Each year as Southern California moves deeper into the late fall and winter months, familiar marine conditions return to the coastline. Cool mornings, gentle offshore breezes and lingering humidity often create dense fog banks that settle across harbors and anchorages. From Dana Point to Ventura, visibility can drop without warning, turning a routine trip in or out of the harbor into a cautious slow speed journey marked by muffled horns and careful listening.
Fog has always been one of the most challenging conditions for boaters, and the Coast Guard has made significant updates in recent years to help mariners navigate safely when their view narrows to only a few boat lengths. One of the most important improvements is the installation of the Mariner Radio Activated Sound Signal system, more commonly known as MRASS. This modern system has replaced the older fog detectors that once triggered sound signals automatically, sometimes inaccurately, and sometimes not at all.
To understand how widely the system has been adopted along the coastline, the Coast Guard explained its current approach to placement. As Lauren E. Giancola noted, “The U.S. Coast Guard has implemented the Mariner Radio Activated Sound Signal (MRASS) system at several key aids to navigation along the Southern California coastline, particularly at harbor entrances and high-traffic areas where fog is common. The Coast Guard has focused MRASS installation on replacing older, continuously operating fog signals. Additional sites are considered as part of routine aid modernization or maintenance projects, based on mariner feedback, traffic density, and environmental considerations.” This growing network ensures the technology is available where it matters most.
MRASS places the control directly in the hands of the mariner. Instead of waiting for a sensor to determine when conditions are foggy, boaters can now activate the foghorn at the end of a jetty or breakwater whenever they need it. By using a standard VHF radio and tuning to the channel listed on their nautical chart or in the Coast Pilot, a boater can key the microphone five or six times in quick succession. This simple action signals the unit to begin sounding. Once activated, the horn typically operates for about one hour before it returns to standby mode.
As the Coast Guard explained, the development of MRASS offers clear advantages when compared to the older fog detection systems still familiar to many longtime boaters. Giancola described the shift by saying, “MRASS improves efficiency, safety, and community impact. Unlike older systems that automatically operated during low visibility, MRASS only activates when requested by a mariner via VHF radio. This reduces unnecessary sound emissions, conserves energy, and extends the lifespan of the equipment. It also empowers mariners with the ability to activate a fog signal when and where it’s needed, providing flexibility while maintaining navigational safety.” These improvements help ensure the system performs reliably during the moments when a mariner truly needs acoustic guidance.
The system is installed at aids to navigation along many harbor entrances throughout the region. It is designed to provide the audible guidance mariners expect while reducing unnecessary sound when visibility is clear. This on demand approach also saves energy and reduces maintenance issues that became common with older automated fog detectors. It ensures the horn will sound precisely when needed, especially in pockets of localized fog that may not be detected by remote sensors.
As the fog season continues, boaters depend on sound seamanship to complement modern tools. To help mariners use MRASS effectively, the Coast Guard offered clear advice meant to reinforce safe practices when visibility diminishes. Giancola emphasized, “Boaters should maintain a proper lookout and safe speed per the Inland and International Navigation Rules, using all available tools—radar, AIS, GPS, and sound signals—to avoid collision. To activate MRASS, mariners should key their VHF-FM radio microphone five times on Channel 83A (157.175 MHz). The system will then sound the fog signal for approximately 30–45 minutes. MRASS should be used as a navigational reference in conjunction with, not as a substitute for, prudent seamanship and onboard navigation systems.”
Although the system is designed to be straightforward, there are still common mistakes among users. To ensure mariners are aware of these pitfalls, Giancola added, “The most frequent issues involve using the wrong VHF channel or incorrect keying sequence. MRASS does not activate on Channel 16 and does not provide two-way communication or confirmation signals. Mariners are encouraged to review operating instructions published in the Coast Guard Light List and applicable nautical charts to ensure proper use. Correct operation ensures the fog signal activates as designed and supports safe navigation near the aid.” Her reminder reinforces how preparation and familiarity remain essential parts of fog navigation.
In a broader sense, the Coast Guard underscored how MRASS fits into its ongoing effort to modernize aids to navigation along the coast. Giancola noted, “The Coast Guard remains committed to enhancing maritime safety through modernized aids to navigation while balancing environmental stewardship and community considerations. MRASS is an important step in improving efficiency, reliability, and responsiveness for mariners operating along the California coast.” This forward looking approach continues to shape how sound signals are used throughout Southern California’s harbors.
For Southern California boaters who frequently encounter gray mornings or sudden banks of whiteout conditions near the breakwater, knowing how to use MRASS is a valuable part of seasonal seamanship. It allows vessels to announce their presence in tight conditions and provides a recognizable reference point when visual navigation markers disappear into the haze.
As fog season continues along the coast, the Coast Guard encourages mariners to review their charts, confirm the correct radio channels for their home harbor, and practice safe operation when visibility is reduced. Slower speeds, proper lighting, consistent horn signals and reliance on radar or GPS equipment all play a role in getting home safely. MRASS is now part of that toolkit, offering reliable sound guidance at the press of a microphone.
This time of year, brings quieter waters, soft morning light and the occasional curtain of mist over the ocean. With preparation and an understanding of the tools available, boaters can navigate these conditions with confidence and maintain the same level of safety they expect in clear weather.
