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estimate of that BER. Major factors that determine the channel
rate are the antenna gain of the satellite and GES, and the RP
power output of the satellite and AES. Level 1 requires one
transmit channel (for R pr T channel) and one receive channel 2, BROADBANDRF
(for P channel). CHARACTERISTICS
1.2.3 The Level 2 capability provides for-channel rates of
0.6 and 10,5 kbitsls for the P channel; and 0.6 and 10.5 kbitsl
s for the R channel and the T channel. The major factor
determining when to use these channel rates is the AES
antenna gain, which must be at least 12 cU3ic when working
with a "global" beam satellite. This capability provides greater
data communications throughput, faster message delivery and
may decrease operational cost. Level 2 requires one transmit
chamel and one receive chmel as in Level 1.
1.2.4 The Level 3 capability adds to Level 2 packet data
communications a ciicuit capability for voice or communications
using a transmit and a receive C channel. These C
channels are capable of operating at channel rates of 10.5 or
21.0 kbits/s to support 9.6 kbits/s vocoders for voice communications.
The C channel can also pmvide circuit-mode data
communications but this is not considered part of the safety
services. The C channel rate used depends on the use of 1/2 rate
forward error correction (FEC) coding (112 rate coding requires
21.0 kbitds). Provision has been made to allow use of lower
vacoder rates that could operate at channel rates of 5.25 or 6.0
kbits/s. Level 3 requires onre transmit channel (R, T or C channel)
and two receive channels (P channel and C channel).
Simultaneous operation of two-way packet and circuit
communicstions is not possible, but raear-simultaneous operation
is possible by switching between transmit channel types.
1.2.5 Level 4 capability adds to Level 3 capability
additional transmit channel capability to provide simultaneous
operation of two-way packet and circuit communications.
Level 4 capability requires two or more transmit channels (R
or T, and C channel) and two or more receive channels (P
channel and C channel), power ,control for each channel carrier
and a linear power amplifier. Channel rates are the same as
Level 3. Both R and T channel capabilities are required on a
transmit channel but not simultaneously.
1.2.6 The AMS(R)S - is the first aeronautical safety
communications service which integmtes both voice and data, -
.
as well as non-safety services. This integration of services
must respect message priority. Preemption of one data
message by another data message of higher priority is handled
easily within the system without loss of information. Preemption
of a voice call by a higher priority data call, or vice
versa, will not normally be necessary in an AES which has two
transmitters. However, in a Level 3 AES which has only one
transmitter such a pre-emption may occasionally be necessary.
2,l. 1 Message cateprics. The tr;msmission sequence at
any aircraft earth station (AES) or ground earth station (GES)
will be ordered in accordance with a given priority scheme. At
the subnetwork interface to the AMSS, the priority scheme for
packet data is as described in Annex 10, Volume m, Part I,
Chapter 4, Tabla 4-26. Wrthin the M S S , this external priority
scheme is augmented with internal priorities assigned to
various signalling and voice-related functions, At the link layer
this augmented priority scheme is referred to as the
Q-precedence number and the resulting internal priority
scheme is given in Table A-3 of chis guidance material. This
"Q-precedence" number list confonns to Annex 10 priorities,
which in turn are derived from Article 51 of the lTU Radio
Regulations. The single Q-precedence List includes both voice
and data traffic, and also includes the signalling necessary to
integrate - voice and data The Qpmedence numbers
associated with the signalling were chosen to optimize the
over-all system perf-ce and integrity.
2.1.2 Receive frequency band. For historical reasons, most
AESs may be capable of receiving more than the required
band of 1 544 to 1 555 MHz but not the full band suggested by
the recommendations. Typically, they cover the frequency
band 1530 to 1559 NMz, and may not cover 1525 to
1 530 MHz.
22 m n c y accuracy and
compensation.
2.2.1 Fnquency accuracy. The Standard contained in
Annex 10, Mlume m, Part I, Chapter 4, 4.2.2 reflects a
requirement on the signal received by the GES. There are
several contributors to the frequency error observed at the
GES. These include frequency errors due to the satellite
　
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