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

海平面的变化对环境有很大的影响。它们调节海岸沉积、侵蚀和形态，调节河口、海湾和地下沃茨的热量和盐分通量，并控制海岸生态系统的动态。海平面变化对沿海航行、沿海基础设施建设和废物管理具有重要意义。在全球海平面上升的背景下，测量、了解和预测海平面变化的挑战尤为重要，到本世纪末，全球海平面上升可能导致数亿人流离失所，海平面测量主要依靠沿海验潮仪和卫星测高。验潮仪以精细的时间分辨率（高达一秒）提供海平面，但只收集沿海地区的数据，而且分布不规则，在南半球和高纬度地区有很大的差距。相比之下，卫星测高的时间分辨率很差（10天或更长时间），但以中等空间分辨率（10至100公里）提供接近全球的覆盖范围。由于应用海况偏差校正的不确定性、沿海潮汐模型的误差和大地水准面梯度大等原因，测高海平面产品在海岸附近存在问题。复杂的海岸线几何形状意味着原始高度计数据必须经过特殊的转换以提供更可靠的海平面测量值，否则将被拒绝，全球定位系统浮标测量的发展现在使得有可能以与测高法相当的精度确定海面高度。全球定位系统浮标与沿海验潮仪相结合，可用作延伸到沿海以外的全球海平面监测网的节点。然而，GPS浮标有几个缺点。它们的部署、维护和恢复困难且昂贵，并且像传统的潮汐测量仪一样，只能提供海洋中单个点的时间序列。我们提出了一个以技术为主导的项目，将全球导航卫星系统（GNSS）技术与最先进的无人驾驶水面飞行器（Wave Glider）相结合。滑翔机利用海浪场作为推进力，利用太阳能在机载设备上运行，并利用卫星通信进行远程导航和数据传输。配备高精度GNSS接收器和数据记录器的Wave Glider实际上是一个完全自主的、移动的浮动验潮仪。任务和路线可以预先编程以及远程更改。由于滑翔机可以从陆地或小船上发射和回收，因此与全球导航卫星系统波浪滑翔机的部署、维护和回收有关的费用相对较小，全球导航卫星系统波浪滑翔机技术的多功能性将远远超过现有的海平面测量方法。GNSS Wave Glider的潜在应用包括：1）测量全球平均海平面和监测海平面变化，2）连接近海和陆上垂直基准，3）校准卫星测高，特别是支持目前重新解释沿海附近现有测高数据的努力，但也支持偏远和难以进入的地区，4）确定区域大地水准面变化，5）海洋模型改进：该项目的主要目的是将最先进的大地测量级全球导航卫星系统接收器和测井系统与国家海洋和海洋中心最近获得的Wave Glider相结合，以创建一个移动的自主全球导航卫星系统潮汐测量仪。到项目结束时，演示GNSS波浪滑翔机将可供NOC和英国海洋界使用。系统性能将根据验潮数据进行验证。进一步的测试将涉及使用全球导航卫星系统波浪滑翔机校准海面高度和来自Cryosat-2的有效波浪高度。

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.