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In order to transmit at the correct time and to ensure global co-ordination between all
participating stations, each station requires an accurate time source. In VDL4, the timeslots
are all synchronised to UTC time, normally provided by a GNSS receiver.
Slots are grouped into superframes, each 1 minute long, with 4500 slots in every
superframe. Thus there are 75 slots per second, and each slot is 13.33 ms in duration.
A position report typically occupies one time-slot while other transmissions, such as
ground station transmissions, may occupy more.
The self-organising concept allows VDL4 to operate efficiently without a centralised coordinating
station, thus eliminating the need for a ground infrastructure. Ground stations
may however serve an important role providing other services that enhance VDL4
operations.
VDL4 is foreseen to operate on specific channels, called Global Signalling Channels
(GSCs), which should eventually be allocated worldwide. In high traffic-density airspace,
the GSCs will be supplemented by Regional Signalling Channels (RSCs), or by Local
Signalling Channels (LSCs). The GSC channels will be used by all participating stations,
and used by ground stations to broadcast Directory of Services (DoS) information about
the services available on the GSC channels, and on regional or local channels.
Frequencies for VDL4 in Europe have been proposed by EUROCONTROL. The plan
foresees making way for the VDL frequencies at the top of the 117.975 – 137.000MHz
band and has been approved by the ICAO Frequency Management Group in Europe.
There is a debate as to whether VDL4 will be able to use frequencies in the 108.000 –
117.975 MHz band. The FAA view is believed to be that VDL4 frequency assignments in
the US would probably have to be in the 117.975 – 137.000MHz band.
It is intended that VDL4 will operate on 25 KHz frequency channels in both the 117.975 –
137.000MHz and 108.000 – 117.975 MHz frequency bands.
VDL4 SARPs were published by ICAO in late 2001. Work on updating the Technical
Manual for improvement of point-to-point communications has recently been completed,
and awaits publication by ICAO. There are a number of patent issues associated with
VDL Mode 4 which are common with the Maritime AIS system (see section 6.10 for a
discussion of AIS and the associated patent issues). The key result of a European
Commission study of the patent issues is that “...the attitude of the patent holders is
reasonable and that information provided on the patents could not be considered as
insufficient to cover the needs of a standardisation body such as ICAO...” (see
http://www.gpc.se/patent/ecpatent.doc).
VDL4 ADS-B MOPS were published as Interim MOPS by EUROCAE in mid-2001. A
European Norm for VDL4 ground stations has recently been produced by ETSI.
The characteristics of VDL Mode 4 are summarised in the Table 5-5 below.
Parameter Value Notes
Service topology Air-air broadcast
Uplink broadcast
Downlink broadcast Airair
point-to-point A/G
point-to-point
ATN compliance Yes
Frequency band 108-136.975 MHz Allocations of required channels could be in either
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