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motion relative to the satellite. Consequently, the proper frame
of reference for measuring the transmit frequency is the satellite.
A practical test of this requirement would use the AES
frame of reference, and the corresponding value in the satellite
Attachment A to Part 1 Annex 10 - Aeronautical Telecommunicutions
frame of reference would be calculated based on the aircraft
position and velocity, and the satellite position.
2.2.2 Frequency compensation by the GES. To reduce
the error due to the spacecraft oscillators, the GES should
listen to an L-band pilot frequency transmitted (at C-band) by
a designated GES and correct its transmission frequency to
minimize the frequency error at L-band. In the from-aircraft
direction a designated GES transmits an L-band pilot
frequency which a11 GESs listen to at C-band and adjust their
receiver local oscillators accordingly. This approach may not
be possible in the case of satellite spot beams where the GES
is not within the footprint of the spot beam of interest.
2.2.3 Doppler shift cornpensarion by the AES. There are
at least two methods of implementing Doppler-shift compensation.
One approach is to use aircraft navigational aids to
estimate the velocity of the aircraft in the direction of the
satellite and then, estimate the Doppler shift from this. A
second approach is to estimate the Doppler shift by measuring
the frequency offset of the received P channel or C channel.
For this latter approach, the frequency of any transmission to
that ground earth station is then the basic channel frequency
offset by the receive frequency, offset with opposite sign and
a scaling factor of approximately 1.07. This approximately
corrects the component of the frequency error due to aircraft
motion Ooppler shift) but does not correct errors in the AES
local oscillator. -.
2.2.4 Frequency ermr budget. The frequency emr
budgets used in arriving at the accuracy requirements for the
GEB-AES link are presented in Tables A-4 and A-5 of this
guidance material. Note that in Table A-4 the Doppler shift
due to aircraft motion is not included, and in Table A-5 it is
assumed to be compensated for.
23 Aircraf't earth station
antenna characteristics
2.3.1 Antems a d level of capability. The Standards
contained in Annex 10, Volume III, Chapter 4 specify high
and low-gain antenna systems. but one should note that these
are not linked directly to a level of capability of AES. A highgain
antenna, with the supwing avionics, will mean a Level
2, 3 or 4 AES installation; and a low-gain antenna, with the
supporting avionics will always lead to at least a Level 1 AES
installation. In the future, different system characteristics may
be capable of providing a Level 2 or higher service. For
example, the mmbination of satellite spot beam antennas and
a medium gain aircraft antenna may be capable of providing
a Level 2 or higher service. The level of service provided to
an aircraft will depend not only on its capabilities but those of
the service providers as well.
2.3.2 Mid-gain antennas. AES antennas may be desirable
for AMSS use that do not conform strictly with "0 dB"
or "12 dB" standards as i n f e d by the AMSS SARPs EIRP
and GK parameters. Where aircraft are limited to mounting a
small antenna even for voice services, the SARPs would not
preclude use of a medium-gain AES antenna.
2.3.2.1 Considering available AEB HPA power, a
medium-gain antenna could be used while maintaining necessary
channel quality; e.g. an 8-11 dB gain would have ample
EIRP to operate at 10.5 kbits/s, and an antenna with a 5-8 dB
gain could operate at 4.8 kbitsls - given that satellite P
channels are operated with sufficient power.
2.3.2.2 Higher-gain future satellites could serve AESs
with lower GIT and EIRF', but may have an effect of potentially
higher service costs and reduced system capacity. AES
antennas with gain less than 12 dBic may be considered
similar operationally to low-gain antennas because they have
too broad a beam to discriminate against other satellite
interference. The breadth of their use will depend on user
requirements for medium-gain antenna installations and
acceptance of resulting limitations.
2.4 Receiver requirements
2.4.1 Gain-ro-noise tempemtuw ratio. The following
factors influence the aircraft eanh station receive system gainto-
noise temperature ratio (GtT):
a) climatic conditions;
b) antenna elevation angles to the satellite;
C) residual antenna pointing errors (including the effects
of errors introduced by the antenna beam steering
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