Why free-inertial performance matters for maritime autonomy 

Autonomous maritime platforms are being pushed to operate for longer, travel further and deliver reliable data in more demanding environments.

Whether conducting offshore surveys, inspecting critical infrastructure, supporting scientific research or running defence missions, today’s vehicles are expected to operate for longer, travel further and perform reliably across a wider range of environments than their predecessors. At the same time, operators want greater confidence in the navigation data their platforms depend on.

That means navigation systems need to do more. The inertial solution needs to be high performance and able to keep the platform on track when operating conditions change.

For me, three principles matter most: a strong inertial foundation, flexible use of aiding sources and the ability to trust the navigation solution in any operating environment.

Those principles sit at the heart of the SPRINT-Nav family. Within that family, SPRINT-Nav F and SPRINT-Nav FX are Sonardyne’s highest-performing hybrid navigators.

Free-inertial performance is the foundation

When discussing navigation systems, it’s easy to focus on the more tangible aids. GNSS, acoustics, Doppler velocity logs (DVLs), sonars and other technologies all have important roles to play.

But the real question is what happens between those measurements. The answer depends on the quality of the inertial solution.

A strong inertial core provides a continuous estimate of position, attitude and motion, allowing a platform to maintain navigation performance even when external measurements become intermittent, degraded or unavailable. The better that inertial performance, the less dependent the platform becomes on any individual aid.

This is the principle behind SPRINT-Nav F and SPRINT-Nav FX, two members of the SPRINT-Nav family. In free-inertial mode, SPRINT-Nav F limits position drift to one nautical mile after 24 hours, while SPRINT-Nav FX achieves the same level of drift after 72 hours (drift is defined as a position error CEP50 after 24/72 hours). That capability has practical implications.

For an autonomous surface vessel on a multi-day offshore inspection campaign, that can mean continuing the mission through changing sensor availability, intermittent acoustic updates or periods where preferred aiding sources are unavailable. A strong free-inertial solution gives the platform the resilience to keep operating through those transitions.

Aiding should be flexible, not fundamental

Historically, navigation systems have often been designed around a limited set of aiding sources. Today’s autonomous platforms need something more adaptable.

No single aiding technology works optimally in every operating environment. An underwater robotic vehicle working close to the seabed may rely heavily on DVL measurements. A surface vessel undertaking a long transit may benefit from different velocity or position aids. Infrastructure inspection, defence and scientific missions can all present unique navigation challenges.

That is why the SPRINT-Nav F and SPRINT-Nav FX members of the SPRINT-Nav family have been designed to work with a deliberately broad range of aiding inputs: external sources of position, velocity or range information that help refine the inertial solution. The navigation filter also can ingest GNSS, USBL, LBL and sparse acoustic arrays.

The key point is that a navigation system should not be constrained by the availability of any single aid. Instead, it should be capable of intelligently incorporating whichever information sources are available to improve the overall solution.

When the inertial core is strong enough, aiding becomes a way to enhance performance rather than something the platform depends on to maintain it.

Navigation integrity matters as much as navigation accuracy

Accuracy will always be important. But as autonomy becomes more widespread, operators need more than accurate position data. They need confidence that the navigation information they are receiving can be trusted.

That means understanding not only where a platform is, but whether the position estimate can be trusted.

This is where high-performance inertial navigation and tightly coupled DVL integration become particularly valuable. By maintaining a trusted estimate of platform motion, the navigation filter can compare incoming measurements with its own prediction of where the platform should be. Measurements that do not match that prediction can then be identified, rejected and flagged to the operator.

The result is a navigation solution that does more than simply process data. It continuously assesses the quality of the information it receives and protects the integrity of the overall solution.

For operators deploying autonomous systems in increasingly demanding environments, that confidence can be just as important as absolute positioning performance.

One navigation architecture, many missions

These principles apply across a broad range of applications. The same navigation architecture can support offshore energy surveys, ocean science missions, autonomous surface vessel operations, infrastructure inspection and defence activities. It is equally relevant above and below the waterline.

While the missions themselves may be very different, the underlying requirement is consistent: platforms need a navigation solution that remains accurate, resilient and trustworthy regardless of the operating environment.

That’s why I believe the conversation around navigation is changing. The most important question is no longer what aiding sources a platform can use. It’s how capable the navigation system remains when those aids change, disappear or evolve.

By combining strong free-inertial performance, flexible aiding and robust integrity monitoring, the SPRINT-Nav Family provides a navigation foundation for the next generation of maritime autonomy, with SPRINT-Nav F and SPRINT-Nav FX offering the highest levels of performance within that Family.