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there may be simultaneous parallel connections over multiple
subnetworks. In these cases, the values for QOS provided by
each subnetwork must be set so as to ensure that the VDL
connection will be used where appropriate.
3.2.5 It is necessary that co-ordination take place
between aircraft operators, ground station operators and ground
system operators to ensure that the right balance is achieved
between different subnetworks.
4. VDL GROUND STATION
NETWORK CONCEPT
4.2 Institutional issues concerning VDL
ground station network operators
4.2.1 An ATS provider wishing to use VDL for air
traffic service (ATS) communications needs to ensure that the
VDL service is available. The ATS provider can either operate
the VDL ground station network itself or arrange for the
operation of the VDL stations (or VDL network) by a telecommunications
service provider. It seems likely that
individual States will make different arrangements for the
provision of VDL service to aircraft. Operation and implementation
of VDL need to be ct~ordinateda t a regional level
in order to ensure acceptable service on international routes.
4.2.2 The use of a VDL ground station network by entities
external to the ATS provider will be subject to service
agreements between the ATS provider and the telecommunications
service provider. These agreements set out the obligations
of the two parties and need, in particular, to be specific
on the quality of service provided as well as the characteristics
of the user interface.
4.2.3 It seems likely that some VDL ground station
network operators will levy user charges. These are expected
to be levied either on the aircraft operators andfor on the ATS
providers. It is necessary to ensure that the use of VDL is
feasible for those aircraft operators intended to use VDL for
ATSIAOC communications. , -
. .
4.3 VDL ground .
station .equipment
4.3.1 A VDL ground station will consist of a VHF radio
and a computer which may be separate or integrated with the
radio, The VDL functionality of the VHF radio equipment will
be similar to that installed in aircraft.
4.3.2 The provision of network status monitoring is an
important element in the maintenance of the highest availability
possible.
4.4 Ground station siting
- 4.4.1 The line of sight limitations of VHF propagation
is an important factor in the siting of ground stations. It is
necessary to ensure that the ground stations are installed in a
mariner which provides coverage throughout the designated
operational coverage area (DOC)..
4.1 Access 4.4.2 The coverage requirements for VDL depend on the
applications that are intended to operate over the VDL. These
4.1.1 A VDL ground station will provide access for applications may function, for example, when an aircraft is at
aircraft to the ground ATN IS using the VDL protocol over a en-route altitude, in a terminal area or on the ground at an
VHF channel. airport.
No. 72
Attachment B to Part I Annex 10 - Aeronautical Telecommunications
4.4.3 En-route coverage can be provided using a small
number of ground stations with a large DOC (for example, the
range of a VHF signal from a station at sea level and an aircraft
at 37 MX) ft is approximately 200 NM). Hence, it is in
fact desirable that the smallest number of ground stations
possible be used to provide en-route coverage in order to
minimize the possibility of simultaneous uplink transmissions
from ground stations which may cause message coilisions on
the VHF channel. The factors limiting en-route coverage will
be availability of landmass and the availability of a communications
link from a ground station to other ground
systems.
4.4.4 Terminal area coverage requires, in general, the
installation of ground stations at all airports where VDL
operation is required in order to ensure coverage throughout
the terminal area.
4.4.5 Aerodrome surface communication coverage must
be provided by a ground station at the airport but, due to the
physical structure of the airport, it may not be possible to
guarantee coverage in all areas with a single station.
45 Ground station
frequency engineering
4.5.1 The choice of the VHF channel on which a ground
station will operate depends on the coverage that the ground
station will be required to provide. Coverage on a particular
channel is provided by a collection of ground stations operating
on that channel and the communications on that channel
will occupy the channel for all the ground stations in a
coverage area.
a CSMA environment, this channel may not be used for
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