Use of GNSS Based Real-Time Tidal Information in Offshore ... · Use of GNSS Based Real-Time Tidal...

Use of GNSS Based Real-Time TidalInformation in Offshore Survey Applications

www.veripos.com 13 March 2012

Dr. David Russell

Technical Sales Manager, VERIPOS

Overview


Overview

What is Tide?

• Tide is the periodic rise and fall of the sea level under the gravitationalpull of the moon and sun

– Caused by attraction (gravity) of sun and moon

– Normally twice per day (but not always)

– Size of tide varies during the solar and lunar cycles

• Tides cause periodical variations in the ‘equilibrium’ sea level calledMean Sea Surface (MSS)


Applying Tides to Depth Data

• Why tide is important– Required for reduction of calculated depths & generally referenced to surface of sea

• LAT – Lowest Astronomical Tide

• MSL – Mean Sea Level

– Always changing due to tide, and by different amounts in different places

• Tide gauges (measuring the depth at a location)– Most accurate

– Sometime difficult to get data


– Sometime difficult to get data

• Predicting tides (by use of software, e.g. Polpred)– Easy and repeatable

– Doesn’t account for environment (atmospheric effects, storm surges, etc.)

• Predicting tide by use of standard ports (tide tables)– More hassle and easier to make mistakes

– Still doesn’t account for environment

• Using GNSS– Includes environment effects (atmospheric effects, storm surges, etc.)

– How to relate GNSS ellipsoid to chart datum - difficult

Vertical Datum

• Very complex – what datum do we use and what historically have we used...

• Approximate Lowest Astronomical Tide

•Approximate Mean Low Water Springs

•Approximate Mean Low Water Tide

•Approximate Low Water

•Approximate Mean Sea Level

•Half Tide


•Mean Lower Low Water

•Mean Low Water

•Mean Low Water Neaps

• Mean Low Water Springs

•Mean Sea Level (MSL)

•Mean Tide Level

•Half Tide

•Highest Astronomical Tide

•Highest High Water Large Tide

•Highest Normal High Water

•Highest High Water

•High Water Springs

•Indian Spring High Water

•Indian Spring Low Water

•Lowest Astronomical Tide (LAT)

•Lower Low Water Large Tide

Tide Terminology – Difference between MSL & MSS

• Mean Sea Surface (MSS) represents the mean state of the ocean and thereforeincludes permanent effects of global currents

– MSS models are developed based on data provided by altimetry satellites. It is notan equipotential surface

• Mean Sea Level (MSL) refers to a ‘level’ water surface, which you would get ifthe sea was perfectly at rest

– It coincides with an equipotential surface, as for example the Geoid. MSL values aremeasured with respect to the level of benchmarks on land


measured with respect to the level of benchmarks on land

• Geoid is an equipotential surface which would coincide exactly with the meanocean surface of the Earth, if the oceans were in equilibrium, at rest, andextended through the continents

– Approximated by Geoid Models as for example EGM96 and EGM2008

• Dynamic Ocean Topography (DOT) is difference between MSS & Geoid/MSL.– It originates from the fact that the major ocean circulation has a (more or less) time-

invariant non-zero component (i.e. a component that does not average to zero overtime)

How Are Tides Determined

• If the oceans were static and not affected by winds and air pressure, then MSSand Geoid would be the same surfaces.

• However, there are steady currents in the ocean, driven by winds andatmospheric heating and cooling, which give rise to differences in sea levelaround the world.

• These local differences between the Geoid and MSS are described by theDynamic Ocean Topography. The DOT values range between (approximately) -


Dynamic Ocean Topography. The DOT values range between (approximately) -2.5m and +1.2 m.

GNSS Positioning Technology

• Positioning is essential technology for offshore operations

– Satellite Navigation is a mature and accepted key technology

• Augmentation data from service providers allows a higher level of accuracy &precision over standalone GNSS

– Services offered range in accuracy from the metre level to thedecimetre/centimetre level

– Use of PPP (absolute) and RTK (relative) technologies


Veripos Apex PPP Veripos Ultra PPP

Advances in Positioning Technology

• New satellite constellations and modernisation of the current systems– GPS / GLONASS / Galileo /Compass

• Will see developments with new position solutions– Different levels of accuracy

– Different combinations of satellite signals

• Combined GPS and GLONASS PPP– Improvement in solution availability and accuracy


– Faster solution convergence

Apex PPP

Apex2 PPP

PPP Convergence

• Using GPS & GLONASS improves the convergence time of the PPP solution

Apex PPP


Apex2 PPP

PPP and Geoid Tides – VERIPOS Implementation

• Reduce the effect of heave, pitch and roll by averaging the PPP high-accuracyheights

• Ensure position is accurate PPP position (not DGPS or converging)– Use QC tag to Error Ellipse/SD threshold of less than 0.1m


• After 39hrs de-tide the heights by using the Doodson filter derived MSS estimate

• Leave actual tidal reduction to client, i.e. antenna offsets, draft & squat

PPP Tide

• Based on the Doodson filter

• Simple X0 filter designed to damp out the main tidal

frequencies

• It takes hourly values, 19 values either side of the

central one

JOB No:CLIENT:PROJECT:

Diagram: The surveyor shall provide a diagram showingthe points of reference and measurements.

Hour # Date TimeObserved

Height (H)

Doodson

Multiplier (M)H x M

0 1 01 0 02 1 03 0 04 0 05 1 0

(By 39 Hourly Observations / Doodson Filter)

MSL TIDAL OBSERVATIONS


central one

• Produces a MSS estimate

• Used at static tide gauges

• Can be used in a dynamic environment

5 1 06 0 07 1 08 1 09 0 0

10 2 011 0 012 1 013 1 014 0 015 2 016 1 017 1 018 2 019 0 020 2 021 1 022 1 023 2 024 0 025 1 026 1 027 0 028 2 029 0 030 1 031 1 032 0 033 1 034 0 035 0 036 1 037 0 038 1 0

Sum = 30 0MSL = S (HM) / 30 = 0

Geoid Tide

- PPP Tide

- can only be calculated after 39 hours

- Unbiased but long initialisation

- Vertical reference: Actual MSS

- Geoid Tide

- can be calculated instantaneously

- Instantaneous but biased

- Vertical reference: Geoid Model


• Ultra Tide = Avg. Ultra/Apex Height – Doodson output (VQC estimated)

• Geoid Tide = Avg. Ultra/Apex Height – Antenna Height (user entered)

Reducing Antenna Height to Sea Surface

• The vertical offset between the vessel waterline and the phase centre need to beknown

– Better than 10cm level, otherwise, bias introduced

• Recommended to repeat the height measurement at regular intervals, particularlywhen there is a draft change

• User measured height is only used to calculate GeoidTide, not for PPPTide


PPP and Predicted Tides






Offshore Results


Offshore Results


Offshore Results – Far East


Tsunami Monitoring – Japan Earthquake March 2011


And…


And…

The Future

• Use DNSC08 Mean Sea Surface Model– Makes GeoidTide unbiased

– Instantaneous Tide relative to MSS - better solution

• Real-time measurement of pitch/roll (INS sensors)

• Other techniques for tide reduction (e.g. direct reduction)

• Tidal harmonic analysis of GNSS tide data to derive tide constituents


• Link to the UK VORF (Vertical Offshore Reference Frame)– Tides relative to the ETRF/GRS80 (~WGS84) can be converted to other

offshore vertical datums in the UK/Ireland

VORF

• Why VORF

– GNSS important in offshore survey

– LIDAR and multibeam

– Datum fusion

– Consistent datum

• Helps improve bathymetric data processing


• Helps improve bathymetric data processing

– Less complex

– Real-time reducing post-processing requirement

– Improved accuracy

– Improved consistency because data can be relate directly to chart datum

– Disadvantage is that only limited to UK waters

Positioned for Success


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