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Objectives
The aim of this project is to investigate the feasibility of high data rate communications from satellite to autonomous underwater vehicles (AUVs) via laser communication. This requirement is in response to the increasing number of AUV and underwater devices being deployed and the requirement for greater capacity information transfer underwater.
Currently available underwater communications via acoustic and RF technologies are suited to long range, low data rate and short-range, high data rate respectively. Additionally, both acoustic and RF technologies do not offer communication channels through the air-water interface. By exploring the opportunities offered by optical communications technology there is potential for long range, high data rate communications through the air-water interface, exploiting to the relatively low attenuation of blue-green light through both the atmosphere and water.
To evaluate the feasibility of bi-directional optical communication links between LEO satellites and AUVs the programme investigated the technical, environmental, and operational challenges of transmitting optical signals through the combined atmospheric–underwater channel. Activities included use case development, a state-of-the-art review, technical specification generation, link budget modelling, feasibility and trade-off analysis, and development of a baseline system concept and recommendations for future development.
Challenges
The key challenges to achieving a successful optical link between space and underwater are:
- Sea water – scattering through water results in significant signal loss as well as spatial and temporal dispersion of the beam. In highly scattering coastal waters system performance is poor, especially in the uplink (AUV to satellite) direction.
- Solar background radiation – when aligned with angle of solar glint off the sea surface the satellite-based receiver will collect large amounts of solar radiation.
- Payload restrictions: AUVs have limited capacity for payload size and power.
System Architecture
A conceptual system architecture was defined that prioritised system performance in select environments over universal operation coverage, as well as ensuring compatibility to satellite and AUV platforms. The system consists of:
- Green (532 nm) pulsed diode-pumped solid state (DPSS) Nd:YAG lasers
- Pulse position modulation (PPM) with low density parity check (LDPC) forward-error correction
- Satellite-based downlink transmitter and large-aperture uplink receiver
- AUV-based compact downlink receiver and high-energy uplink transmitter
- Use of a satellite constellation to ensure recurring link opportunities
Modelling results show that the system can operate bidirectionally in clear waters (Jerlov IB to III) down to 70m depth. Reaching high downlink data rates (up to 400kbps) and lower uplink data rates (up to 40kbps) enabling data transfer sizes, per satellite pass, between 1–15 MB (downlink) and 0.1–1.5 MB (uplink), depending on environment. However, in coastal water with high scattering conditions the communication system will not operate successfully.
Plan
The project ran between March 2025 and March 2026, during which four main technical areas were explored:
- Identification AUV use cases and review of current communication methods
- Generation of technical requirements for space to underwater communications
- Feasibility study and trade off analysis of potential technical solutions
- Evaluation of identified technical solutions
Current Status
The feasibility study is complete and has demonstrated that space to underwater optical communications are scientifically viable and technologically promising, particularly for downlink applications in clear, deep water. However, the uplink faces fundamental physical limitations that currently restrict applicability in coastal environments and therefore market readiness.
The technology readiness level (TRL) has been assessed to be at level 2, to increase TRL the following activities are recommended:
- Identify if there is commercial demand for the technology or if further development required.
- Develop a detailed model to represent the environment and system.
- Demonstrate proof of concept in representative environments.
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