R and frequency information needed by AES's wishing to log on to the
R system. Because of its importance for system integrity, the
R Psmc-Channel has the most robust transmission link design of all
R the channels.
R Pd-Channels are used for signalling and to carry ground-to-air data
R messages. The system has been designed to support a number of
R Pd-Channels in anticipation of continuous traffic growth.
R Both types of channels, Psmc and Pd, may be combined on a single
R physical P-Channel.
R
R - R-Channel (Random access channel):
R The R-Channel is used for air-to-ground signalling and data
R communications. As in the case of the P-Channel, there are two
R types of R-Channel: Rsmc and Rd.
R The Rsmc-Channel is used for system signalling functions such as
R log-on and call requests. The Rd-Channel is used for short duration
R data transmissions. Again, the system has been designed to support
R a number of Rd-Channels.
R
R - C-Channel (Circuit-mode channel):
R The C-Channel is used to provide full-duplex voice or data
R communications. Because any given channel can pass traffic in only
R one direction, C-Channels are assigned in pairs, one for the uplink
R (ground-to-air) and one for the downlink (air-to-ground). The
R C-Channel is capable of passing digitized voice at 9.6 kilobits per
R second (or less) or data at 10.5 kilobits per second (or less).
R C-Channels also contain a sub-band data channel which is
R time-multiplexed with the voice. This sub-band channel operates at
R approximately 200 bits per second and carries system control and
R signalling information as well as user data.
R
R - T-Channel (Time division multiple-access channel):
R The T-Channel is the air-to-ground data communication channel used
R for the transmission of longer messages (unlike the R-Channel which
R is used for short messages). The GES selects a T-Channel and,
R working on the first come, first served basis, reserves time slots
R on it as soon as an AES makes a call attempt. The AES then
R transmits in its reserved time slots.
R (2) System operation
R
R (a) Log-on procedure
R
R Log-on is initiated by the SDU after selection of the log on
R command on the MCDU SATCOM Menu (Ref. para. MCDU-SDU interface).
R The unit is programmed with two data tables. One called the owner
R requirement table (ORT), contains information on all GES's in
R each satellite region and the order of preference in which they
R are to be selected. The second table, called the system table,
R contains system management and control information including all
R GES Psmc and Rsmc Channel frequencies, GES identifications and
R satellite positions. Each system table also has a version number
R assigned to it.
R The SDU is also programmed with a technical address which is the
R telephone number of the AES.
R When Log-on begins, the SDU receives position and orientation
R information from the aircraft inertial navigation system and
R carries out an initial search:
R - it tunes to each Psmc Channel according to the preference
R specified in the ORT,
R - the BSU translates azimuth and elevation commands received from
R the SDU in ARINC 429 digital format into signals needed to
R select antenna elements.
R The combined signals result in the steerable beam pointing in
R the direction maximizing the signal,
R - it checks the quality of the signal by means of the received
R strength and/or bit error rate,
R - it rates the frequency if it is the best quality signal of the
R frequencies so far checked.
R At the end of this procedure, the AES returns to the Psmc
R frequency giving the best quality signal.
R The Psmc-Channel carries an electronic bulletin board which also
R contains system table information. The AES compares the version
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