optimising ultra-short baseline (usbl) positioning from ......responder on vessel pre-calibrated...

Geraint West

Global Business Manager - Oceanographic

Optimising Ultra-Short BaseLine (USBL)

Positioning from Research Vessels

Optimising USBL Positioning from Research Vessels

USBL for demanding science

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What’s the right solution for my ship?

RV Falkor

RV Neil Armstrong

RV Celtic Explorer

RV Thomas G Thompson

Co-mounted with multibeam

Crane-mounted

RV Gordon C Sproul

ASV C-Worker USV

Over-the-sidemounting

NOAA ShipFairweather

RRS James Clark Ross icebreaker option


USBL System Overview

All I

Output to Viewpoint Software

Output Telegram to DP Desk

GPS in

Ranger 2 Software, Navigation Computer, NSH

DP ReferenceTransponder (DPT 6, Compatt 6)

Subsea Vehicle Transponder(WSM6, WMT, DPT6)

OR

• USBL is a range and bearing system - position calculated by measuring reply signals at the transceiver:

– time (range)– phase (angles) of

• Requires high quality vessel motion sensors and precise timing

• Accuracy defined in relation to percentage (e.g. 0.12 – 0.5%) of Slant Range


So what does an accuracy of 0.12 – 0.5% of Slant Range mean?

eg - a 0.5° error in transceiver pitch or roll would equate to 17.5 m positional error at a slant range of 2,000 m (0.88% of slant range)

• GNSS – Noise - will be directly translated into the subsea position, but only

affects precision.– Offset Error or Invalid GNSS Corrections - won’t affect precision, will

affect accuracy

• AHRS – Uncompensated motion

• Pole flex, flap• Hull flex• MRU mounting, latency or low quality• Predominantly affects precision…….Will affect accuracy if it causes a fixed

bias.

– Misalignment of MRU to vessel frame or to USBL - fixed bias that will affect accuracy

• Sound Speed (Timing)– Scaling or TAT Error– Ray Refraction– Bad Surface SV

• Bad Depth (Pressure)


USBL System Position Error


Installation: Analysis of movement of pole AHRS with relation to bridge MRU

<0.3° pitch bias directly affects position accuracy

Pole resonates at 2Hz approx


RV Falkor – Installed USBL v Lowered OTS USBL

Courtesy of Blair Thornton, Universities of Southampton & Tokyo

• Collects USBL data as the vessel moves around a transponder deployed on the seabed;– Nominal horizontal offset of 1/3 x water depth

(Max 500m)– Full range of headings maximises any heading

errors– Sailing closer to the transponder’s vertical

maximises pitch and roll errors– Reciprocal headings maximise offset errors.

• Observations analysed:– Quantify errors– Provide corrections– Accurate position of the seabed transponder


Calibration of Attitude Sensors in the USBL System (CASIUS)

• Random:– Range ‘jitter’ due to acoustic noise– Transponder timing– AHRS/Gyro sensor drift– Angle measurement error

• Systematic– Sound speed error– Incorrect offsets– Uncalibrated on incorrect calibration– Sensor latency

• Mistakes– Incorrect TAT– Multipath– Reference beacon movement– Tracking the wrong beacon


Errors and how to avoid them

Use good equipment

Calibrate the equipment and use it properly

Employ good people and train them well


Long Range Tracking

• 2005 - Sonardyne LUSBL first installed on the Chikyu

– Using 5th Generation MF Compatts(super directional)

– Wideband range and bearing measurements of seabed transponder array relative to USBL transceiver position.

• 2012 - Upgraded to Sonardyne Wideband DP-INS in 2012

– Tightly coupled acoustically aided inertial position accuracy better than 0.5m

– Update rate: Acoustically 0.1Hz, AAINS Typically 1Hz

• Achieved drilling record drilling 854m below seafloor at a depth of 6897m.


Chikyu DP-INS system

Lodestar: Provides

inertial tracking

aided by USBL

measurements.

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Long Layback Tracking with iUSBL

Directional WMT MF responder on vessel

Pre-calibrated Integrated Gyro and Inverted USBL (GyroiUSBL) on towfish collects acoustic measurements away from noise of the vessel.• Attitude and heading data with perfect timing

collected with the acoustic measurements on the towfish – avoids MUX latency issues.

Tracked to a slant range distance of 9,022m with good SNRLimited by tow cable, not signal

Responder transmitting 202dB from the vesselSNR >20dB.

22-23 May 2017

RV Rachel Carson

Trial site: Monterey Bay, California

Water depth: 3,500m

25 Nov – 6 Dec 2017

RRS James Cook

Trial site: SW of Canary Islands

Water depth: 4,300m

Cooperation of MBARI and NOC is gratefully acknowledged


LMF Trials 2017

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• Transceiver – LMF (15.5kHz Wideband2™ carrier) GyroUSBL

– Integrates a high accuracy AHRS with the USBL transceiver –provides perfect timing

– Out of the box “Calibration free”

– ~0.3% slant range precision improving to up to 0.1% slant range after the first and only calibration

– GyroUSBL can be moved from vessel to vessel without a need to re-calibrate with only a quick spin test to verify alignment required

• Beacon – on the Seabed

– 6th Generation (6G) LMF Compatt (omni) Compatt transponder

– Omni-directional (SL 193dB re 1μPa @1m) used in the MBARI trial

– Directional (SL 201dB re 1μPa @1m) used for the NOC trial


Gyro USBL combines USBL and attitude sensors in one instrument

Frequency(kHz)

Absorption (dB/km)

Δ from 14kHz(dB/km)

14 1.93 -

17 2.71 0.78

19 3.27 1.34

25 5.12 3.19

34 8.01 6.08


Absorption – Benefits of lower frequencies


Noise Comparison Between RV Rachel Carson and RRS James Cook

10

15

20

25

30

35

40

45

50

55

60

0 10,000 20,000 30,000 40,000 50,000 60,000

Spe

ctra

l le

vel (

dB

)

Frequency (Hz)

James Cook (NOC)

Rachael Carson (MBARI)

LMF MF


Frequency Effect on Measurement Precision

A greater phase change across the aperture of the array is observed at high frequencies resulting in higher angular precision

But SNR of LMF eventually more significant than phase resolution of MF:• This is SNR based, so vessel noise

base determines at what range this takes place


Interference with other acoustic sensors

0 –

62

.5 k

Hz

Time


RRS James Cook – Comparison of thruster states

Stationar

y

MF unit 20 kHz 36 dB



LMF unit 12

kHz

40 dB

LMF unit 15

kHz

40 dB

LMF unit 18

kHz

32 dB

Stationar

y

Underway

MF unit 20 kHz 36 dB 36 dB



LMF unit 12

kHz

40 dB 40 dB

LMF unit 15

kHz

40 dB 40 dB

LMF unit 18

kHz

32 dB 32 dB

Stationar

y

Underway Station

keeping

MF unit 20 kHz 36 dB 36 dB 65 dB



LMF unit 12

kHz

40 dB 40 dB 70 dB

LMF unit 15

kHz

40 dB 40 dB 65 dB

LMF unit 18

kHz

32 dB 32 dB 60 dB


RRS James Cook – Noise level at different thruster states

15

25

35

45

55

65

75

0 4 7 11 15 18 22 26 29 33 37 40 44 48 51 55 59

Spe

ctra

l no

ise

leve

l (d

B /

ro

ot

Hz)

Frequency (kHz)

SNR vs Precision


SNR v Precision

In band SNR Precision in metres (1DRMS)


RRS James Cook CASIUS LMF HPT


LMF USBL Trials Results

Tracking

stopped for

telemetry tests

Tracking

stopped for

telemetry tests

CASIUS Max slant range (m)

Depth % of depth

1drms (m)

RV Rachel Carson 3,500 0.12% 4.2 11,300

RRS James Cook 4,300 0.17% 7.3 11,100

Rates achievable are dependent on configuring the delays between packets to minimise errors and consequent retries, which in turn reduce data real-world data-rates.


RRS James Cook LMF Telemetry Trials

Rate (bps)

Theoretical bit rate (4300m) (7200m)

Actual rate Energy per bit (mJ) Actual rate Energy per bit (mJ)

200 192 154 260 154 260

400 352 282 142 256 156

900 737 579 69 535 75

3000 3003 2765 14 2259 18

3500 3281 3021 13 2384 17

6000 6007 3686 11 1327 30

9000 9121 5150 8 - -

• Optimum solution begins with optimum installation:

– Optimise noise environment

– Optimise equipment (including ancillary sensors)

• Understand the errors and calibrate as per best practice

– Train your team


Summary


Keep pushing the boundaries…

Any questions?

optimising ultra-short baseline (usbl) positioning from ......responder on vessel pre-calibrated...

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