Alternative Navigation for Underwater Vehicles Using Gravity

The Challenge

Unmanned Undersea Vehicles or Autonomous Under Water Vehicals (UUVs or AUVs) are increasingly used for tasks ranging from seabed mapping to defence surveillance and offshore energy inspection. Yet these platforms face a persistent challenge: navigation without GPS / GNSS.

Standard inertial navigation systems (INS) drift over time - especially during long-duration missions or when acoustic updates (e.g., USBL, DVL) are unavailable or unreliable. This poses critical limits on positioning accuracy for mission planning, sensor alignment, and data correlation.

Why Gravity?

The Earth’s gravity field is varies significantly at local and regional scales due to subsurface geological differences. This variability can be turned into a navigational advantage. By comparing gravity measurements with a known gravity map, AUVs can localize their position with improved accuracy—even in the absence of external signals. This is akin to using elevation data to confirm your position when mountaineering. While elevation alone won't pinpoint your exact location, comparing it with a detailed topographic map improves navigational accuracy.

This method, known as Gravity-Aided Inertial Navigation (GAIN), enables:

• Reduced inertial drift over time
• Passive, non-radiating navigation
• Enhanced mission autonomy in GNSS-denied environments
• Navigation in deep water or areas with limited acoustic access
• Gravity is particularly valuable for long-range missions

Recommended Approach

The biggest challenge for Gravity-Aided Inertial Navigation (GAIN) is achieving high-accuracy, low-drift measurements in a form factor that is suitable for autonomous mobile platforms. A high-accuracy gimbal is also essential to align the sensor precisely with the gravity vector - ensuring measurements remain independent to the roll and pitch of the vehicle. High-resolution resonant MEMS gravimeter technology presents a unique opportunity to reduce the Size, Weight, and Power (SWaP) requirements compared to traditional gravimeter systems, while still meeting accuracy requirements. SMGs advanced gimbal technologies ensure our sensor has minimal tilt errors no matter the position of the vehicle. Further unique features of resonant MEMS design also offer potential for active drift correction in the field.

When integrated with an onboard INS our gravity technology enables the vehicle to detect small changes in gravitational acceleration along its route, which can be continuously used to improve navigational accuracy and reliability.

• In-situ gravity measurements are continuously logged during transit
• High accuracy readings can be made when vehicle is stationary or has grounded / landed on stable ground
• These measurements are compared to a pre-surveyed gravity reference map (from satellite, shipborne, or gradiometer data)
• Kalman filters or similar algorithms fuse the gravity data with inertial estimates to improve location

Key considerations:

• Gravimeter resolution: ideally <5 µGal for meaningful corrections
• Gravity map resolution: the higher the better
• Platform integration: compact, low-power sensors like the physics core of GAIA-FIELD offer integration potential with underwater nav stacks
• High-accuracy gimbal: strapdown solutions correct for known tilt errors but high-accuracy gimbals significantly reduce compensation errors

Learn More

Contact us to discuss how gravity-based navigation can help transform the operation of your autonomous systems in subsea, GNSS-denied, and sonar quiet environments.

Applications include:

• Defence & ISR AUV navigation
• Offshore inspection and construction
• Scientific seafloor mapping
• Exploration under ice sheets or in complex subsea terrains

Contact our team to discuss integration options or partner on a trial deployment.