Canadian Coast Guard Boosts Marine Survey Activities

The St. Lawrence River, the busy North American waterway that flows 3,800 kilometres to connect the Great Lakes to the Atlantic Ocean, is a vital component of the commerce and economy of Canada and the United States. Some 36 million metric tons of cargo moved through the Montreal-Lake Ontario section of the river in the nine-month navigation season of 1999, while the US locks at Messina, New York, saw nearly 3,167 vessel transits during the same period.

Because it is a limited-depth waterway carrying as much traffic as it does, the St. Lawrence needs to be managed in a way that takes into consideration the dual priorities of safety and efficiency. This means figuring out the optimal uses for the river in terms of vessels, speeds and traffic scheduling without permitting unnecessary safety hazards.
In the heavily navigated, 300-kilometre Laurentian section of the river between Montreal and Quebec, where large-ship traffic is exceptionally congested, dredging operations are essential and are conducted regularly. Ice formed during the winter flows downstream in spring and displaces huge amounts of rock and debris which, in turn, significantly alter water depth in some spots. These depth changes require constant vigilance to ensure ships’ safety.

Finding a New Marine Survey Solution
The Canadian Coast Guard has long worked together with the Canadian Hydrographic Service in conducting bathymetric surveys to accurately determine water depth, prepare for dredging operations, and to determine the effectiveness of such operations after completion.
Until recently, the Coast Guard Hydrographic Office used a combination of technologies for these surveys: a Differential Global Positioning System (DGPS) for horizontal positioning and a network of tide gauges to determine the vertical position of the survey vessel with reference to chart datum. This method, however, presented a few problems. For one, the tide gauges give local measurements taken close to shore on the assumption that the water at the vessel’s location is the same as that indicated by the nearest tide gauge. Unfortunately, currents, tidal effects and other hydrodynamic phenomena acting on the vessel cause hollows and swells at the water surface. ‘Squat’, the tendency of a ship’s draft to increase as it moves through water, is a good example of this. This is a hydrodynamic phenomenon in which displaced water creates an increase in current velocity past a ship’s moving hull. That, in turn, causes a reduction in pressure resulting in a localised reduction of the water depth - an important factor in determining precise depths for navigation.
In addition, the old system was impractical, relying as it did on a series of eighty tide gauges from which cumbersome data collection was an ongoing process. Matters were further complicated by the need to interpolate measured depths from several gauges at some points along the river.
Nearly four years ago, the coast guard began to look for a real-time kinematic GPS solution that would provide vessel positioning data and greater overall accuracy, and would take into account those hydrodynamic effects not measured by the old system.

Solution: Stable and Accurate Positioning Data
We considered several options and, and after an impressive demonstration that achieved Real-Time Kinematic (RTK) initialisation at 65 kilometres from the base station, settled on the Thales Navigation Long Range Kinematic (LRK) solution. This system takes full advantage of dual-frequency GPS technology, while reducing initialisation times to just a few seconds. The system maintains optimal real-time positioning accuracy to within a centimetre at a range of up to forty kilometres, even with a reduced number of available satellites, and gives a real-time three-dimensional positioning of the survey vessel - something never achieved under the old system. Positioning is expressed in the WGS84 (World Geodetic System 1984) datum and can be converted to chart datum with the help of ‘seamless’ datum, a grid model that gives the differences between the WG884 datum and chart datum.
The LRK positioning of survey vessels is done by Thales Navigation’s Aquarius 5502 MK receivers, using DGPS signals from the on-the-fly OTF network deployed by the Canadian Coast Guard in the Laurentian region.
After selecting the Thales equipment, the next step was to work with Thales to build the St. Lawrence OTF network and monitor its stability over an 18-month period. Validation studies were conducted with an Aquarius 5002SK station and a 6000 RK relay station. The network was up and running in September 2001.
Later, the topographical department decided to invest in three Thales 6502 SK/MK systems for use by its land survey teams, the SK station to be used only when surveyors are outside the OTF network’s range.

Next Up: Studying ‘Squat’ Effects
This year, the coast guard plans to begin experiments to analyse and quantify the effect of squat on various vessels’ draft depth. A physical model is currently used to determine these effects based on vessel speed, vessel load and waterway depth. The coast guard OTF network will allow verification of the model’s validity through full-scale experiments.
In these experiments a small vessel with little squat exposure serves as a reference, navigating just ahead of a larger vessel being monitored. The larger vessel will be fitted with as many as six DGPS receivers that provide a real-time LRK position. These highly accurate positions will provide data for calculating the fore and aft squat measurement, as well as torsion movements of the vessel.
The LRK technique, with its stable and accurate positioning readings, is an ideal means of conducting these experiments. Centimetre-level accuracy is essential in assuring the relevance of experimental conclusions.
Most important, of course, is the ability to provide accurate and reliable charts of channel depth characteristics, taking into account tidal effects, during the busy navigation season. The Thales system is accomplishing just that, along with the welcome phasing-out of the old tide gauge system.