Determination of real-time tide information using GPS

David Russell and Pieter Toor report on developments in the offshore positioning and navigation markets.

Real-time observation of tidal variations using high-accuracy global positioning systems (GPS) offers many benefits over conventional methods, ie the deployment of tide gauges or the use of tidal prediction software.

Veripos Tides uses highly stable and accurate positions from the Veripos Ultra positioning service. The service is based around a positioning technique known as Precise Point Positioning that minimises or removes all of the main GPS errors sources such as satellite orbit, satellite clock, troposphere, ionosphere and multi-path.

Since the service provides the user with real-time decimetre level accuracies it is possible to use these positions to detect height variations relative to a vertical reference offshore.

To achieve this, the real-time tide estimation process has to manage height variations caused by vessel motion and by inaccuracies in the GPS position calculation process. Information and results are presented to the user via numerical and graphical displays within the Veripos Verify-QC software.

Vertical reference terminology

- Mean Sea Surface (MSS). Represents the height of the ocean surface (measured from the surface of some reference ellipsoid), averaged over some specific time interval (and corrected for ocean and earth tides). MSS models are based on data provided by altimetry satellites.

- Mean Sea Level (MSL). There are two different interpretations: 1. In geodesy, MSL usually means the local height, ie vertical offset, of the global Mean Sea Surface above a level reference surface called the Geoid; 2. In tidal analysis, MSL means the still water level averaged over a period of time (eg, month or year) so periodic changes in sea level (eg, tides) are also averaged out. MSL values are measured with respect to the level of benchmarks on land. As such a change in MSL can result from either a real change in sea level or a change in the height of the land on which the tide gauge is located (eg, from isostatic rebound).

- Geoid. A surface over which the gravity potential is constant (ie, water does not 'flow' on the Geoid but it remains in equilibrium). It also is the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global MSS.

- Dynamic Ocean Topography (DOT). Difference between MSS and the Geoid. It originates from the fact that the major ocean circulation has a time-invariant non-zero component (ie, a component that does not average to zero over time).

If the oceans were static and not affected by winds and air pressure, MSS and Geoid would be the same surfaces. However, steady currents in the ocean driven by winds and atmospheric heating and cooling, give rise to differences in sea level around the world.

These local differences between the Geoid and MSS are described by the Dynamic Ocean Topography whose values can range between -2.5m and +1.2 m.

Therefore, Geoid models like EGM96 and EGM2008 can be considered an approximation of MSS, but only to the 2-3m level.

Verify-QC does not consider chart datums and therefore can't convert tidal values relative to MSS to other tidal references like Lowest Astronomical Tide (LAT).

The relationship between Chart Datum/LAT and MSS is a vertical offset 'Z0' which can only be determined by deploying a seabed tide gauge and logging over at least two Lunar Cycles.

Z0 is not constant and varies from place to place and the vertical offset between LAT and MSS is published for specific ports around the world in the Admiralty Tide Publications.

The Veripos Tides process provides two real-time tide observations relative to different vertical references.

The first method estimates the tide relative to a Geoid Model, either EGM96 or EGM2008, which is a global approximation of MSS. This observation, called Geoid Tide, is available instantaneously and is suitable for short projects.

The second method first derives MSS using a VERIPOS Tide filter and then estimates the tide relative to this surface. This observation, called Ultra Tide, is independent of uncertainties in the Geoid model. It requires 39 hours of continuous data and is only suitable for longer-term offshore projects.

The Ultra Tide observation is more accurate since its vertical reference (MSS) is accurate to the cm level.

The Veripos Tides estimates provide the real-time total variation from a vertical reference (MSS for Ultra Tide or EGM model for Geoid Tide).

The fact is that the sea surface varies not only due to tidal effects but also due to additional effects like atmospheric effects (variations in pressure and wind) which in extreme cases will cause tidal surges.

The benefit of estimating a real-time total variation from a vertical reference is that a single vertical correction can be applied to marine survey depth information.

In addition, the system allows tides to be estimated at a high frequency (every minute) such that a very detailed sea surface variation pattern can be derived.

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David Russell is Technical Support Manager and Pieter Toor is R&D Manager with VERIPOS Westhill, Aberdeenshire, Scotland, UK.

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