WAYPOINT Navigational data risk: a question of integrity

30 Jan 2023 The Navigator

George Shaw from the Royal Institute of Navigation looks at how seafarers can balance accuracy and integrity in positioning

Risk evaluation for safe navigation is underpinned by mariners’ awareness of positioning data quality, allowing them to maintain situational awareness and avoid hazards. Good positioning is characterised by accuracy, availability, continuity over an operation and integrity. Appreciation of position accuracy is inherent in good seamanship, but integrity is harder to assess. Yet integrity is crucial as it alerts the seafarer to the possibility of misleading information.

What is integrity?

Integrity is the measure of the (generally small) likelihood of position errors that are unacceptably large but which do not raise an alarm – what you might call ‘hidden errors’. It is critical for risk assessment and forms the basis for positioning uncertainty, bridge alerts and appropriate data use in digital services (e.g. under-keel clearance). Accuracy and integrity are interdependent, and must be carefully balanced to to meet the seafarer’s needs in increasingly complex sea spaces.

Vessels principally use GPS navigation, which is accurate to within a few metres. Regional space-based augmentation systems (SBAS) or differential GPS beacons can improve accuracy to one to two metres, providing integrity warnings if the GPS service malfunctions. However, existing SBAS systems have been designed specifically for aviation. There is currently no service integrity ‘guarantee’ for GPS maritime positioning.

Electronic position data is extremely precise, even when in error, and is very compelling when portrayed on ECDIS without any alarm. This invokes a false sense of security if integrity is poor. The use of multi-constellation GNSS capability (including Galileo, Glonass or Beidou) should enhance accuracy, availability and continuity of positioning, but makes questions of integrity even more complex.

Maritime SBAS services designed to support integrity for GNSS positioning solutions are not straightforward. This is because GNSS signal reception is more challenging for ships than for aircraft. Receiver Autonomous Integrity Monitoring (RAIM) automatically measures integrity and alerts users if there is a problem, but it needs improvement, specifically in adapting the methods that are being evolved for aircraft for maritime use.

Mitigating risk

GNSS are vulnerable to interference, either naturally or via deliberate jamming and spoofing and this may cause unacceptable undetected positioning errors. Improved accuracy with SBAS or Precise Point Positioning (PPP) cannot detect or counter these effects and may even make the problem worse, as it increases trust without necessarily increasing safety. While current technology cannot provide adequate integrity protection, seafarers must be able to mitigate navigational risk by maintaining traditional skills and frequently cross-checking different positioning sources. Established triangulation methods are still very valuable to the navigator. A ‘cocked hat’ of bearings provides an intuitive ‘pool of errors’ for position uncertainty – although remember that if you have just three sightings, the true position is likely to lie outside the indicated area of intersection.

In future, positioning integrity would benefit from a wide variety of independent sensors, not all space-based, deployed in a ‘system-of-systems’ solution with maritime specific RAIM. As autonomous vessels emerge, and more and more obstacles (e.g. wind turbines) fill crowded waters, initiatives like these are growing urgent. Mariners are encouraged to insist on resilient positioning capability to provide accuracy with integrity, and advocate the necessary changes in standards and type approval for multisystem, multi-constellation bridge equipment.

Contact RIN at: www.rin.org.uk | 1 Kensington Gore, London, SW7 2AT | Tel: +44 (0)20 7591 3134