Optical sensor monitors fog for San Francisco Bay mariners

Jan 23, 2014 04:59 PM

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The introduction of the sensor is welcomed by the bar pilots who work in San Francisco Bay.

“It’s a good addition to all of the other ways of telling us about the visibility around the bay,” said Bruce Horton, a San Francisco Bay bar pilot.

“Until now we’ve used either other pilots on ships or those around the general area, or tugboats traveling in the area to get information,” Horton said. “We’d call on them and get a pretty good idea of what the visibility was.”

Horton said that the weather has not been foggy enough as of late to assess the full value of the sensor. However, the bar pilots have already used the sensor to assess when to leave the dock, he said.

“More than anything, this may be a better tool for the Coast Guard,” said Horton. “That way they have a better idea of what’s going on, because it is more instantaneous.”

Horton said captains are not supposed to move from safe anchorage or a pier in seven critical maneuvering areas in San Francisco Bay unless there is more than a half-mile visibility.

“If you’re moving already and fog sets in, it’s up to you whether to try to anchor somewhere,” Horton said. “We’ve transited in the middle of San Francisco Bay with visibility less than a quarter-mile, because we have to get to a safe anchorage. You cannot stop (just) anywhere in the bay.”

For containerships destined for the Port of Oakland or leaving it, the technology will prove useful, he said. Many micro-climates are encountered in San Francisco Bay.
“You can have clear visibility in Oakland, but no visibility when approaching the entrance to the Oakland Estuary,” said Horton.

That entrance is a critical area for bar pilots because of the cross currents pilots must contend with passing through it.

Horton welcomes the eventual use of the sensor in other areas of San Francisco Bay, especially at the Carquinez Strait, where the Benicia-Martinez Bridge and a Union Pacific bridge await passage.

“In Benicia, they’re looking at putting it near the Union Pacific railroad bridge — which is a very narrow transit,” said Horton. “That’s one of the critical maneuvering areas where you’re not supposed to move if you have less than a half-mile visibility, unless you get caught in fog and you have no other option other than to transit.”

The FS11 sensor is made by Vaisala, a Finnish environmental measurement company. The sensor looks like a high-tech version of bull horns mounted on a long base. Its technology works when fog and other atmospheric particles pass between a transmitter which shoots a beam toward an optical receiver. The more the beam is scattered, the more the presence of fog and lower visibility at that location, said Janne Räsänen, Vaisala product manager.

“The sensor is particularly advanced in that it can compensate for optics contamination,” he said.

To do so, the sensor detects the amount of contamination in its sensors and makes corrections. The sensor has wider applications than shipping, said Räsänen. At present the sensor is widely used at airports as a visibility measurement and runway visual range assessment tool. The sensor is installed at airports in the U.S., Germany and South Africa, said Räsänen.

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