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channels.
3.3.2 The relative motion of the aircraft and the satellite
means that any signal reflections from aircraft wings or tail, or
the sea or grwnd below can result in time-varying multipath.
This is in part due to the rather broad beamwidth of the AES
antenna. The chamcteristics 'of this multipath depend on a
number of characteristics including the aircraft velocity, the
look-angle of the satellite with respect to the aircraft and the
slope of the reflecting surface. The rate of these variations
(Doppler bandwidth) increases with aircraft velocity and
elevation angle to the satellite. However, the multipath
intensity is inversely proportional to the aircraft velocity and
the satellite elevation angle. Consequently, it is primarily
below elevation angles of 20 degrees that multipath is a
significant problem. At elevation angles of 5 to 20 degrees the
Doppler bandwidth can vary from 20 to 100 Hz or more and
the multipath power can be as much as -7 dB relative to the
direct path signal.
3.4 Acquisition delay
This delay requirement is a high-level specification composed
of a number of components due to various sub-systems
including satellite acquisition, fcequency and bit acquisition,
and frame synchronization, The total delay of 16 seconds is
the worst-case delay allowed. This is the time from when one
first commands the AES to find the satellite until the time at
which the AES can attempt to log-on. Once logged on, typical
times for setting up a voice call or transmitting a packet data
message will be much less than this.
4. CHANNEL FORMAT TYPES
AND DATA RATE3
41 General
4.1.1 System timing. All timing of the different
transmission channels is derived from the P channel. If
required, synchronization of the P channels of each ground
earth station (GES) to universal co-ordinated time (UTC) is
one way of ensuring a world-wide timing reference for all
aircraft using aeronautical mobile-satellite service (AMSS).
The synchronization of all P channels to UTC is currently not
required because there has not been an application identified
which would benefit significantly enough to warrant the
increased cost.
4.1.2 Channel spacings. The channel spacings in
Table A-9 of this guidance material make adequate provision
for separation to reduce adjacent channel interference and to
ensure correct channel tuning in the presence of Doppler shifts
due to all causes. In the case of the channels at the lowest bit
rate, the possible spacings for the to-aircraft direction
(P channels) and from-aircraft directions (R and T channels)
are different. This is due to the uncorrected Doppler shift on
to-aircraft channels, and the use of automatic frequency
control (AFC) to minimize Doppler shift in the from-aircraft
direction. Note that the requirement on the aircraft earth
station (AES) to be. capable of tuning in steps of 2.5 kHz
accommodates all the potential channel spacings listed below
and allows the interleaving of channels between adjacent
satellite spot beams.
4.1.3 P channel synchmnizution/lusddegradution
Actions by the AES management depend upon indications of
the signal quality of the received P channel. These include an
indicator of when the AES is synchronized to the P channel,
and when it is degraded andlor lost. The AES must
synchronize to a P channel before it can receive P channel
signal units, or transmit over the R or T channel. A degradedl
10s: P channel indicates reduced operational performance and
is usually an indication hat a switching operation, for
example, a satellite or spot beam handover, should be
performed.
4.1.3.1 These indicators of signal quality generally are
based on physical measurements exceeding a threshold,
However, the measurements used and the threshold settings
depend upon the AES implementation. P channel
synchronization is declared whenever the P channel unique
word is detected reliably. Synchronization is lost whenever the
unique word is not detected reliably.
4.1.3.2 Either of two conditions will cause a declaration
of degradatiodloss of the P channel. 'he first condition
corresponds to a "degraded" P channel and has two cases: 1)
if the bit emir rate rises above 1 o4 in a three-minute averaging
period; and 2) if synchronization is lost ten or more times
during a three-minute period. This three-minute averaging
period provides confidence that the degradation is not
temporary, due, for example, to an aircraft manoeuvre. The
second condition corresponds to a "lost" P channel, which is
declared if synchronization is lost continuously for ten
　
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