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POS MV Vertical Positioning

March 2004

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Where we fit in!

“Other sensors (notably modern heave-pitch-roll sensors) can contribute to achieving such 3-D georeferencing accuracies, but non-periodic and very low frequency vertical transducer motions, such as due to tides, squat, long-period heave, and other dynamic draft effects, are adequately measured only by RTK GPS1.”

1 Proposal for “Hydrography Research for Marine Vertical Positioning and Seabed

Classification”, Dr. Stephan D. Howden and Dr. Denis A. Wiesenburg.

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Topics

1. Positioning

2. Real-Time Heave

3. TrueHeave™

4. TrueHeave II – The Next Step

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Position

POS MV Position is a blend of Inertial and GPS data

• Position measurements are computed in a strapdown navigator using inertial data

• Errors in navigator are estimated and corrected by Kalman filter using data from aiding sensors

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Position

• Today POS allows for replacement of real-time heave with RTK position in echosounder data channels

• Loss/degradation of GPS, or corrections, causes vertical accuracy to degrade

POS/MV RTK Position Error Vs GPS Outage Duration

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1.00

2.00

3.00

4.00

5.00

6.00

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0 10 20 30 40 50 60 70 80 90

GPS Outage (seconds)

RM

S p

osi

tio

n e

rro

r (m

eter

s)

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Position – Tightly Coupled

POS MV Tightly Coupled Position• Kalman filter processes

inertial, GPS ranges & range rates & raw corrections

• GPS aiding < 4 SVs• Instant RTK lock recovery• Better multipath rejection

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Real-Time HeaveCharacteristics of Real-Time Heave

• Made up of two components• DC (pitch X lever arm)• AC (vertical motion)

• Zero mean – filtered vertical motion• Long settling times (3-7 minutes)• Poor accuracy in longer period swells due to phase error in

filter• Unacceptable accuracy during transitions of wave encounter

period

Improvements Offered• Filtering at “sweet spot”• Adaptive setting for filter corner period

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Vertical Velocity

Vertical Displacement

Vertical Acceleration

Real-Time Heave Filter

• The heave measurement can be viewed as a high pass filter operating on a double integration of vertical acceleration

• The high pass filter limits the accelerometer sensor noise in the vertical measurements channel

• Filter parameters are set to limit the phase error and settling time of the heave measurement

Information from Information from the Strapdown the Strapdown

NavigatorNavigator

1st Integration

High Pass FilterHigh Pass Filter

Heave EstimateHeave Estimate

2nd Integration

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TrueHeave – Basic Principles

Heave Motion Characteristics:• wave periods are generally from a few seconds to

as long as 30 seconds• high frequency energy 0.5 to 4 Hz.• longer waves with higher amplitudes (generally)• vessel characteristics affect vertical motion

TrueHeave Filter Objectives:• remove processing artifacts but not real motion• no tuning required for varying sea conditions• provide online quality measurement

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TrueHeave – How it works

• Additional data channel used to augment the real-time heave channel

• Raw vertical acceleration data, complete with unestimated sensor errors, is stored in memory

• Once three minutes of data is acquired, a FIR ~zero phase filter is passed over the data

• The delayed heave, real-time heave and time tag are output together over Ethernet for logging

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Real-Time TrueHeave

Data Available to Filter

Filter Point Filter Point

Data Available to Filter

Signal Filtering

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TrueHeave – 30 sec

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30 minute run, X Scale is IMU time, Y-scale is cm

12 cm heave precision observed in Real-Time

1 cm heave precision can be achieved with TrueHeave

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TrueHeave – Rapid Turn

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10 minute run, X-Scale is IMU time, Y-Scale in cm

Over 20 cm Heave variation seen in Real-Time

6 cm heave precision can be achieved with TrueHeave

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Real-Time Heave (3x)

TrueHeave (3x)

Courtesy D. Lockhart, Fugro

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TrueHeave Benefits

• Significantly reduced heave error due to ~zero phase error filter• particularly true during long period swells

• Shorter turn times, line changes, run-ins, and easier shoreline surveys• due to faster settling time of the FIR filter

• Produces a quality measurement on heave data

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TrueHeave Challenges

• Algorithm still has no ability to measure very long periods (>30 sec) or squat, dynamic draft & tides• Higher accuracy (and cost) IMUs can push the

limit to ~60 sec

• QC measurement is compared to real-time heave channel

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TrueHeave II – The Next Step

Objectives• To integrate RTK GPS and IMU data to obtain:

• the best estimate of the echosounder head relative to mean sea level

• a robust elevation which will survive GPS/RTK outages and data quality problems

• To produce an objective QC statistic for Heave

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TrueHeave II – The Next Step

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TrueHeave II – The Next Step

KF HeaveEstimator

Tightly-Coupled POSNavigator

TrueHeaveAlgorithm

RTKGPS

IMU

TrueHeave DelayedEllipsoidHeight

Pressure

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TrueHeave II – The Next Step

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Final Thoughts

• TrueHeave is not the whole solution to solving the vertical motion problem

• IS using POS MV to measure squat, dynamic draft and tides interesting?

• If yes, Applanix is interested in working with other parties to further explore the opportunity

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