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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