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Parameter Value Notes
RF Channels Multiple 25 kHz channels the 117.975 – 137.000MHz or 108.000 – 117.975
MHz bands. No global allocation has yet been
determined.
Transmitter power Airborne – 20W
Ground – 32W
Modulation scheme Binary GFSK
+/- 2400 Hz
Bit rate 19200 bit/sec
To support ADS-B applications VDL4, four
channels are recommended 2 and 2 regional.
Additional local channels would also be required
for airport surface operation.
Note that for small aircraft (GAT), the power is
4W (VDL Mode 4 MOPS).
Channel access method STDMA Self-Organising TDMA with nominally 75 slots per
second per channel.
Table 5- 5 Characteristics of VDL Mode 4
5.7.2.3 Prospects for VDL3 and VDL4
The general view in Europe is that, since 8.33 kHz will provide Europe’s voice needs until
2015, the potential advantages of VDL3 are reduced. The airline community do not favour
VDL3 – hence the view of the study team is that VDL3 is unlikely to enter operational
service.
The view on VDL4 is more complex since VDL4 offers a wider range of communication
services than other VDL modes and hence is also a candidate for ADS-B.
EUROCONTROL is considering VDL4 as a potential communication and ADS-B link for
implementation from 2010 onwards. No decisions have been made. From the point of
view of communication modes, VDL4 has a number of advantages over VDL2 including
an access mechanism that provides efficient channel re-use and hence potentially greater
effective data rates in en-route scenarios. Crucially, VDL4 supports priority and preemption
and hence is well suited to tactical data links. Hence, the wholescale transfer of
controller pilot dialogue is possible.
A key issue for the aeronautical community is the proliferation of data link technologies.
The existence of three different VHF data link technologies is a barrier to consensus
building. However, several manufacturers have begun to address this issue through the
production of multi-mode radios which are capable of supporting 25 kHz, 8.33 kHz and
combinations of the VHF digital links. Hence a migration path 25 kHz to 8.33 kHz
analogue for voice together with data modes progressing ACARS to VDL2 to VDL3/4 is
technically possible.
5.7.3 Possible use of Mode S SSR as a data link
Mode S Secondary Surveillance Radar (SSR) was discussed in Section 3.3 as a means
for provision of surveillance data. Mode S SSR has also been standardised to provide a
communications link that is compatible with the ATN. The aeronautical community has
given consideration in the past to use of this link for ATS communication services. The
current status is that such use is not being actively pursued. The reasons for stopping
development of Mode S data link included:
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