What to check on a VHF Radio.
GMDSS combines several communication layers covering different ranges, and VHF is the closest-range, most-used layer of all of them. Its range is limited by line of sight between antennas, typically 20 to 30 nautical miles ship to ship or ship to shore depending on antenna height, which is far short of MF, HF or satellite coverage. What VHF does better than any other GMDSS element is immediacy: Channel 16 remains the international distress, safety and calling frequency monitored continuously, and Digital Selective Calling on Channel 70 can trigger an automated…
What Sets VHF Apart Within GMDSS
GMDSS combines several communication layers covering different ranges, and VHF is the closest-range, most-used layer of all of them. Its range is limited by line of sight between antennas, typically 20 to 30 nautical miles ship to ship or ship to shore depending on antenna height, which is far short of MF, HF or satellite coverage. What VHF does better than any other GMDSS element is immediacy: Channel 16 remains the international distress, safety and calling frequency monitored continuously, and Digital Selective Calling on Channel 70 can trigger an automated distress alert to all VHF DSC-equipped vessels and coast stations within range far faster than raising anyone by voice. VHF is also the primary tool for routine bridge-to-bridge coordination and pilot and port communication, work that satellite systems are not built for and not intended to replace.
Main Components
Fixed VHF Transceiver
The bridge-mounted radio, required to include DSC capability under GMDSS carriage requirements, providing both voice and digital calling functions.
DSC Controller
The module handling Digital Selective Calling, which can send an automated distress alert including the ship's position, provided it is interfaced correctly to a GPS source.
Antenna and Coaxial Feed
Mounted as high as practicable to maximize line-of-sight range; feeder cable condition directly affects transmitted power and received sensitivity.
Portable Survival Craft VHF
Separate, waterproof, battery-powered handheld sets carried for use in lifeboats and liferafts, independent of the ship's main power and fixed installation.
Power Supply and Battery Backup
An emergency power source ensuring VHF distress and safety communication remains available if main ship power is lost.
Selection and Sizing
- Number and type of VHF sets required by GMDSS sea area and ship type, including fixed sets, portables and survival craft sets.
- Antenna height and cable run length, which set achievable range more than transmitter power does within the fixed 25-watt limit.
- Integration requirements with GPS for DSC position reporting and with the bridge alarm system for distress alert handling.
- Battery backup duration required to meet the applicable GMDSS power supply provisions for the ship's sea area.
Regulations and Class
VHF DSC carriage is mandated under SOLAS Chapter IV for GMDSS-equipped vessels according to sea area, with Channel 70 DSC and Channel 16 voice watch requirements set out in the ITU Radio Regulations. Survival craft portable VHF sets are required under SOLAS life-saving appliance provisions in numbers tied to the ship's lifeboat and liferaft complement. Annual radio survey confirms functional testing of DSC alerting, and equipment must carry type approval demonstrating compliance with the relevant GMDSS performance standards.
Typical Faults
| Fault | Cause | Consequence |
|---|---|---|
| DSC not triggering an alert | GPS interface not connected or MMSI not programmed correctly | Distress call transmitted without position data, delaying rescue response |
| Reduced transmit range | Corroded antenna connector or degraded coaxial cable | Loss of contact at ranges the set should reliably cover |
| Dead survival craft handheld batteries | Battery past expiry date or not checked at scheduled intervals | No communication capability from lifeboat or liferaft in an actual emergency |
| Channel 16 not monitored | Set left on a working channel instead of the required dual-watch or distress channel | Missed distress or safety calls directed at the vessel |
What to Look for in a Supplier
- Type approval certificate confirming compliance with GMDSS performance standards for the specific sea area classes the vessel operates in.
- Confirmed GPS interface compatibility for DSC position reporting, tested at commissioning rather than assumed.
- Survival craft handheld sets with battery expiry clearly marked and a replacement schedule the crew can follow without ambiguity.
- Local service and spare parts support able to meet annual radio survey deadlines.
Test DSC distress alerting through the proper test procedure at each survey interval, a set that transmits normal voice traffic fine can still have a DSC function that was never actually verified to send a position-tagged alert.
