This PhD project, hosted at the University of Southampton within the EPSRC Centre for Doctoral Training in Quantum Technology Engineering, focuses on advancing inertial navigation for GPS-denied environments such as underwater, space, and subterranean domains. The research aims to overcome navigational drift by integrating fast, drift-prone classical inertial sensors (gyroscopes and accelerometers) with highly stable quantum sensors. The student will develop and test fusion algorithms, explore sensor configurations, and validate system performance through both simulation and hardware-in-the-loop testing. The project will involve integrating a single-axis quantum sensor with classical 3-axis MEMS or optical-based inertial sensors, investigating how sensor orientation and configuration affect navigation accuracy. Multi-axis quantum setups will also be explored to further constrain drift. The research will be validated using real-world data, with a particular emphasis on marine applications. The student will gain expertise in hybrid navigation, sensor fusion, and real-time state estimation, learning to translate raw multi-sensor data into reliable navigation outputs. The project is supported by industry advisors, providing exposure to real-world challenges and professional networking opportunities. The position offers substantial training in scientific, technical, and commercial skills, preparing graduates for careers in autonomous navigation technologies. Funding is available on a competitive basis, with UK students eligible for a 4-year UKRI TechExpert stipend of approximately £31k per year, and studentships at the UKRI base rate available for EU, Horizon Europe, and international students. Overseas applicants with external funding are also encouraged to apply. Entry requirements include at least a UK 2:1 honours undergraduate degree or international equivalent. Applications should be submitted via the University of Southampton online portal, including a CV, two academic references, degree transcripts/certificates, and English language qualification if applicable. The university is committed to equality, diversity, and inclusivity, and welcomes applicants seeking part-time study. The application deadline for UK students is 31 July 2026, while international applicants must apply before 31 March 2026.