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oscillator, due to relative motion htween the aircraft and
spacecraft, due to the local oscillator of the GES (for a closed
loop compensation system) and the local oscillator of the AES.
Efforts are ma& to reduce the m rca used by the first two as
described below. This Standard characterizes that portion of
the frequency error which is due to the AES and the aircraft
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
7/11/96
No. 71
At&c/unent A to Part I Annex 10 - Aernmutkd Telewrnmunic&ns
frame of mference would be calculated based on the aircraft
position and velocity, and the satellite position. p-1,,5.v- , .
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 e-mr at L-band. In the from-aircraft
direction a designated GES transmits an L-band pilot
frequency which d l 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 compensation 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 appmach 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 (Doppler shift) but does not correct errors in the AES
local oscillator.
2.2.4 Frequency ermr budget. The frequency error
budgets used in arriving at the accuracy requirements for the
GES-AES link are presented in Tables A-4 and A-5 of this
guidance material. Note that in W l e A-4 the Doppler shift
due to aircraft motion is not included, and in Table A-5 it is
assumed to be compensated for.
2.3 Aircraft earth station
antenna characteristics
2.3.1 Antennus and level of cupabiliry. The Standards
contained in Annex 10, Volume 111, Chapter 4 specify high,
intermediate and low-gain antenna systems, but one should
note that these are not linked directly to a level of capability
of AES. A high-gain antenna, with the supporting avionics,
will mean a Level 2, 3 or 4 AES installation; and a low- or
intermediate-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 combination of
satellite spot beam antennas and a low- or intermediate-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 Higher-gain future satellites could serve AESs with
lower GlT and EIRP, but may have an effect of potentially
higher service costs and reduced system capacity. AES
anmmm 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.
2.4 Receiver requirements
2.4.1 Gain-to-noise tempmnrre mtio. The following
factors influence the aircraft earth station receive system gainto-
noise temperature ratio (Gm):
a) climatic conditions;
b) anterma elevation angles to the satellite;
c) residual antenna pointing errors (including the effects of
errors introduced by the antenna beam steering system);
d) the noise contribution of the receiver low noise amplifier
at the operating temperature;
e) the transmitter power amplifier output level;
f) the attenuation and noise temperature contributions of a
radorne, where a radome is fitted; and
g) the RF environmental conditions in which the aircraft
earth station is intended to operate.
2.4.2 Qpicul link carrier to noise densities. Tables A-6,
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