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functions is designate ,d,,R while an R channel being
used for other functions is designated &. The functional
designations R,,, and Rd do not necessarily apply to separate
physical channels.
ReMle link service (RLS). A data communications service
provided by the subnetwork which automatically provides 4.1.1.2.3 T channel. Reservation time division multiple
for emor control over its link through emor detection and access (TDMA) channel, used in the from-aircraft direction
requested retransmissiorl of signal units found to be in error. only. The receiving GES reserves time slots for transmissions
requested by aircraft earth stations (AESs) according to
message length. The sending AES transmits the message in the
Satellite region A geographically defined sub-area within the reserved time slots according to
view of a satellite within which services can be provided by
that satellite.
4.1.1.2.4 C channel. Circuit-mode single channel per
carrier (SCPC) channel, used in both to-aircraft and from-
Satellite service area A geographically defined sub-area aircraft directions. hi^ is time division multiplexed to
within the view of a satellite within which services are provide a for voice or data traffic and a subprovided
by that satellite. Note that a satellite service area band channel for signalling, supervision and data messages.
might be sub-divided in terms of operational characteristics. The use of the channelis conmlled by assignnieanntd release
conditions, or limitations for a variety of reasons. signalling at the start and end of each transaction.
Signal unit (SU). A time-ordered, contiguous set of data octets
used for signalling and control, and for user packet data
transmissions. Standard-length SUs are 96 bits (12 octets)
used in P, T and C channels. R channel SUs are 152 bits
(19 octets), and the T channel uses a header SU of 48 bits
(6 octets).
Spot beam. Satellite antenna directivity whose main lobe
encompasses significantly less than the earth's surface that
is within line-of-sight view of the satellite. May be
designed so as to improve system resource efficiency with
respect to geographical distribution of user earth stations.
Subsequent signal unit (SSU). In a series of SUs, the signal
unit(s) following the initial signal unit.
4.1.2 General
4.1.2.1 When aeronautical mobile-satellite service
(AMSS), using near-geostationary orbiting satellites, is
installed and maintained in operation as an aid to air traffic
services, it shall conform with the provisions of 4.1 to 4.10.
4.1.2.2 Requirements for mandatory carriage of AMSS
equipment including the level of system capability shall .be
made on the basis of regional air navigation agreements which
specify the airspace of operation and the implementation timescales
for the carriage of equipment.
Superframe. A recumng, time-structured set of data trans- 4.1.2.3 The agreements indicated in 4.1.2.2 shall provide
at least two years' notice of mandatory carriage of airborne
mission frames, which also includes a superframe marker
(see also the definition of "frame"). systems.
4.1.2.4 Recommendation.- Civil aviation authorities
should co-ordinate, with national authorities and service
4.1.1.2 DESCRIPTIOOFN providers, those implementation aspects of AMSS which will
CHANNEL TYPES permit its world-wide interoperability and optimum use, as
appropriate.
4.1.1.2.1 P channel. Packet mode time division
multiplex~M)c hannel transmitted continuously from the Note.- Pmvisions on the ~Zlocutiona nd assignment of 24-
aeronautical ground earth station (GES) in the to-aircraft bit aircrafc addresses ,for use by the M S S are contained in
direction to carry signalling and user data. A P channel being Chapter 9.
4/11/99
No. 74
part r Annex 10 - Aeronautical Telecommunications
4.1.3 System capabilities
Note.- A system providing aeronautical mobile-satellite
service (AMSS) comprises the AES,, the satellite and the GES.
A Level 1 (2, 3 or 4) system consists of an AES with Level I
(2, 3 or 4) capability with one or more satellites and one or
more GESs having the capabiliries to operate compatibly with
all capabilities of the AES. Multiple service providers of such
systems may coexist. A basic level of interoperability among
different systems is provided.
4.1.3.1 Scope. A level of system capability shall include
the performance of the AES, the satellite and the GES. All
AESs, as a minimum, shall have a Level 1 capability and shall
continuously monitor the P channel after log-on to the GES.
Each GES shall provide at all times when operating AMS(R)S,
a Level 1 capability as a minimum.
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