VHF Radio Coverage Area calculation for maritime applications

Radio communication equipment is typically integrated into VTS applications to provide the VTS operators with a real‐time assessment of the situation in the VTS area of responsibility as well as a means to deliver timely services to VTS participants. Information collected and disseminated via this equipment can assist in assembling the traffic image and in supporting safe navigation of the VTS area.

Radio communication is used to collect position, safety, and general information from shipboard personnel and remote sensing devices. This mean of communication is also the primary means through which services are delivered to VTS participants.

VTS radio communication comprises both voice and data services using equipment consistent with the GMDSS Sea Areas.

Area A1 ‐ Within range of VHF coast stations with continuous DSC (digital selection calling) alerting available (about 20‐30 nautical miles);
Area A2 ‐ Beyond area A1, but within range of MF coastal stations with continuous DSC alerting available (about 100 nautical miles);
Area A3 ‐ Beyond the first two areas, but within coverage of geostationary maritime communication satellites (in practice this means INMARSAT);

This covers the area between roughly 70°North and 70°South.

Area A4 ‐ The remaining sea areas. The most important of these is the sea around the North Pole (the area around the South Pole is mostly land).

Geostationary satellites, which are positioned above the equator, cannot reach this far.

Radio Service Coverage Area of VHF Radio systems is one of the most important performance elements for a ship to shore and a shore to ship radio communications.

The Maritime VHF band comprises a number of channels within the frequency range of 156 MHz to 162.025 MHz.

The VTS Authority should ensure that the VTS radio infrastructure provides adequate coverage for the VTS area. VHF radio reception is generally dependent upon the line‐of‐sight distance between VTS receive site and the ship antenna heights. As a minimum requirement, the radio communications range should facilitate VTS ship communications before the ship enters a VTS area of responsibility.

According the IMO Resolution A.810 (19) the selection of VHF coast stations for sea are A1 should be based on the following principles:

Each sea area designated as A1 requires a continuous VHF guard and should have the minimum number of stations necessary to provide VHF coverage in the costal area of the Government concerned.
In certain Area, several Governments may collectivity provide complete coverage along their coasts.

Criteria for establishing Sea Area A1

The communication range of stations operating in the mobile VHF band is likely to be limited by propagation factor rather than lack of radiated power.

Sea area A1 is that sea area which is within a circle of radius A nautical miles over which the radio propagation path lies substantially over water. The radius A is equal to the transmission distance between a ship’s VHF antenna at a height of 4 m above sea level and the antenna of the VHF coast station which lies at the centre of the circle.

The following formula should be used to calculate the radius A in nautical miles:

H is the height of the coast station VHF receiving antenna and h is the height of the ship’s antenna which is assumed to be 4 m.

The formula given above applies to line-of-sight cases but is not considered for cases where both antennae are at a low level.

High terrain features may block line-of-sight. In this case we need to take into account the terrain. Using electronic terrain maps and the Global Mapper software we can draw a coverage area.