each antenna.
The polarization loss is determined completely by the axial
ratios and relative orientation of their major axes. The worst
case situation is when the major axes are orthogonal, i.e.
9 = 90". Various forrns of equation [AS] are possible, depending
upon the assumptions about the reference antenna. For link
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