Looking out of the window was not really an option for the pilot conducting the 28, 372 GRT containership Cap Blanche on the Fraser River, British Columbia, Canada, on 25 January this year. With fog reducing visibility to 150 metres he could not even see the bow of the 221.62 LOA vessel, but he did have his trusty portable pilotage unit, PPU, which he relied upon exclusively for navigation and connected it to the vessel’s AIS. But the AIS had a secret, one which put Cape Blanche on the silt at the river’s Steveston Bend.
The accident report from Canada’s Transport Safety Board brings to light a little known aspect of navigation by GPS yet one that might not have led to the grounding had the pilot not been essentially left to his own devices even when his actions conflicted with the vessel passage plan.
The PPU had a predictor function that projects the vessel’s future position by performing geometric calculations based on the vessel’s current rate of turn, position, heading, course over ground, COG, and speed over the ground, SOG. The COG and SOG are derived from GPS values that continuously fluctuate, even when the vessel maintains constant speed and course due to inherent errors and inaccuracies in the GPS. To stabilize these values, a GPS smooths these inputs to provides the user with a more stable COG and SOG.
One can often see the GPS fluctuations on a GPS-equipped tablet computer or smartphone.