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5.6.2 Movement of HF voice services to HFDL
The implementation of HFDL may reduce the number of voice messages being passed
(and the duration of each message), leading to a more efficient use of the spectrum.
5.6.3 Movement of HFDL to Satcom
Satellite communications may be used for remote/oceanic regions. However, coverage is
poor near the poles, and the cost/benefit case for users (in particular, airlines) means that
many continue to prefer HF communications.
A low-cost satellite communications solution is not likely in the near future. As such, the
cost-benefit case for HFDL continues to be a driver for airline equipage.
5.6.4 VHF datalink: upgrade of ACARS to VDL Mode 2 and initial transfer
of ATIS and controller pilot dialogue
The move to datalink communications may increase the performance for VHF (with VDL
Mode 2, Mode 3 and Mode 4 offering potential improvements in spectrum efficiency
compared to VHF voice).
VDL Mode 2 (VDL2) provides an air/ground bit-oriented point-to-point VHF data link which
is compatible with the aeronautical telecommunication network (ATN). VDL2 is designed
to support both AOC and ATS applications, and for many airlines is seen as an upgrade
to ACARS for their AOC communications needs.
Airlines and the data link service providers ARINC and SITA are pushing ahead with an
initial implementation of VDL2 which uses VDL2 as an improved carrier for the ACARS
service. This is termed ACARS over AVLC (AOA). Substantial levels of implementation
are expected by 2010.
EUROCONTROL has progressed the VDL2 system for use for ATS applications under the
PETAL II, Euro VDL2, and Link 2000+ programmes.
The Link 2000+ programme aims to use VDL2 to support initial data link services such as:
• Data Link Initiation Capability
• Digital ATIS
• ATC Communication Management.
• ATC Clearances and Flight Plan Consistency are also planned to be
progressively introduced in the en-route environment in a second phase of the
Link 2000+ programme.
VDL2 uses a Differential 8-Phase Shift Keying (D8PSK) modulation scheme and a CSMA
medium access protocol (Collision Sensitive Multiple Access) similar to the VHF ACARS
scheme. One VHF channel is shared by many aircraft and ground stations.
Interference testing of VDL2 to assess frequency compatibility with other onboard
systems has been carried out by EUROCONTROL and is now complete. EUROCONTROL
has progressed frequency planning at recent ICAO FMG meetings, and has specified a
number of steps to vacate the necessary frequency channels.
According to the EUROCONTROL plan presented at FMG, in Europe VDL2 is planned to
operate in four frequencies at the top of the COM band: 136.975, 136.925, 136.875, and
136.825 MHz. This involves clearing existing users of the channels away from these
channels, and into other channels lower in the spectrum.
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