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DYNAMIC POSITIONING CONFERENCEOctober 9-10, 2012

SENSORS SESSION II

Including GNSS Based Heading in Inertial AidedGNSS DP Reference Systemy

Arne Rinnan, Sverre Bratberg, Nina Gundersen, Marit SigmondKongsberg Seatex AS

Including GNSS based heading in inertialIncluding GNSS based heading in inertial aided GNSS DP reference systemsBy Arne Rinnan, Kongsberg Seatex

OutlineOutline

• Introduction: GNSS based DP reference systems

• Problem description: GNSS and gyro limitations

• Technology: Dual antenna & IMU integration

• Validation: Operational test installations

• Conclusion

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INTRODUCTION

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GNSS based DP reference systemsGNSS based DP reference systems

• High precision GNSS solutions are used as DP reference systems

• Provides position input to DP of 10-20 cm accuracyp p y

• Large vessels requires more accurate heading than from traditional gyros

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GNSS based DP reference systemsGNSS based DP reference systems

• GNSS systems are line-of-sight, radio navigation systems

• Measurements are subject to signal outages and interference

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PROBLEM DESCRIPTION

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Antenna offset calculationsAntenna offset calculations

• Inaccurate heading from the gyro results in errors in the GNSS antenna offset calculations with traditional DP reference systems

• Problem increases with the length of the GNSS antenna offset vectors

• Ex:• Ex: – Antenna vector 100 m – 0.1 deg -> 18 cm– 1 deg -> 1.8 m

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DP reference system monitoringDP reference system monitoring

• Monitor performance prior to use by DP

• Monitor performance during use

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DP reference system failure detectionsDP reference system failure detections

• Freeze test– Alive assessment

Variance test• Variance test– Long-term assessment

• Prediction test• Prediction test– Short-term assessment

• Slow drift test– Assembly assessment– Divergence test– Median test (voting)Median test (voting)

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DP offset calculationsDP offset calculations

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Position estimates and measurement updatesPosition estimates and measurement updates

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Operating at high latitudesOperating at high latitudes

• Accuracy of traditional gyros degrades at high latitudes

5

4

5

deg)

2

3

eadi

ng e

rror

(d

1

RM

S he

00 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Latitude

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GNSS signal obstructionsGNSS signal obstructions

• Large structures on board may block satellite signals

• Interference from satellite communication equipment

• Shadowing from nearby vessels may block satellite signals

• Ionospheric activity may block or degrade satellite signals

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The ideal antenna location does not exist!The ideal antenna location does not exist!

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TECHNOLOGY

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A hybrid GNSS based position, attitude andA hybrid GNSS based position, attitude and heading system

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Dual GNSS antenna solutionDual GNSS antenna solution

• Increased GNSS availability on vessels with permanent or various shading/signal blocking

Fl ibl t i t ll ti ti t ki diti• Flexible antenna installation ensures optimum tracking conditions. Antenna separation 2.5 m – 100 m

Mitigates interference (vertical and horizontal separation of the antennas)• Mitigates interference (vertical and horizontal separation of the antennas)

• Two antennas separated with more than 5-10 m will experience different mulitpath conditionsmulitpath conditions

• Accurate lever arm compensation (compensates GNSS antenna offsets for heading pitch and roll)for heading, pitch and roll)

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GNSS based headingGNSS based heading

• Relative GNSS techniques can be used to calculate accurate vessel heading

H di d t 0 05 d (95 %)• Heading accuracy down to <0.05 degrees (95 %)

• No need for calibration

• No degradation at high latitudes, unlike traditional gyros or compasses

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GNSS/IMU integrationGNSS/IMU integration

• GNSS systems are line-of-sight radio navigation systemsGNSS systems are line-of-sight, radio navigation systems, and the measurements are subject to signal outages and interference

• The IMU is a self-contained system that is completely independent of the surrounding environment, and hence virtually immune to external disturbancesy

• Complementary properties of GNSS system and IMU allow for more reliable and precise navigation solutions

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GNSS/IMU integrationGNSS/IMU integration

• IMU integration allows for position calculation in periods with reduced GNSS availability (less GNSS satellites, reduced geometry)

• The IMU will continuously provide navigation information even during short-term GNSS loss of signal

• Integrity check of GNSS data

• Enhanced RAIM capabilities (reduces the effect of multipath)

• More reliable and accurate lever arm compensation of position and velocity

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FeaturesFeatures

• Combined use of a dual antenna GPS + GLONASS system and accurate motion measurements

Calculates multiple positions based on data from the two GNSS receivers• Calculates multiple positions based on data from the two GNSS receivers and the IMU, ensuring a high level of robustness and integrity in the position solution output to the DP

• The dual antenna configuration provides great advantages by improving the availability of GNSS signals

• Measures the heading of the vessel with higher accuracy than traditional gyros

• The IMU will continuously provide navigation information when both GNSS antennas experience short-term loss of signals

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VALIDATION

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Operational test installationsOperational test installations

Petronordic

Balder Bourbon Topaz

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Example: Balder (April - September 2012)Example: Balder (April September 2012)

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InstallationInstallation

Antenna separation

V: 16.6 m H: 35.5 m

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Antenna shadow sectorsAntenna shadow sectors

Side view Top view

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Vessel movements – roll, pitch, headingVessel movements roll, pitch, heading

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Antenna movementsAntenna movements

GNSS antenna 1

GNSS antenna 2

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Position calculations (simulated)Position calculations (simulated)

GNSS antenna 1

GNSS antenna 2

Nor

th (m

) System

Time (h)

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GNSS heading vs gyro headingGNSS heading vs gyro heading

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GNSS error propagation and heading errorsGNSS error propagation and heading errors

Lever arm compensation using GNSS heading and

Lever arm compensation

roll/pitch compensation

Lever arm compensation

using gyro heading and roll/pitch compensation

Lever arm compensation using gyro heading only

Error propagation between two GNSS antennas

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Error propagation between two GNSS antennas

GNSS based hull flexing measurementsGNSS based hull flexing measurements

• Vessel movements

• Vibrations

• Temperature

• Weight/Ballast

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CONCLUSION

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ConclusionConclusion

• GNSS based heading combined with inertial aided GNSS gives superior results for the most complex installations

• The dual antenna solution minimizes risk of interference and improves availability in periods with signal shading

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