Shipborne GNSS for geoid validation

Stavanger, 22.11.2018

Shipborne GNSS for geoid validation

- Examples from the FAMOS project -

Mirjam Bilker-Koivula

Maaria Nordman, Jaakko Kuokkanen, Timo Saari

Hannu Koivula, Pasi Häkli, Sonja Lahtinen, Ulla Kallio

Finnish Geospatial Research Institute, National Land Survey of Finland

Acknowledgement: Jyrki Mononen, Finnish Transport Agency


Background & Motivation

• FAMOS project:

– Improve infrastructure for future navigation in the Baltic Sea

– E.g. Hydrographic surveying, gravity surveys, new Baltic

Sea geoid model, GNSS augmentation tests

• Baltic Sea Chart Datum 2000, BSCD2000

– Common height reference for the whole Baltic Sea

– Based on EVRS, epoch 2000

– Zero level defined by the geoid

• Quality assessment of geoid models at sea needed

– GPS-levelling at sea


Derive geoid heights from GNSS

observations at sea

dN = SSH – ADT – N


Shipborne GNSS for geoid validation -

Contents

• Background & Motivation

• Calculation process

– GNSS processing

– Reduction to sea surface

– Sea surface modelling

• 3 examples from the FAMOS project

– 2015 Airisto campaign

– 2017 Aranda campaign

– 2018 Geomari campaign


Calculation process

GNSS post-processing

Result: Ellipsoidal heights at the vessel’s reference

point in the processing coordinate system

Height translation from vessel reference point

to the sea surface

Result: Ellipsoidal heights at the sea surface

Translation from sea surface to geoid surface

Result: Ellipsoidal heights at the geoid surface

Comparison with geoid models


GNSS post-processing

GNSS base station processing

Result: Consistent reference coordinates for the

processing system

Kinematic processing of vessel GNSS/IMU data

Result: Ellipsoidal height of vessel reference point

in processing coordinate system

Coordinate transformation

Result: Ellipsoidal height of reference point in the

systems related to the geoid models in use

Height reduction from reference point to the

sea level

Result: Ellipsoidal height of the sea level in the

systems related to the geoid models in use

Campaign reference

frame & epoch, e.g.:IGb08 2015.754770

IGS14 2017.432877

IGS14 2018.39315

Geoid model ref.

frame & epochETRF96 1997.000

ETRF2000 2000.000


Height translation from vessel reference

point to the sea surface

• Reductions

– Pitch and Roll (small effect)

– Heave: short term vertical movements of the vessel

– Static draft: Impact of ships load changes to draft

– Dynamic draft: Squat (velocity effect to draft)

• Result: ellipsoid height at the sea surface

= Sea Surface Height, SSH


Translation from sea surface to geoid

surface

Height transformation from sea surface to geoid level (zero height)

- remove Absolute Dynamic Topography (ADT)

Sea surface modelling:

• Tide gauge method

– Tide gauges from surrounding countries

– Fit surface

• Physical model method

– Sea surface from Baltic Sea physics analysis and forecast

(Copernicus Marine Environment Monitoring Service ,CMEMS)

– Fit surface to tide gauges


26.9.2015

Corrected

physical

model

surface

Tide

gauge

surface

Original

physical

model

surface

Computed

correction

surface


3 FAMOS measurement campaigns

• 2015 Airisto

– Survey vessel

– Dedicated gravity campaign

– GNSS data from vessel’s own navigation system

• 2017 Aranda

– Environmental research vessel

– Piggy-back campaign

– GNSS equipment installed by FGI

• 2018 Geomari

– Geological research vessel

– Dedicated gravity campaign

– GNSS equipment installed by FGI


Airisto Campaign 2015

• Survey vessel Airisto

(Meritaito Oy)

• Dedicated gravity survey

• 26.9. – 7.10. (Umeå) – 12.10.

• Vessel’s GNSS data stored

• Well determined

– Internal coordinate system

– Squat table

– Static draft readings


FAMOS 2015 Airisto campaign - GNSS

data analysis Applanix POSPac MMS 7.1

Smooth Best Estimate Trajectory


Reduction to sea surface

Roll

0 – 5 cmPitch

0 – 4 cm

Static draft

10 cm

Squat

0 – 20 cm


Reduction to geoid surface

Height transformation from sea surface to

geoid level (zero height) - remove ADT

Sea surface modelling:

• Tide gauge method

– 6 Swedish tide gauges

– 10 Finnish tide gauges

• Physical model method

– Baltic Sea physics analysis and forecast

(Copernicus Marine Environment

Monitoring Service ,CMEMS)

– Fitted to tide gauges



Comparison with geoid models: dN = SSHGNSS – ADTmodelled – Ngeoid model

Mean 9.5 cm

SD 5.0 cm

Mean 9.5 cm

SD 1.8 cmMean 15.5 cm

SD 20.9 cm

Mean 15.6 cm

SD 19.1 cm

Mean 0.3 cm

SD 11.5 cm

Mean 0.2 cm

SD 3.2 cm

Mean -3.6 cm

SD 58.8 cm

Mean -3.5 cm

SD 60.4 cm




• 7 lines rejected

• Grand mean and standard deviation

Non-filtered FIN2005N00 model Non-filtered NKG2015 model

Tide gauge surface Physical model Tide gauge surface Physical model

mean (cm) sd (cm) mean (cm) sd (cm) mean (cm) sd (cm) mean (cm) sd (cm)

4.7 10.9 3.7 11.2 3.2 10.9 2.1 11.1

Filtered FIN2005N00 model Filtered NKG2015 model

Tide gauge surface Physical model Tide gauge surface Physical model

mean (cm) sd (cm) mean (cm) sd (cm) mean (cm) sd (cm) mean (cm) sd (cm)

4.7 4.3 3.7 4.3 3.1 4.3 2.1 4.2


Famos Aranda, 5.6-10.6.2017

• Environmental

research vessel

• Piggyback campaign

• GNSS/IMU

equipment installed

by FGI

• GNSS trajectory calculations

with Inertial Explorer Software


Squat estimation from GNSS data


Static Draft

• Readings at three locations during stops at sea

• 1.3 - 2.9 cm/day


Tide Gauges

• 14 stations in Finland

• 6 stations in Estonia

• 9 stations in Sweden

Geoid Models

• FIN2005N00

• NKG2015

Reduction to geoid level

CMEMS Baltic Sea

physics analysis and

forecast


Geoid height differences

ΔN = SSH – ADT – N(FIN2005N00)


GNSS-Geoid comparison Aranda

DOY 156-161

Tide Gauges (cm) Physical Model (cm)

FIN2005N00 NKG2015 FIN2005N00 NKG2015

Mean Std. Mean Std Mean Std Mean Std

Line 1 0 10 -1 9 1 10 0 9

Line 2 -3 9 -4 9 -1 9 -1 9

Line 3 -9 10 -10 9 -9 11 -11 11

Line 4 -17 11 -18 12 -19 10 -20 10

Line 5 -5 8 -7 8 -5 8 -7 8

Line 6 -8 11 -10 11 -8 11 -10 11

Line 7 -8 6 -6 5 -8 6 -6 6

All (1 min) -7 10 -8 10 -7 11 -8 11


2018 Geomari Campaign

• 21.-25.5.2018

• Dedicated gravity campaign

• GNSS/IMU equipment installed by FGI

• Location measurements in ship coordinates

• Static draft reading in harbours


Squat estimation

• Majority of time constant speed

• Not much accelerations/decelerations

• No reasonable squat values obtained


At 5 knots:

< 1 cm


Tide Gauges

• 4 stations in Finland

• 5 stations in Estonia

Geoid Models

• FIN2005N00

• NKG2015

Reduction to sea level

CMEMS Baltic Sea

physics analysis

and forecast


-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

dN_TG dN_PM

Geoid height differences

Monday Tuesday Wednesday Thursday Friday(m)

ΔN = SSH – ADT – N(FIN2005N00)


DOY 141-145

Tide Gauges (cm) Physical Model (cm)

FIN2005N00 NKG2015 FIN2005N00 NKG2015

Mean Std. Mean Std. Mean Std. Mean Std.

Monday 5.2 4.1 15.6 4.6 4.6 4.1 15.0 4.6

Tuesday -3.8 5.7 6.3 6.7 -4.9 5.7 5.2 6.8

Wednesday 3.4 3.1 11.6 3.6 2.9 3.1 11.2 3.8

Thursday 7.4 4.3 14.3 4.7 7.5 4.7 14.5 5.2

Friday -5.1 3.7 4.5 4.6 -5.6 3.5 4.1 4.2

All 2.4 6.4 11.2 6.4 2.0 6.7 10.8 6.7

GNSS-Geoid comparison Geomari


It is possible to recover geoid heights from GNSS

observations at sea and validate existing geoid models

• Agreement between geoid heights derived from GNSS

observations and those from geoid models better than 5 cm

in the Bothnian Sea

• Less good agreement with big vessel and piggyback-mode

over large area

• In Eastern Gulf of Finland results are promising. Here geoid

height differences may reveal differences between geoid

models that are due to differences in input gravity data.

Conclusions I


Important:

- Positioning of GNSS antenna

- Common processing of base stations

- Coordinate transformation to systems related to geoid model

- Vessel with well determined internal coordinate system

- Knowledge on relative locations of instruments and water line

- Pitch and roll

- Static draft and squat

- Well-controlled area with good monitoring networks and models

- Elsewhere modelling of tides may be needed

Conclusions II


www.famosproject.eu

Shipborne GNSS for geoid validation

Documents

Transcript of Shipborne GNSS for geoid validation