News»View all»GPS puts pilot in a fog
Posted on 7 November 2014

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.

Smoothing is user‑defined in most cases, and may vary from 2 to 30 seconds. As a result of GPS smoothing interval settings, values for COG and SOG may lag. In addition, the actual values of the vessel position may lag, in some cases up to a ship’s length, resulting in inaccurate depictions of the vessel’s position on navigational equipment, such as on a pilot’s PPU. The use of WAAS‑based DGPS antennas provides more accurate data on the vessel’s position. These antennas use a default GPS smoothing interval of 4 seconds for SOG and 6 to 8 seconds for COG, making the smoothing interval a known value that can be accounted for when using the PPU.

Predicting a future position without being able to account for smoothing intervals cannot be done with accuracy.

Use of the PPA‑issued navigational equipment is at the discretion of the pilot, with the exception of voyages through Second Narrows, where the use of the PPU and WAAS‑based DGPS antenna is required by the PPA. At the time of the grounding, the pilot’s PPU was obtaining GPS data from the Cap Blanche‘s AIS, which had an unknown smoothing interval, and the predictor was set to display six vessel positions, each at 30-second intervals.

The pilot did not have his WAAS‑based DGPS antenna with him on this voyage, as he had experienced technical problems with it on prior voyages. These problems were not reported to the PPA, and the PPA does not have a formal process to track problems with navigational equipment issued to pilots. The PPA does have spare equipment, and pilots can switch out equipment when it is defective.

The pilot’s PPU was receiving inaccurate GPS positions from the AIS with no way of knowing the level of the anomalies, not something the rest of the bridge team did not know but were relying upon for safe navigation..

What should the other members of the bridge team have known?

Due to the large volume of water flowing through the Fraser River, an average of 3 million cubic metres of sediment is deposited in the navigable channel of the Fraser River each year. As such, silting is evident in certain areas of the river between Sand Heads and Fraser Surrey Docks, and is most pronounced when the freshet starts to subside from mid-August to early September. At the time of the incident, the Steveston Bend had silting on the south side that extended into the navigable channel,  reducing the design depths of 10 m between kms 5 and 6 and 10.1 m between kms 6 and 7. Buoy S10, which is approximately where the vessel grounded, is located on the south side of the channel and is slightly east of km 6.

Although both electronic and paper charts warn “Depths shown on this chart are subject to change as a result of silting, scouring and dredging. For the latest available depths in the dredged area, consult the Canadian Coast Guard website Avadepth Water Depth Forecasting for the Fraser River (http://www2.pac.dfo-mpo.gc.ca) the bridge team could not access it because the vessel was not equipped with Internet.

The pilot did have the latest information but it was not shared with the ship’s navigation officers, nor did they ask for it.

A passage plan had been prepared which provided for a safe speed of 10 knots in the Fraser River. Under pilot’s orders this speed was exceeded but no-one questioned the pilot.

A study on pilot/ship officer relationship published by the TSB in 1995 noted that pilot’s decision making “can become the weak link in a system prone to single-point failure; i.e., in the absence of effective monitoring, there is little safety backup for the pilot in the navigation of the vessel.

And so it proved.

  1. If a navigator relies on a single piece of navigational equipment, there is a risk that potential errors or inaccuracies will go undetected.
  2. If information that may affect the safe passage of the vessel is not communicated between bridge teams and pilots, there is a risk that unsafe situations and conditions may persist.
  3. If pilots do not make use of the most accurate navigational equipment available to them, there is a risk they will make decisions based on imprecise information.