VHF direction finding in the age of AIS



PLATH GmbH, Hamburg, Germany


When, in 2002, the International Conference on Maritime Security concluded with the implementation of Automatic Identification Systems (AIS), it seemed as though the combination of GPS and AIS would offer total reliability for the assignment of a ship’s position in a VTS area. However, this is frequently turning out not to be the case. This article reports on a solution to this which involves the aid of a radio direction finder.

AIS is certainly an appropriate means of conveying a large and varied amount of information about shipping movements to users onshore, without interruption and irrespective of time, in order to regulate the transport chain and hence keep everything moving smoothly. But it is precisely in determining the ship’s location at a certain point in time, as shown on a VTS console, that the weaknesses of the system become apparent. It is not only a matter of AIS failures due to technical deficiencies, but also of verifying the bearing of a particular ship or locating it at the moment when a VHF transmission is received by the VTS operations room.


A VHF radio direction finder is of inestimable value for making shipping lanes safer, especially when monitoring vessels representing a particularly high risk (HRVs).

It provides directional information about the origin of the VHF transmission at the time of the enquiry. The radio direction finder offers additional assistance when putting pilots aboard via helicopter. In this case the helicopter can be guided with the aid of a land-based VHF direction finder to the ship lying in the roads, which is of great assistance particularly in poor visibility.In a VTS area, radar provides an overview of the objects located within the VTS area. AIS, on the other hand, continuously provides the positions of AIS-equipped objects within its reception range. With both systems, any confusion, or even the very presence of a ship can only be resolved or confirmed – provided that the vessel has a VHF transmitter – in the course of the verbal exchange of information via VHF, best of all with the aid of a radio direction finder.

Ship-to-shore radiotelephony takes place in what is known as the lower maritime waveband, i.e. in the frequency range between 156 and 157.5 MHz. The upper frequency range is reserved for radio traffic in the shore-to-ship direction, and lies between 160.6 MHz and 162.55 MHz. As radiotelephony transmission is only of interest in the ship-to-shore direction for radio direction finding, and the reception of independent local transmitters reduces sensitivity, the so-called upper maritime frequency range is filtered out. Also located in this upper frequency range is AIS in which a very large number of automatic signals from countless directions are coordinated, and is therefore unsuitable for radio direction finding.

 Radio direction finders in use in coastal waters

When finding a transmitter for location purposes, it is important to use an antenna that permits good all-round reception. The antenna is to be installed at the highest point on the mast, away from reflected fields, so that only the signal that spreads out on the direct path between the transmitter and the receiving antenna is tracked. Any residual errors are to be kept stable, so that they can be easily compensated for by computer at the receiver. Operation of modern radio direction finding systems is based on the concept of a server-client architecture and WinDF software, which makes integration into a service provider’s VTS console fundamentally easier and means that multiple operations, data and audio can be carried out from various points.

The modern radio direction-finder in the VTS monitors up to eight radio frequencies at a time, which can of course be freely selected within the scope of the overall frequency range. This means that, in addition to the maritime frequencies, other frequencies can be selected in the civil aviation waveband between 118 and 137 MHz and the military emergency aviation frequency of 243 MHz.

The radio direction finding antennas can be easily installed on a mast in the form of space-saving H-dipole antennas.

The choice of the location of the antennas is also extraordinarily important. The shape of the coastline and the topography of the hinterland play a role here, as does the presence of conducting obstacles e.g. antennas, bridges or silos etc. For the direction finding or location of vessels under way, the location of one or more radio direction finding stations in relation to one another and to the primary direction of shipping movements also plays a crucial role. As only the VHF maritime frequency band is received in a VTS system, and used to determine direction, the antenna can be tuned in terms of its physical and electrical properties for a particular frequency range in this case. The 4-element Adcock A6243H4 antenna for civil aviation and maritime VHF radio was designed with special attention given to lightning protection and to installation on light masts in an environment that has a high salt content. Antennas have been examined that have never suffered a failure, despite years of use in exposed locations and traces of lightning strikes. This makes the antenna ideally suitable for use in maritime environments, and its excellent lightning-conducting properties give it a long MTBF, i.e. long availability and minimal maintenance requirements.

 Precisely in modern traffic guidance concepts, the use of a VHF radio direction finder is an indispensable tool for maintaining maritime safety.

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