Monitoring Ocean Water Level In Remote Shoreline Locations Using Gps Reflectometry

The Global Positioning System (GPS) was originally developed to provide accurate navigation and timing services. Since then, a plethora of additional applications have been developed. This paper describes the deployment of a dual-frequency GPS receiver developed primarily for ionospheric space weather monitoring activities, but which is also capable of continuously measuring variations of water level and thus deducing tidal height. The approach uses GPS signals reflected from the water surface.Accurate water level observations are fundamental for storm-surge forecasting, informed emergency response, ecosystem management, safe navigation, and efficient coastal and nautical mapping and charting. Installations of water level gauges for making coastal tidal measurements are often restricted to areas with coastal infrastructure, and incorporate in-water stilling wells. Newer systems make primary water level measurements using downward looking acoustic sensors that still incorporate a protective well, but one that is more open to the local dynamics in order to avoid removing shorter-period wave signals. Such systems are implemented operationally by the National Oceanic and Atmospheric Administration as part of the National Water Level Observation Network (NOAA NWLON) and are the preferred method when the most exacting measurements are needed, such as for safe maritime navigation. However, NWLON units are expensive to install and maintain, and can be impractical for regions that lack coastal infrastructure and experience icing, leaving remote shorelines critically under-instrumented for water level observations, for example along the western and northern coastline of Alaska.Areas lacking sufficient water level information could benefit dramatically from increased water level observations despite a slightly reduced accuracy, for example a water level measurement to within ten centimeters instead of tenths of a centimeter. GPS receivers provide a viable alternative for water level observing at this accuracy level, they are much less expensive to install and maintain since there is no contact with the water, and they do not require coastal infrastructure, such as piers, for installation.ASTRA performed a successful pilot study with partners, the Alaska Ocean Observing System (AOOS) and the NOAA National Weather Service (NWS), demonstrating the efficacy of GPS receivers for water level measurements. The study confirmed that receivers originally developed for ionospheric space weather monitoring can also be used to accurately and reliably measure water levels and tides to generate a quality water level measurement adequate for many water level data needs, which for this demonstration was within 5 cm. We present details of the pilot study, and development of automated processing algorithms for GPS reflectometry water level observations in real time.