GNSS Wave Glider: A new tool for sea level and sea state measurement

Variations in sea level have a great environmental impact. They modulate coastal deposition, erosion and morphology, regulate heat and salt fluxes in estuaries, bays and ground waters, and control the dynamics of coastal ecosystems. Sea level variability has importance for coastal navigation, the building of coastal infrastructure, and the management of waste. The challenges of measuring, understanding and predicting sea level variations take particular relevance within the backdrop of global sea level rise, which might lead to the displacement of hundreds of millions of people by the end of this century.

Sea level measurement relies primarily on the use of coastal tide gauges and satellite altimetry. Tide gauges provide sea levels at fine time resolutions (up to one second), but collect data only in coastal areas, and are irregularly distributed, with large gaps in the southern hemisphere and at high latitudes. Satellite altimetry, in contrast, has poor time resolution (ten days or longer), but provides near global coverage at moderate spatial resolutions (10-to-100 kilometres). Altimetric sea level products are problematic near the coast for reasons such as uncertainties in applying sea state bias corrections, errors in coastal tidal models, and large geoid gradients. The complicated shoreline geometry means that the raw altimeter data have to either undergo special transformations to provide more reliable measurements of sea level or be rejected.

Developments in GPS measurements from buoys are now making it possible to determine sea surface heights with accuracy comparable to that of altimetry. In combination with coastal tide gauges, GPS buoys could be used as the nodes of a global sea level monitoring network extending beyond the coast. However, GPS buoys have several downsides. They are difficult and expensive to deploy, maintain, and recover, and, like conventional tide gauges, provide time series only at individual points in the ocean. This proposal focuses on the development of a unique system that overcomes these shortcomings.

We propose a technology-led project to integrate Global Navigation Satellite Systems (GNSS i.e. encompassing GPS, GLONASS and, possibly, Galileo) technology with a state-of-the-art, unmanned surface vehicle: a Wave Glider. The glider farms the ocean wave field for propulsion, uses solar power to run on board equipment, and uses satellite communications for remote navigation and data transmission. A Wave Glider equipped with a high-accuracy GNSS receiver and data logger is effectively a fully autonomous, mobile, floating tide gauge. Missions and routes can be preprogrammed as well as changed remotely. Because the glider can be launched and retrieved from land or from a small boat, the costs associated with deployment, maintenance and recovery of the GNSS Wave Glider are comparatively small.

GNSS Wave Glider technology promises a level of versatility well beyond that of existing ways of measuring sea levels. Potential applications of a GNSS Wave Glider include: 1) measurement of mean sea level and monitoring of sea level variations worldwide, 2) linking of offshore and onshore vertical datums, 3) calibration of satellite altimetry, notably in support of current efforts to reinterpret existing altimetric data near the coast, but also in remote and difficult to access areas, 4) determination of regional geoid variations, 5) ocean model improvement.

The main thrust of this project is to integrate a state-of-the-art, geodetic-grade GNSS receiver and logging system with a Wave Glider recently acquired by NOC to create a mobile and autonomous GNSS-based tide gauge. By the end of the project, a demonstrator GNSS Wave Glider will be available for use by NOC and the UK marine community. The system performance will be validated against tide gauge data. Further tests will involve the use of the GNSS Wave Glider to calibrate sea surface heights and significant wave heights from Cryosat-2.

SEA LEVEL SCIENCE COMMUNITY. Engineers and scientists working on sea level problems will be the main beneficiaries of this project. The proposal has connections with funded projects that have aimed at improving the quality of altimeter data along the coast, such as ALTICORE, COASTALT/COASTALT-2, and PISTACH. The project is also of interest to the ELLIP-C community, that is creating a database of ellipsoidal heights of mean sea level at the coast, and indeed to any other projects for which the accurate determination of sea level and sea level variability is a requirement, for example, the recently NERC-funded project "Weighing the Ocean".

We will reach the scientific community through publications in the peer-reviewed literature, and by attending conferences and workshops. Our plans for the GNSS Wave Glider will be presented in the Challenger Conference 2012, and further progress will be reported at at least two international conferences in the final year of the project. For the final project workshop we will convene a specialist 2-day meeting on sea level observation, to which we will invite academics and stakeholders.

OCEAN OBSERVING COMMUNITY. We expect that the Wave Glider technology will make a substantial contribution to future GOOS (Global Ocean Observing System), GLOSS and SOOS (Southern Ocean Observing System) observation programmes. As the site of the Permanent Service for mean Sea Level, NOC maintains strong links with GLOSS. Future GLOSS needs that could be met with GNSS Wave Glider technology (such as providing near real time sea level data from remote locations for assimilation in operational ocean models) will be explored and discussed in our end-of-project workshop. Likewise, NOC contributes to SOOS through our sustained observations in Drake Passage. The Wave Glider has potential as a platform from which observations could be conducted in this remote area.

ENVIRONMENTAL AGENCIES. Agencies such as the UK Climate Impacts Programme, the Met Office, Defra, the Environment Agency, the Scottish Environment Protection Agency or the Chagos Conservation Trust have an interest in sea-level monitoring. We will inform these agencies of our activities and will seek their advice as to possible ways of utilising our technology for measuring and monitoring applications of interest to them. The PI and co-Is will have the task of liaising with these institutions.

INDUSTRY. The project will develop an innovative technology that will eventually be exploited commercially. Two strong industrial partners will be involved in the project providing advice, expertise and in-kind resources, such as the Liquid Robotics European Wave Glider facility. In addition to e-mail exchanges and video-conferences with these partners, we plan to hold annual meetings with them at the ocean technology fairs Ocean Business and Oceanology International.

GENERAL PUBLIC. We will engage the general public through magazine articles and lectures. The British Science Festival will take place in Newcastle in September 2013. We will apply to host a workshop at this festival to highlight the key role of high-end technology in the present and future of ocean science and ocean exploration.

Prior to our work at Holyhead, we will organise classroom visits to local schools targeting students who are at the stage of choosing their GCSE subjects, encouraging them to choose science. Visits of school groups to our tide gauge site at Holyhead will be organised to witness our Wave Glider trials.

We will seek media opportunities to explain the importance of our science to the general public and to advertise NERC science to the broadest possible audience. This will involve NOC's and Newcastle's PR departments to assist with developing media opportunities.

Project results will be publicly available through dedicated web pages. We will also operate a restricted web forum for discussion with the wider community