Touchdown monitoring is a fundamental requirement during subsea cable installation. Its primary engineering objective is to ensure the as-laid cable position is within the specified engineering corridor.

This process demands exceptional positional accuracy to avoid obstacles (such as boulders), prevent damage to seabed ecosystems, and record precise touchdown position data for future cable inspections.

Navigational challenges specific to touchdown monitoring 

The environment and the cable itself introduce critical challenges that must be overcome to maintain precision at the seabed interface:

• Magnetic interference: Subsea cables contain ferrous material (iron/steel) that interferes with instruments relying on magnetic sensors for heading, often rendering them extremely unreliable and ineffective when operating close to the cable.
• Shallow water acoustics: Achieving accurate positioning is often challenging when using traditional acoustic positioning methods in noisy shallow waters.
• Positional drift: Maintaining accuracy during extended monitoring missions requires continuous positional refinement to limit drift over time and distance.

SPRINT-Nav for precision output 

The SPRINT-Nav family is engineered to provide continuous, high-performance navigation specifically addressing these challenges:

• Integrated hybrid system: SPRINT-Nav Mini fuses an inertial navigation system (INS), Doppler velocity log (DVL), attitude and heading reference system (AHRS) and depth sensors into a single, compact and lightweight instrument. This hybridization ensures fast, precise, and robust navigation.
• Magnetic field immunity: To counter the interference from ferrous cable material, the SPRINT-Nav family uses ring laser gyros (RLGs) and fibre-optic gyros (FOGs). These gyrocompasses determine heading by sensing Earth’s rotation, so they do not rely on magnetometers and are immune to magnetic interference. Read more in our case study.
• Factory calibration: All units are factory-calibrated (ready to go on deployment), eliminating time that would otherwise be spent performing on-site calibration manoeuvres.
• Depth sensing: An integrated pressure sensor provides depth data.

Deployment and integration

SPRINT-Nav provides precise subsea positioning of the cable as it meets the seabed when integrated onto the following platforms:

• Aiding system integration: The system is often coupled with Sonardyne’s Ranger 2 USBL (Ultra-Short Baseline) positioning system. Ranger 2 provides external acoustic position aiding, which is used to limit positional drift of the SPRINT-Nav’s inertial navigation system (INS) over extended monitoring periods, enhancing the overall accuracy. 

Operational performance metrics

The integration of SPRINT-Nav provides the following verified performance benefits for touchdown monitoring:

• Uninterrupted accuracy: Delivers uninterrupted pin-point accuracy for touchdown position calculation.
• Data reliability: Provides the necessary accurate heading, attitude, and positional data for reliable calculations of cable touchdown points.
• Validation: Client operational checks confirmed that the touchdown position provided by the SPRINT-Nav Mini was “spot on!” (find out what they said here).
• Compliance assurance: Real-time monitoring capability ensures the cable position is accurately known.
• Size/payload efficiency: The SPRINT-Nav Mini is highly compact (213 mm x 148 mm diameter) and lightweight (0.7 kg in water), minimizing negative impact on the manoeuvrability of dedicated monitoring assets like the CableFish

Read our SPRINT-Nav for cable lay blog

Learn how SPRINT-Nav ensures subsea cables are laid within the engineering corridor, avoiding obstacles, protecting seabed ecosystems and enabling accurate future inspections.