Sonardyne Sprint vs iXblue Rovins: Choosing the Right Subsea INS for Your ROV

Inertial navigation systems have become standard equipment on survey-grade ROV spreads, and two systems dominate the market: Sonardyne's Sprint-Nav (integrating Sprint INS with a DVL and altimeter) and iXblue's Rovins (a standalone INS unit designed for ROV integration with external DVL and USBL aiding). Understanding the technical differences between these systems matters for pilots, survey engineers, and project managers who need to specify the right system for a given job — and who need to understand what the navigation output actually means in terms of positional uncertainty.

Core INS Technology: FOG vs RLG

The Sprint uses a fibre-optic gyroscope (FOG) based inertial measurement unit. The Rovins uses an FOG-based unit as well, but iXblue's FOG technology is proprietary — iXblue manufactures their own gyroscope fibers in-house, which they argue gives them tighter performance consistency between units. In practice, the gyroscope noise floor of both systems is similar: both achieve heading accuracy of approximately 0.01 degrees RMS when stationary and aided, and attitude accuracy of 0.01 degrees RMS roll and pitch. The difference between the systems becomes apparent in extended free-inertial (unaided) performance and in the way each system handles aiding source transitions — the behavior when USBL or DVL is lost and then re-acquired differs between platforms and has operational significance on jobs with intermittent aiding.

USBL Aiding Integration

• Sprint-Nav integrates natively with Sonardyne's own USBL systems (Ranger 2, AvTrak 6) through the Sonardyne Lodestar AHRS/USBL interface — this tight integration allows the Sprint to apply USBL updates with minimal latency and with proprietary calibration factors that account for vessel motion
• Rovins integrates with any USBL system via standard NMEA or binary position input, making it hardware-agnostic — this is a genuine advantage on spreads using Kongsberg cNODE or Teledyne Benthos transponders where the Sonardyne ecosystem integration is not available
• Sprint's USBL aiding rejection algorithm is more conservative than Rovins' — it will reject a USBL fix that deviates by more than a configurable threshold from the INS prediction, which reduces the impact of multipath-corrupted fixes but can cause the system to run unaided for longer in poor acoustic conditions
• Rovins applies a looser USBL acceptance window by default, which means it will incorporate more fixes including some that may be multipath-affected — the result is a position that tracks USBL more closely but with more high-frequency noise
• For deepwater operations below 1500m where USBL accuracy degrades, both systems are effectively equivalent because the USBL contribution to position solution becomes secondary to DVL-aiding quality

DVL Integration and Velocity Aiding

DVL bottom-track velocity is the primary external aiding source for both systems during normal ROV operations. The Sprint-Nav integrates a Sonardyne DVL within the same pressure housing, with a fixed and factory-calibrated lever arm between the DVL transducer array and the IMU — this eliminates the lever arm survey step that is required when integrating a standalone Rovins with an external DVL. The iXblue integration with a Teledyne Workhorse Navigator or RDI DVL requires a lever arm measurement during installation calibration, and any change to the DVL installation position (after a hard landing or following maintenance that disturbs the mounting frame) requires a re-calibration. In practice, Sprint-Nav integration is faster to commission on a new spread, while Rovins with a separately mounted DVL is more flexible for non-standard vehicle configurations where the DVL must be offset from the vehicle center.

Accuracy Specifications in Operational Terms

• Both systems specify position accuracy as a function of distance traveled when operating in DVL-aiding mode without USBL — Rovins achieves approximately 0.1% of distance traveled (1 m CEP per 1000 m traveled), Sprint-Nav achieves a similar specification
• In USBL-aided mode with good acoustic geometry, both systems achieve absolute position accuracy limited by the USBL system performance — typically 0.2-0.5% of slant range CEP
• Heading accuracy while underway (dynamic heading) is where Rovins has a documented advantage — iXblue's FOG technology achieves slightly lower heading drift under dynamic conditions, which translates to better pipeline tracking accuracy on survey runs
• The practical accuracy difference between Sprint-Nav and Rovins on a typical pipeline survey is smaller than the uncertainty introduced by suboptimal USBL geometry, acoustic multipath, or vehicle navigation behavior — system selection should not hinge on sub-centimeter specification differences
• For structure-of-interest approach positioning (within 5m of target), both systems are equivalent — the critical variable is the calibration quality of the aiding geometry, not the INS hardware

Calibration Requirements

• Sprint-Nav requires a 'figure-of-eight' calibration dive to align the DVL to the IMU frame when first commissioned — this dive takes approximately 45 minutes and should be repeated if the unit is removed from the vehicle
• Rovins requires a full lever arm survey (IMU to DVL, IMU to USBL transponder, IMU to vehicle CRP) before first use and after any component position change — surveying these offsets with a total station is recommended for positions where error is greater than 5 cm
• Both systems require a 'coarse alignment' period after power-on before they deliver full navigation accuracy — Rovins achieves full alignment in approximately 5 minutes stationary; Sprint-Nav achieves alignment faster (approximately 2 minutes) due to the integrated design
• Alignment at depth after deployment requires a minimum period of straight-and-level vehicle motion — sudden direction changes during the alignment window increase heading error; pilots should be briefed not to execute heading changes until the system reports 'aligned'
• Full ZUPT (zero velocity update) calibration should be performed during the alignment window — instruct the pilot to hold the vehicle stationary for a minimum of 90 seconds after reaching working depth before beginning navigation-critical tasks

Field Reliability and Practical Considerations

Sprint-Nav has a strong reliability record in deep water, particularly in applications where the integrated unit design reduces the number of subsea connectors in the signal chain. The most common field issue with Sprint-Nav is the DVL bottom-track loss at altitude above 100m — the Sprint uses a 600 kHz DVL that has a maximum bottom-track range of approximately 80-100m depending on bottom type. Rovins installations using a 150 kHz external DVL (such as the Teledyne Navigator 150) extend bottom-track range to 400m, which is decisive on operations above open-water-column segments of pipeline routes. For the typical deepwater IMR or construction support job, this altitude range difference is irrelevant — but for pipeline route surveys crossing steep topography with significant altitude variation, the DVL frequency choice can determine whether INS-aided navigation is achievable on certain route segments.

Which System for Which Job