Autosub6000. Results of its Engineering Trials and First Science Missions

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1 Autosub6000 Results of its Engineering Trials and First Science Missions Stephen McPhail, Maaten Furlong, Veerle Huvenne, Peter Stevenson, Miles Pebody, James Perrett NOC, Southampton, UK

2 Objectives of the sea-trails on RRS Discovery Standard AUV Stuff Navigation....Control Launch and recovery.. Speed performance. Acoustic telemetry and tracking. Interesting issues specific to deep diving AUV Buoyancy change: As the AUV dives Navigation: A solution to the initial position problem? Energy: Field test of the new rechargeable batteries..

3 Specifications of Autosub m long, 0.9 m Diameter 1500 kg dry weight Range : 180 km, 36 hr at 5 km/hr (ultimate x 3 of this ) Battery recharge time of 8 hours from totally exhausted 6000 m Depth Navigation with Teledyne RDI 300kHz DVL, Ixsea Oceano PHINS, and USBL (Linkquest TrackLink 10000) Aim to achieve and maintain GPS standard accuracy over several days without an external position fix [or LBL transponder network] Payload space of 0.5 m 3 and Up to 250 W available for sensors. Sensors -Currently: EM2000 Multibeam, ADCP (RDI 300kHz) Seabird SBE 52 MP

4 Key to the Range / Depth performance of Autosub6000 Lithium Polymer Pressure Balanced Batteries Up to 12 battery packs, each of 4.5 kw hr can be fitted within the centre section of Autosub6000. Charge in 8 hours

5 AUV Navigation: Problem 1: The Initial Position Problem On the surface the AUV can get GPS fixes. Near (within 200 m) of the seabed the AUV can dead reckon navigate with good accuracy using its Doppler Velocity Log (DVL ) and Gyro compass 6 km But during the descent - the AUV navigation effectively drifts with the currents - it could be out by hundreds of metres??

6 The Solution Range Only Navigation As the AUV circles at depth (bottom tracking) under the ships position.. Get many ranges from the ship mounted interrogator to the acoustic transponder on the vehicle Combine Ranges, AUV s Navigation, and Ship s positions, to yield a single, high quality position fix for the AUV Multiple Ranges measured by acoustic transponder as AUV circles 6 km 1 km

7 But But. But isn t the geometry terrible for this? Position errors are very sensitive to range measurement errors as shown, 1m of range error gives..12 m of horizontal error.. What about sound speed profile? (is 0.01% needed?) What about refraction? What about depth sensor error, and pressure to depth conversion? What about the fish vertical movement? Multiple Ranges measured by acoustic transponder as AUV circles 6 km 1 km

8 However.. 1)Refraction effect is surprisingly insignificant for range only 2)Systematic errors.. Sound speed, depth error, can be solved for given the that there are many (e.g. 100) measurements. 3)Fish vertical movement causes random errors (are averaged out significantly) HORIZONTAL ERROR (m) OFFSET ANGLE FROM VERTICAL (degrees) Horizontal error vs offset angle for worst case constant ms-1 sound velocity gradient. At 6000 m depth. 4) And fish vertical movement can be measured and we did

9 Practical results: First Deep Dive D ep th [m ] X position (North) [m] Y position (East) [m] Autosub6000 spiralled down to 1000m, 2500m, then 4556 m...why? Autosub6000 then ran a 1km side box around the centre position (while we collected range and position data). After 20 minutes we repeated this box (to check the repeatability of the method) Figure 3: 3D Navigation plot for the first deep Autosub6000 mission. The AUV spiraled down to 4556 m depth, and then, after receiving a continue command sent by acoustic telemetry, executed a 1 km side box.

10 How to check robustness of the RoN results? One Fix, no independent reference Mission 13 End. Navigation solutions with reduced data sets. All data used 3 sides of box used 2 sides of box used 1 side of box used 10

11 Buoyancy Change..the worrying unknown Buoyancy change as the vehicle dives is caused by the vehicle parts compressing at a different rate to seawater as the pressure increases. The Autosub AUVs are usually ballasted at about + 10 kg buoyant on the surface (only 0.3% of its displacement) The biggest area of doubt surrounded the syntactic foam for the vehicle buoyancy We made some measurements on thermal and compressive moduli of the materials But not possible to be sure enough about the full scale changes.. So we had to proceed with caution..and ballast at + 20 kg buoyant.

12 Buoyancy Measurement Free ascent method worked well. Good for monitoring changes during ascent, and between missions dz dt Buoyancy Autosub6000's Depth [m] Drag Lift filter vertical velocity [m] Mission Time [s] Figure 2: The vertical ascent speeds during Mission #6, run alternately at 300 W and 10 W propulsion power. From this data we can calculate the depth dependant buoyancy variation and vehicle drag. Key to the success of this method is that the vertical speed dz/dt can be measured very accurately. Results show increase in buoyancy from 20 kg at surface to 26 kg at 4500 m...acceptable.. 12

13 RRS James Cook 027 (PI Dr Russell Wynn, NOC,S) Investigate Geohazards. E.g. Tsunami generating events such landslides and deep sea bed slippage, by : Piston Coring of deep Sea sediments Simultaneously micro-bathymetric surveys using the Autosub6000 fitted with an EM2000 multibeam bathymetric sonar AUV positioned at start of missions using Range Only Positioning, and recorded data for TERCOM like navigation.

14 Cruise Track and Autosub6000 Survey Areas Tenerife 14

15 Dive 1: Agadir Canyon Scours 4 km x 4 km 100 m Altitude. 200 line spacing. 2 m gridded m 4280 m Image processed using CARAIBES IFREMER Software by Dr Veerle Huvenne (NOCS) 15

16 Dive 2: Giant Scour in Horseshoe Abyssal Plain 4 km x 5 km 100 m Altitude. 300 m line spacing. 2 m gridded. Image processed using CARAIBES IFREMER Software by Dr Veerle Huvenne (NOCS) 16

17 Dive 3: Scours in Mouth of Setubal Canyon 4 km x 6 km 100 m Altitude. 300 line spacing. 2 m gridded. Image processed using CARAIBES IFREMER Software by Dr Veerle Huvenne (NOCS) 17

18 Dive 4: Cascais Canyon 18

19 Dive 4: What went wrong?.. 60 degree dive Mission Aborted over-depth Cause -failed sternplane actuator potentiometer (Take care to set up safety systems.. ) 19

20 Dive 5: Mouth of Whittard Canyon Image processed using CARAIBES IFREMER Software by Dr Veerle Huvenne (NOCS) 20

21 Navigation during the Autosub6000 Bathymetric survey missions Range Only Navigation worked effectively at the start (and end of) mission But what about the navigation drift during the mission? 21

22 SMTERCOM (Simultaneous Mapping and TERCOM Navigation) Section of Bathymetry from on NS baseline. 2 m pixels Same area bathymetry run on EW line 22

23 SMTERCOM The idea is to measure the offset between the sub-image and the main image

24 SMTERCOM Simple qualitative look. (Sum of Absolute Differences) -1. Suggests the general method can be very effective (as long as there is some relief on the sea floor) We are working on automating this for post processed navigation and optimising for lower S/N. 24

25 Summary and Future plans Autosub6000 shown to work reliably for Multibeam survey in deep water For initial positioning problem: Range only Navigation works reliably and accurately. For navigation drift during a survey : SMTERCOM looks very promising and simple to implement for suitable target areas. Future Plans for Autosub6000 : Development of collision avoidance capability. Safely getting in closer, safely in more hostile terrain. Develop SMTERCOM for routine use on surveys Extend range of AUV by up to 3 X with more batteries NERC funded research Hydrothermal activity and deep ocean biology of the Mid-Cayman Rise, with cruise in 2010 to hunt for and explore hydrothermal vents. This is working together with ISIS ROV. 25

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