Autonomous Underwater Vehicle

Rationale: Deep-sea survey operations require extended autonomous operation because acoustic communication bandwidth (1kbps) is insufficient for real-time piloting, and surface vessel time is the primary cost driver at £25k/day. 24-hour minimum enables single-dive coverage of typical survey blocks (10km x 2km at 3 knots).

Rationale: AUV replacement cost exceeds £2M and loss at 6000m depth makes recovery impractical. Autonomous surfacing is the primary loss-prevention mechanism. The 120-second threshold ensures the vehicle begins ascent before cascading faults can disable the emergency systems. This is the single most critical safety requirement for any untethered deep-sea vehicle.

Rationale: Primary end users are marine scientists and hydrographic surveyors who require IHO S-44 Order 1 compliant bathymetry and georeferenced imagery for habitat mapping, infrastructure inspection, and geological survey. Data that cannot meet publication standards has no value — the entire mission cost is wasted.

Rationale: AUVs operate from vessels of opportunity — research vessels, offshore supply vessels, and naval auxiliaries. Requiring specialised launch equipment limits operational availability and increases mobilisation costs. A-frame deployment with standard rigging is the industry baseline for vehicles under 500kg.

Rationale: Operations in marine protected areas and environmentally sensitive sites require compliance with NOAA/NMFS acoustic exposure guidelines. Thruster noise and low-frequency sonar emissions are the primary contributors. Exceeding 180 dB SPL triggers marine mammal harassment thresholds under the US Marine Mammal Protection Act and equivalent EU regulations.

Rationale: Marine classification society approval is required for operation in international waters and by most research institutions. DNV-ST-0512 is the primary standard for autonomous underwater vehicles. Without classification, the vehicle cannot be insured or deployed from most research vessels.

Rationale: Research vessels operate on tight schedules with limited technical staff. AUV turnaround between missions must be achievable with the ship science party. Requiring specialist facilities or large teams for routine maintenance would severely limit operational availability during expedition cruises.

Rationale: The vehicle must operate in polar through tropical waters covering the full oceanographic temperature range. Deck storage on open vessels in Arctic or equatorial ports exposes the vehicle to extreme air temperatures. Sea State 4 is the practical limit for crane operations from typical research vessels and defines the minimum weather window for deployment and recovery.

Rationale: Power budget analysis: propulsion at 3 knots draws 400W, navigation sensors 80W, survey payload 150W, vehicle management 50W, comms 20W = 700W total. 24h × 700W = 16.8kWh gross, but with 15% abort reserve and 85% battery depth-of-discharge limit, the required installed capacity is approximately 10kWh usable from a 13kWh pack.

Rationale: Survey data georeferencing requires knowing vehicle position to within the resolution of the multibeam sonar footprint. At 100m altitude, the multibeam footprint is approximately 1m. Over a 24h mission at 3 knots the vehicle travels ~130km, so 0.1% DTT gives 130m drift — acceptable with periodic DVL bottom-lock and INS aiding, and within post-processing correction capability using terrain-relative navigation.

Rationale: The 120-second surface-and-activate-beacon requirement from STK-OPS-002 includes ascent time plus beacon activation. From 6000m, passive buoyant ascent at approximately 1m/s takes 100 minutes — far exceeding 120s. The 5-second initiation requirement ensures no delay is added by the safety system itself. Independence from the VMC is essential because the VMC may be the failed component.

Rationale: Software watchdogs can be defeated by the same fault that disables the VMC. A hardware watchdog on an independent microcontroller with its own power supply ensures that total VMC failure (hardware crash, power rail loss, software hang) always results in surfacing. The 60-second timeout allows for VMC reboot attempts while preventing extended uncontrolled descent.

Rationale: IHO S-44 Order 1 requires total horizontal uncertainty of 5m + 5% depth and vertical uncertainty of 0.5m at 95% confidence. At 100m altitude, a 400kHz multibeam with 120-degree swath covers approximately 200m width with 0.5m beam spacing. This resolution, combined with the 0.1% DTT navigation accuracy, satisfies the horizontal uncertainty budget for depths to 6000m.

Rationale: Data rate budget: multibeam at 50MB/s, side-scan at 30MB/s, 4K video at 100MB/s, CTD at 0.1MB/s = 180MB/s aggregate. 72h at 180MB/s = 46TB theoretical maximum, but with compression (4:1 typical for sonar) and duty-cycled camera operation, 4TB provides adequate capacity. 200MB/s write speed includes 10% margin over aggregate sensor rate.

Rationale: Standard oceanographic A-frames (e.g., on R/V class vessels) have a safe working load of 2-5 tonnes and a throat clearance of 3-5m. 350kg is well within the SWL including dynamic loading from sea state 4 conditions. The 4.5m length constraint ensures the vehicle fits within the A-frame width and can be handled on a working deck with standard rigging points.

Rationale: Post-mission or emergency surface recovery requires the support vessel to locate the AUV. Iridium SBD provides global coverage position reporting independent of vessel range. 5-minute interval balances power consumption against drift rate (surface currents typically 0.5-1 knot = 150-300m between reports). Xenon strobe at 2nm visibility is the COLREG standard for small vessel lights and enables visual acquisition in final approach.

Rationale: Marine mammal hearing sensitivity peaks between 10Hz-1kHz for baleen whales. Propulsion noise at 130 dB SPL at source attenuates to below harassment threshold (120 dB RMS for continuous noise per NOAA guidelines) within 3m. Multibeam operation above 100kHz is outside the hearing range of most cetaceans (upper limit ~80kHz for most species). Combined, these constraints enable operations in marine protected areas without triggering permitting requirements.

Rationale: 6000m depth rating covers 97% of the ocean floor, enabling full-ocean-depth survey capability excluding only the hadal trenches. The 1.5 safety factor on yield for Ti-6Al-4V is consistent with DNV-GL rules for submersible pressure vessels and provides margin for material variability, cyclic fatigue from repeated dive profiles, and manufacturing tolerances on wall thickness.

Rationale: A 24-hour mission cycle with deployment costs exceeding 50000 USD per ship-day demands high reliability. 2000 hours MTBCF provides less than 1.2 percent probability of critical failure per mission, consistent with mature AUV platforms such as Kongsberg HUGIN and MBARI LRAUV. This value drives component selection, redundancy architecture, and screening requirements.

Rationale: Pre-dive checks are mandatory in all operational AUV programmes to prevent deploying a vehicle with latent faults. The 120-second budget reflects the practical constraint of launch windows from research vessels where deck time is limited. Every safety-critical subsystem must be exercised because a latent fault in the emergency system could lead to vehicle loss.

Rationale: Seawater is a highly aggressive electrolyte. Dissimilar metal junctions, particularly titanium hull to aluminium fittings or stainless steel fasteners, create galvanic cells that cause rapid corrosion. MIL-STD-889 provides the accepted galvanic compatibility guidance. A 10-year service life reflects typical AUV fleet investment horizons and drives material selection for hull, fasteners, connectors, and fairings.

Rationale: The AUV houses sensitive acoustic receivers, magnetometers within the INS, and precision analogue front-ends for CTD in close proximity to a 250W BLDC motor drive switching at 20 kHz. Without EMC discipline, motor harmonics couple into sensor cables and degrade measurement quality. Internal EMC is the primary concern rather than external regulatory compliance since the vehicle operates far from other electronic systems.