Outline. Application of AUVs for Hydrography. AUVs for hydrographic surveying AUV horizontal mapping accuracy

Transcription

1 Application of AUVs for Hydrography Øyvind Hegrenæs, Ph.D. AUV Department Outline AUVs for hydrographic surveying AUV horizontal mapping accuracy HUGIN 1000 with HISAS 1030 SAS HiPAP 500 USBL or GPS surface fix AUV vertical mapping accuracy Hydrographic accuracy requirements Conclusions / 2 /

2 Hydrographic Surveying with AUVs Ship Survey System AUV Survey System Real-time data collection and storage on ship Real-time data collection and storage in AUV system Survey company data processing system Client / 3 / Hydrographic Surveying with AUVs The survey end-product has no better accuracy than the least accurate component in the measurement chain, from surface navigation all the way down to the acoustic seabed footprint. Main conceptual differences between ship-based and AUV-based hydrography: Subsea navigation more challenging than on the surface. No GPS under water. Water level How to find the depth (d) from the surface to the sounding equipment. W d Sounding equipment Compensation for waves and swell. D Chart datum (K0) Operational height above the seabed. Relevant to e.g. ray-bending. D K0 = D + d - W DK0 Seabed B as ed on figure by the Norwegian Hydrographic S ervice / 4 /

3 Hydrographic Surveying with AUVs Increased data quality Stable platform Low platform noise. Acoustic synchronization of sensors Possibility to operate below difficult water layers and to optimize altitude and stand-off Increased mapping resolution Advanced sensors are brought in optimal position for detailed seabed mapping Fine track-control and short turns Increased mapping efficiency compared to tow-fish and ROV. Comparable to surface vessel in shallow water Simultaneous recording of full geophysical sensor suite and oceanographic data Only solution for demanding applications Deep water detailed surveys Under ice survey Naval Some missions can (must) be carried out autonomously / 5 / HUGIN 1000 Multi-Purpose AUV Navigation INS (inertial navigation system) Acoustic positioning (USBL, UTP) Surface GPS Pressure sensor DVL bottom-track DVL water-track Model aiding DPCA micronavigation Terrain navigation Feature based navigation Compass (for redundancy) Payload MBE SAS (synthetic aperture sonar) SSS (sidescan sonar) SBP (sub-bottom profiler) ADCP (acoustic Doppler current profiler) CTD (conductivity-temperature-depth) Fishery research instrumentation Optical camera Turbidity sensor Other FLS (forward looking sonar) Forward altimeter Downward altimeter Acoustic up and down links Radio link WLAN Iridium / 6 /

4 HISAS 1030 Multi Aspect Interferometric SAS Specifications: Range knots; knots; or 10 x altitude Image resolution better than 5x5 cm (theoretical: 2x2 cm) Typical SAS bathymetry resolution 10x10 cm multi-aspect capability Design: Phased-array transmitter-receiver (16 Tx/Rx elements) 2 receiver arrays per side (1.2 m, 32 Rx elements) Frequency and bandwidth configurable within 50 to 120 khz; khz typical configuration Prototype integrated on HUGIN in March systems built, 3 under production / 7 / HISAS 1030 Multi Aspect Interferometric SAS Comparable area coverage rate to surface ships in shallow water when AUV the is fitted with SAS about 2 km 2 /h. Resolution is depth and range independent. MBE HISAS Merged bathymetry / 8 /

5 HISAS 1030 Multi Aspect Interferometric SAS An AUV with SAS is well suited for detecting and classifying obstructions and features on the seabed. Extensive experience from naval operations. / 9 / HISAS 1030 Multi Aspect Interferometric SAS An AUV with SAS is well suited for detecting and classifying obstructions and features on the seabed. Extensive experience from naval operations. HISAS 1030 Range 80 m 3.9 m long boatwreck TileCam Altitude 3 m (high turbidity) Mosaic from 8 images / 10 /

6 HISAS 1030 Multi Aspect Interferometric SAS Resolution matters 400 khz multi-ping SSS Range m HISAS 1030 ( khz) Range m / 11 / Horizontal Mapping Accuracy HUGIN 3000 mapping a known wellhead at 1300 m depth (GoM in 2001) / 12 /

7 Horizontal Mapping Accuracy Horizontal DTM or chart accuracy depends on (not exclusively) AUV position uncertainties Position measurement accuracy Sound velocity profile in the case of USBL Survey vessel attitude and installation accuracy in the case of USBL Performance of the integrated navigation system. Ability to enhance the accuracy by estimating errors in the position measurements and in the aiding sensors Sounding position uncertainties Measurement accuracy Sound velocity profile AUV attitude accuracy Transducer mounting alignment relative to AUV attitude sensor What horizontal mapping accuracy can be expected from a state-of-the-art AUV system? HUGIN 1000 AUV with HISAS 1030 SAS used as case study / 13 / Horizontal Mapping Accuracy AUV position uncertainties (1σ) GPS (typical values) GPS SPS/PPS 1-5 m Currently 1.8 m on HUGIN DGPS < 0.5 m RTK m PPP (post-processing) Similar to RTK USBL (HiPAP 500) Slant range: < 20 cm Azimuth/elevation: 20dB 10dB 0dB / 14 /

8 Horizontal Mapping Accuracy AUV position uncertainties (1σ) In case of GPS-USBL, uncertainties are also attributed to: SVP errors Survey vessel attitude errors System installation errors Neither are in general observable for straight line motion (worst case scenario). AUV navigation system (typically aided INS) is however capable of estimating zero-mean time varying errors with faster dynamics than the AINS error drift e.g. high-frequency measurement noise. / 15 / Horizontal Mapping Accuracy AUV position uncertainties (1σ) Comprehensive aiding toolbox available to the HUGIN AUVs Kongsberg Maritime proprietary INS using Honeywell HG9900 IMU (0.8 nmi/h) NavP: in-situ navigation system NavLab: simulation and navigation post-processing sw including smoothing functionality / 16 /

9 Horizontal Mapping Accuracy AUV position uncertainties (1σ) Total horizontal AUV position uncertainty of GPS-USBL aided INS: Assumed that 80% of the USBL measurement error is white, and 95% of the surface ship GPS error is colored. AUV to surface ship offset: x = 50 m, y = 50 m Uncertainty parameters (all 1σ): HiPAP USBL SNR: 20dB Ship horizontal GPS: 0.2 m Ship attitude: 0.01 (r, p) and 0.1 (y) Transducer installation: 0.05 (r, p) and 0.1 (y) Sound velocity profile: 1 m/s GPS-HiPAP uncertainty Real-time AINS with GPS-HiPAP Smoothed AINS with GPS-HiPAP / 17 / Horizontal Mapping Accuracy Sounding position uncertainties (1σ) Measurement uncertainty of the HISAS due to: Position accuracy of the footprint itself (uncertainty in Γ and Φ). The position of the sounding within the footprint/resolution cell. The second factor is predominant / 18 /

10 Horizontal Mapping Accuracy Sounding position uncertainties (1σ) HISAS bathymetry resolution (ground-range independent): 16.9 cm along-track 22.5 cm across-track Uniform distribution 8 cm horizontal sounding position accuracy. Any errors in the transducer alignment or in the AUV AINS attitude induce a pointing error and hence also a sounding position error. An error in the sound velocity profile also causes a position error. / 19 / Horizontal Mapping Accuracy Sounding position uncertainties (1σ) Total horizontal sounding (relative) position uncertainty: Assuming flat seabed and 25 m AUV altitude. Uncertainty parameters (all 1σ): HISAS measurement: ~0.08 m AUV attitude: (r, p) and 0.04 (y) Transducer installation: 0.03 (r) and 0.1 (p, y) Deep water sound velocity profile: 1 m/s Shallow water sound velocity profile: 2 m/s HISAS bathy with shallow water SVP HISAS bathy with deep water SVP HISAS 1030 on HUGIN 1000 complies with the IHO special order for the full swath! More in a few slides / 20 /

11 Required Total Mapping Accuracy Minimum IHO Standards for Nautical Charts Area type Max total horizontal uncertainty (95%) Max total vertical uncertainty (95%) Feature detection (full seafloor search) Special order Areas where under-keel clearance is critical (usually not deeper than 40 m) 1a order Areas shallower than 100 m where under-keel clearance is less critical but features of concern to surface shipping may exist. 2 m 5 m + 5% of depth Depth dependent, e.g m, m Cubic features having dim > 1 m Depth dependent, e.g m, m Cubic features having dim > 2 m, in depths up to 40 m; 10% of depth below 40 m Special order vertical requirement a challenge to both surface ships and AUVs. Special order horizontal position accuracy achievable for AUVs. AUV will typically run with surface fix GPS or GPS-USBL positioning. Resolution and horizontal accuracy often more important than extreme absolute vertical accuracy (when not making nautical shallow water charts). / 21 / Achievable Horizontal Mapping Accuracy Total horizontal DTM position uncertainty (95%) Combined uncertainty from HISAS bathymetry and AUV positioning: Straight line motion (worst case): AUV GPS accuracy 1.8 m (1σ) Other parameters as in earlier slides GPS fix (15 min), shallow water SVP GPS-USBL (50 m AUV depth), deep water SVP GPS-USBL (50 m AUV depth), shallow water SVP GPS-USBL (500 m AUV depth), deep water SVP / 22 /

12 Vertical Mapping Accuracy Results from depth accuracy analysis in 1999: 0.26 DTM depth uncertainty (1σ) AUV depth: 300m AUV altitude: h = 30 m (blue) h = 50 m (green) h = 70 m (red) DTM DEPTH ACCURACY (m) MBE BEAM ANGLE (deg) In-depth analyses of vertical mapping accuracy currently being carried out for HUGIN 1000 with HISAS / 23 / Vertical Mapping Accuracy Final DTM or chart depth accuracy depends on (not exclusively) AUV depth accuracy Pressure sensor measurement accuracy Pressure distribution along the hull (sensor placement) Atmospheric pressure uncertainty UNESCO pressure to depth conversion, particularly the sensitivity to temporal and spatial variations in the density profile Dynamic pressure field due to waves or swell (hydrostatic conditions in UNESCO) Sounding depth accuracy Sounding equipment measurement accuracy Ray-bending and the sensitivity to temporal and spatial variations in the sound velocity profile (SVP) AUV attitude accuracy Transducer mounting alignment relative to AUV attitude sensor Depth reduction accuracy Determination of water level relative to the desired datum Datum uncertainty / 24 /

13 Vertical Mapping Accuracy Example of Wave Induced Pressure Fields in Shallow Water Pressure NavLab post-processed sensor (AINS) Water depth: 17 m AUV depth: 6 m Wave length of the swells causing the oscillations: ~100 m -Depth [m] Time [s] / 25 / Conclusions There is an increasing interest to use AUVs traditional hydrographic surveys, e.g. for creating nautical charts. Achievable DTM mapping accuracy eminent to the end-client. An in-depth analyses of the different components contributing to the horizontal DTM position accuracy have been carried out, using the HUGIN 1000 AUV fitted with HISAS 1030 as a case study. State-of-the-art AUV systems with appropriate acoustic positioning and SAS can be made to comply with the IHO S-44 Special Order in terms of total horizontal position uncertainty. For AUV systems with GPS surface fix as the only external positioning there is still some way to go, though satisfying the requirements of the IHO S-44 Special Order seems feasible. Ship can carry out independent work (e.g. force multiplication). In-depth analysis of vertical accuracy underway. / 26 /

14 Kongsberg Maritime / 27 /