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(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 S 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 dday
This delay requirement is a high-level specification composed
of a number of components due to various sub-systems
including satellite acquisition, frequency 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 RATES
45 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 coordinated 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 baircraft direction (P
channels) and horn-aircraft directions (R and T channels) are
different. This is due to the uncorrected Doppler shift on
to-aircraft channels, afld 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 (Am)
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 synchronizatwrJIoss/degradation.
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 degradedllost P channel
indicates reduced operational performance and is usually an
indication that a switching operation, for example, a satellite
or spot beam handover, should be performed.
Annex 10 - Aeronautical Telecommunictztions Volume I11
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. The first condition
corresponds to a "degraded" P channel and has two cases:
I) if the bit error rate rises above loa 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
seconds. There is a single indicator for either of these conditions
and, consequently, the action by the AES management
for either event is the same.
4.2 P channel
4.2.1 General. At least one P channel is transmitted
continuously by each GES that forms part of the AMSS
service. Each AES must continuously monitor the P channel
transmitted by the GES to which it is logged on. The P
channel implements a time division multiplexing strategy to
send small packets of information to the AESs that are
　
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