Depending on the GPS (age and specific type) they may be only accurate to +/-30M. This puts you inside a circle 60m wide. Then add to the fact that many GPS take anywhere from 2-15 seconds to update, gives a bit of margin for error. ESP in the channel near green island, and when travelling fast.
Fog does not USUALLY have much effect on GPS. HOWEVER anything that obstructs the line of to the satellites can effectively lower the accuracy or cause it not to work at all. in the forest, under an overpass, in a building or tunnel. Also canyons are notorious, and the channel at green island IS effectively a canyon. Even Heavy clouds or Fog may completely block reception.
Radar often (mostly) does not pick up things low in the sea, Logs, wash rocks, small boats, lobster pot buoys, or cargo containers floating in the water. It may miss vessels completly also, the smaller and the less metal present the less likely to show on radar.
It is difficult for an island not to show but I was once looking in a radar when an island the size of Green Island appeared to our naked eye and was not on the radar. The first thing I saw was a tree floating in the fog, about 10m up. Fog interference with the radar, and a significant local magnetic disturbance combined to almost plow us into the island. Fortunately that boat only did 10 kts and we avoided it. This was before GPS was available to anyone but military.
All passenger vessels world wide are now required to use AIS.
http://en.wikipedia.org/wiki/Automatic_ ... ion_System
It is a GREAT method of aiding collision avoidance in these conditions. It uses VHF radio waves in conjunction with GPS data for ships to broadcast location, heading, and speed info then the NAV system shows the ships and lists them by distance/time of closest approach to your vessel. Provided they keep going in a straight line!
I suspect that AIS, Radar, and the masters good sense to slow down avoided catastrophe in both cases we are talking about in this thread.
If you want to see something crazy look at the live AIS data for HK here. Choose area #904 and zoom in, it's crazy!
http://www.marinetraffic.com/ais/
The law of the sea is to travel slow enough to stop in the distance of visibility. I prefer to be able to stop in 1/2 that distance. I mean what about the other guy coming at me? Also required for power vessels underway in limited visibility to give one blast of 4-6 seconds on the horn every 1 minute. AND to reduce speed to the speed necessary to avoid collision.
I am an experienced sailor, powerboater, and navigator, previous holder of a USCG 100TON Masters License - and I prefer NOT to go out in less than 1/2 mile visibility period. Anything less than that on any vessel and I will have to consider - conditions, sea state, crew experience, electronics, other traffic, and anything else I can think of that might make me decide to stay home!
As I side note - By international law a vessel MUST use all available means to avoid collision. That means your EYES at all times, no matter what else you have.
This requires also that ALL vessels equipped with radar use it at ALL times.
Something I noticed that NO ferries were doing in the daylight when I got here. Until that one hit a concrete post with a light on it at Cheng Chau, about Sept. Starting THAT morning all the ferries I have seen are using radar 24x7. The skipper of that ferry is going to be in deep doo doo as he said he did not have his radar on, and therefore was not using ALL AVAILABLE MEANS TO AVOID COLLISION.
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