Don’t Take Your Satnav Position for Granted
20 09 2023

NLAI Associate Director Capt Phil Buckley highlights the need to improve maritime navigation system integrity.

The Blue Economy is underpinned by an oft overlooked but critical capability. Since the very first humans set out to sea the ability to understand position and time has allowed safe navigation. Local knowledge and familiarity with coastal terrain was supplemented by advances in knowledge and then technology. Astronavigation, the compass, Harrison’s clocks – all steps in the evolution of our ability to be increasingly precise about where and when we are whilst at sea. Space technology now gives us what many call GPS (Global Positioning System) or GNSS (Global Navigation Satellite System) and many of us, on land and sea, put our unwavering faith in this solution.  

As humans advance their Blue Economies, we are deploying more infrastructure at sea. Offshore wind has grown rapidly over the last 20 years. Aquaculture fish farms are now the source of more fish protein than wild caught fish. We ship more goods to more ports than ever before. Autonomous vessels are becoming more prevalent. The seas have never been busier or more complex and so it is crucial that we ask the question – to what extent can users really trust the position, speed, heading and time displayed by their electronic chart or ‘satnav’?  

Resilient positioning systems face numerous challenges at sea. Signal interference can be caused by natural phenomena such as ionospheric scintillation and geomagnetic storms. These disturbances can disrupt the signals received by GNSS receivers, leading to degraded accuracy or complete loss of positioning information.  

Another challenge is the vulnerability of GNSS signals to intentional or unintentional interference. Malicious actors can jam or spoof GNSS signals, causing ships to lose their position or be misled into dangerous areas (a recent example of this took place in the waters off Crimea in May). Additionally, harsh weather conditions and saltwater corrosion pose challenges in maintaining the physical infrastructure of resilient positioning systems.  

How confident, then, should users be when making decisions based on this information? In other words, what is the integrity of the system? 

 Autonomy economy 

In each of the growing sectors named above, significant investment is being poured into robotics and autonomous systems that offer reductions in cost and/or improvements in safety at sea. In the offshore wind market, for example, a UK-Singaporean consortium recently received grant funding to develop an autonomous system that can inspect offshore wind farms without human intervention. Drones and advanced imaging technology will detect and assess any damage or maintenance needs on the wind turbine monopiles and jackets. A similar commercial pilot project was set to begin offshore in Germany in July. The project aims to develop and test drones and unmanned surface vehicles (USVs) to perform various tasks such as inspections, maintenance, and data collection for the operation and maintenance of the Deutsche Bucht offshore wind farms.  

It’s not just on and under the water that such systems are providing value. Uncrewed Aerial Vehicles (UAVs) are also beginning to flourish, especially to support aquaculture and fisheries operations. Applications here include monitoring fish populations, assessing water quality and detecting potentially illegal fishing activities. They can collect real-time data, such as water temperature, salinity and pH levels, to aid decision-making processes. Drones equipped with cameras and sensors can identify fish species, estimate their sizes, and even track their migration patterns. Additionally, autonomous drones are capable of patrolling and protecting aquaculture farms, identifying predators, and minimizing the risk of disease outbreaks.  

Such innovative approaches promise to improve efficiency, reduce costs, and enhance safety. However, the continued rise of autonomous systems puts even more pressure on the need for reliable navigation systems.    

In April, for example, an Inmarsat I-4F1 satellite outage occurred in the Asia-Pacific region, leading to disruptions in GPS services for farmers and for some maritime safety systems. The outage was said to have affected farmers who rely on GPS technology for precision agriculture, such as monitoring crops, guiding farm machinery, and optimizing resource usage – all activities that marine and maritime operators are replicating across the Blue Economy. The outage also affected the Global Maritime Distress and Safety System and other communications at sea. 

INSPIRing thought and action 

An 18-month project led by Tailor Airey is exploring integrated navigation as a system of systems for PNT resilience (INSPIRe) to define practical measures that could be implemented to quantify the level of confidence a user should have in their PNT solution. INSPIRe is funded by the European Space Agency’s Navigation Innovation and Support Programme(NAVISP) and builds on the earlier NAVISP-funded MarRINav project led by Blue Economy solutions company NLA International.  

An integrity measure will tell the user if their PNT data is sufficiently reliable to be used for the manoeuvres in progress at the time and warn the user if it is not. GNSS integrity is seen to have two components: system level integrity and user level integrity. System integrity would indicate any compromise in the raw signals transmitted by the satellites and user level integrity would detect signal degradation at the point of use caused by local factors such as multipath reflections from port infrastructure. 

Integrity should not be confused with accuracy: an imprecise position may in some circumstances be useful if integrity measures can reliably assure the user that the true position lies within a defined boundary to a defined confidence level. 

We know of examples from the automotive sector where positioning systems provide dangerously false assurances that location accuracies are within defined limits. A critical requirement for INSPIRe is that integrity indications must themselves be trustworthy.  

An engaging approach 

Resilient positioning systems emerging within the maritime industry include the Differential Global Positioning System (DGPS), which uses a network of ground-based reference stations to correct the errors in GNSS signals. DGPS improves positioning accuracy, making it suitable for applications that require precise positioning, such as hydrographic surveying and offshore construction. 

Building and maintaining resilient positioning systems require a comprehensive approach that encompasses technology, infrastructure, and human resources. Thus, the INSPIRe project is also studying RAIM (Receiver Autonomous integrity Monitoring) algorithms for single and dual frequency GPS, and dual frequency multi-constellation (DFMC) GNSS. INSPIRe will also study EGNOS (European Geostationary Navigation Overlay Service) monitoring, RAIM availability prediction, a DFMC integrity monitor and feasibility of using signals of opportunity to improve integrity.  

The INSPIRe team have been proactive to engage with end users in the Blue Economy and maritime space. Stakeholders have confirmed that there is some awareness of the importance of integrity metrics needed to provide assurance that GNSS positions are safe to use. Many participants at a recent workshop could cite personal experience of GNSS outages. 

We are also working with the team who are studying a possible UK SBAS (Space-Based Augmentation System) to ensure that INSPIRe is properly aligned to that project. 

We all know and love the blue circle depicting error limits on Google Maps, but typically the area encompassed by a particular limit will be a more complex shape such as an ellipse or “cocked hat”. Mariners we consulted said they would find a spatially accurate representation of position error bounds useful to complement a simple ‘traffic light’ indication. 

A system to improve integrity must provide value to the user, so the project will perform a cost benefit analysis to quantify this and will propose routes to implementation.  

A call to action 

In conclusion, resilient positioning systems are fundamental to the safe and efficient operation of the maritime industry. By providing accurate positioning and timing information, these systems enable a wide range of maritime activities, from commercial shipping to offshore exploration. Through the integration of diverse technologies and the implementation of robust infrastructure, resilient positioning systems can overcome the challenges posed by the maritime environment and ensure the reliability and integrity of navigation and timing at sea. Embracing resilient positioning systems is not only essential for the safety and efficiency of the maritime sector but also for the sustainable and profitable growth of the blue economy. 

We would always like to hear from those with an interest in PNT integrity and the value of accurate integrity information, or views on how that information should be provided to users. If you would like to get involved or to sign up for project newsletters, please contact us through the INSPIRe website. To learn more about the role importance of PNT in Blue Economies write to 

This article was first published in the Institute of Chartered Shipbrokers’ Shipping Network magazine.  Captain Phil Buckley is a retired Royal Navy submarine Commanding Officer and former harbour master at Jersey and Southampton. He is currently an Associate Director at NLA International, assisting with Blue Economy solutions for a range of maritime authorities. The article is written on behalf of the INSPIRe stakeholder engagement work package team.

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