2.15 Radar & Marine Radio (VHF / GMDSS)

Radar and Marine Radio (VHF / GMDSS)

Two pieces of electronics deserve their own section because the Rules of the Road and federal regulation both lean on them: radar for seeing in the dark and fog, and the VHF radio for talking, arranging passings, and calling for help.

Radar

Radar transmits pulses of radio energy and measures the range and bearing of whatever reflects them back. It is used for collision avoidance - the Rules invoke radar in safe speed, risk of collision, and conduct in restricted visibility - and for piloting, fixing your position by the range and bearing of a charted point of land.

Radar is not all-seeing. Know its limits:

• A minimum range / blind zone close aboard, where a nearby small boat can hide.
• Sea clutter (wave returns) and rain clutter that mask targets; the gain, sea (STC) and rain (FTC) controls exist to manage them, but overuse can erase a real target.
• Shadow sectors behind your own masts, or behind a headland.
• Weak returns from small, low, wooden, or fiberglass craft - a passive radar reflector greatly improves a small boat's own visibility to others.
• EBL (electronic bearing line) and VRM (variable range marker), and on plotting sets ARPA/MARPA, compute a contact's CPA and TCPA. Rule 7 warns you not to act on scanty radar information - use both long and short range scales and plot systematically before you assume a target's intentions.

Most small-boat sets display in relative motion, head-up, with your own boat fixed at center - so a target that holds a steady bearing while its range decreases is on a collision course, exactly the radar version of the constant-bearing rule. Switching between range scales matters: a long scale gives early warning of distant traffic and landfall, a short scale shows close-in detail as you enter a harbor. A charted RACON (radar beacon) paints a coded Morse flash on the screen to mark a key aid or landfall, a useful radar fix.

VHF marine radio - the channels

VHF is line-of-sight range; more antenna height means more range. Standard fixed sets transmit at 25 watts (high) or 1 watt (low). Memorize these channels:

Keep routine chatter and radio checks off channel 16; use a working channel (68, 69, 71, 72, etc.). Domestically, a recreational VHF requires no individual station license, but a vessel operating internationally or on certain commercial voyages does.

GMDSS and DSC

Modern VHF sets carry a Digital Selective Calling capability tied into the Global Maritime Distress and Safety System. Pressing the guarded red distress button sends an automated digital distress alert on channel 70 that includes your MMSI and, if the set is connected to GPS, your position. Always follow the DSC alert with a spoken Mayday on channel 16. To use DSC you must first register the radio and program in your MMSI number. Beyond VHF, a 406 MHz EPIRB (Emergency Position-Indicating Radio Beacon) alerts search-and-rescue via satellite from beyond VHF range and, once registered, transmits your identity and position - the backstop when you are out of radio contact with shore.

Distress, urgency and safety calls

Three spoken signals, each said three times, set the priority:

• MAYDAY, MAYDAY, MAYDAY - distress. Use only for grave and imminent danger to life or to the vessel (sinking, fire, uncontrolled flooding, a person overboard in danger) requiring immediate help.
• PAN-PAN (PAHN-PAHN), three times - urgency. The safety of the vessel or a person is at risk but there is no immediate danger to life - a disabled but not sinking boat, or a non-life-threatening injury.
• SÉCURITÉ (SAY-CURE-I-TAY), three times - safety. A navigational or weather warning - a hazard to navigation, a large vessel maneuvering in a narrow channel, a storm advisory.

A distress call on 16 follows a set order: Mayday x3, "this is [vessel name x3 and call sign]," then position, nature of distress, assistance required, number of persons aboard, and a description of the vessel. The exam's favorite trap: a non-life-threatening problem is a Pan-Pan, not a Mayday - reserve Mayday for grave, immediate danger to life or the vessel.