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AMSS for channel rates greater than 2 400 bits/s. A-QPSK
is a modulation technique which maps a " Q into a Q
degrees and "1" into a 180 degrees, or "0 into 90 degrees
and "1" into 270 degrees, alternating between the two
options on successive bits. Thc encoded A-QPSK data
stream is then filtered such that the modulated spectrum
-meets the amplitude mask of Table A1-3 and the phase
mask defined in Table A1-2 in Appendix 1 to Chapter 4.
Burst. A time-defined, contiguous set of one or more related
signal units which may convey user information and
prococols, signalling, and any necessary preamble.
Cyclic redundancy check. The last ~ w oby tes of each signal
unit form a cyclic redundancy check of the whole signal
unil as follows. The check bits for error detection are
calculated from the first 10 octets of a standard length
signal unit, or frbm the first 17 octets of an extended length
signal unit or from the tirst 4 octets of the burst identifier,
using the following generator polynomial:
Nore,- See CCITT (Red Bonk) Reconmzendution X.2.3,
Section 2.2.7for the method of calcubntion rrnd the bit ode,:
Direct link service (DLSj. A data communications service
which makes no attempt to automatically correct errors,
detected or undetected, at the link layer of the air-ground
communications path. (Error control may be effected by
end-user systems.)
Epoch. A span of time related to the beginning and end, or
lifetime, of an event or a sequence of associated events.
Frame. A structured, repeating time-segment of a communication
link architecture that provides for time-predictable
communication activities between its beginning and end.
Global beam. Satellite antenna directivity whose main lobe
encompasses the entire earth's surface that is within line-ofsight
view of the satellite.
Initial signal unit (ISU). The first of the series of signal units
followed by SSUs.
Link interface control information (LZCZ). The control information
exchanged between the link Iayer and any of its
service users as part of the link interface data unit (LIDU).
Link interface &tu unit (LZDU). The total information rransferred
in a single interaction across the interface between
the link layer and a link service user. Each LIDU contains
link interfacc control iriformation (LICI) and may also
contain a single link service data unit (LSDU).
Link service data unit (LSDU). A part of the link interface
data unit (LIDU) and is the same as the subnetwork protocol
data unit (SNPDU).
Lone signal unit (LSU). A single signal unit comprising the
total message.
Near-geustotionary orbits. Satellites operating in neargeostationary
orbits have an orbit period of 24 hours with
an inclination of up to five degrees from the equatorial
plane.
Network co-ordination station (NCS). The entity of the overall
AMS(R)S system that has functionai responsibilities to:
co-ordinate communications traffic and satellite conncctivity
within its satellite region; and provide inter-system
co-ordination with adjacent satellite regions served by other
satellites.
P channel synchrunitution. The state of the P channel
demodulator when the P channel unique word is reliably
detected.
ANNEX I0 - VOLUME 111
Annex I0 - Aeronautical Telecommunications Volume III
P channel degradatio11/7uss. A declaration that is niade when
the P channel bit error rate rises above lo4 over an
averaging period of 3 minutes, or more than 10 short-term
interruptions (loss of P channel synchronization for less
than 10 seconds) are experienced in any 3-minute period; or
when the P channel synchronization is lost for more than 10
seconds.
Q number, Q level, Q precedence. A definition of the transmission
precedence of a message or signalling sequence,
using numbers from 0 to 15 (with 15 transmitted first).
Reliable link service (RLS). A data communications service
provided by the subnetwork which automatically provides
for error control over its link through error detection and
requested retransmission of signal units found to be in
error.
Sdellite region. A geographically defined sub-area within the
view of a satellite within which services can be provided by
that satellite.
Satellite service area. A geographically defined sub-area
within the view of a satellite within which services are
provided by that satellite. Note that a satellite service area
might be sub-divided in terms of operational characteristics,
conditions, or limitations for a variety of reasons.
Signal unit (SU). A time-ordered, contiguous set of data octets
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