CoAuthors

Bruce Haines (NASA JPL, USA); Josh Willis (NASA JPL, USA); Luca Centurioni (Scripps Institution of Oceanography, USA); Rick Lumpkin (NOAA AOML, USA)

The overarching idea of this project is to establish a new ocean observing system for monitoring global and regional mean sea level (MSL) changes. This system will consist of a global array of thousands of water-following drifting buoys, or drifters, tracked by a Global Navigation Satellite System (GNSS) which will continuously provide the geographical positions and the height of the sea surface along the buoys’ trajectories. The Lagrangian sea surface height data collected in this way, averaged over regional basins and the global ocean, will provide daily estimates of regional and global mean sea levels. This new system will be independent, resilient, sustainable, and hopefully relatively economical. Additionally, the new system could provide opportunities for developing globally-distributed, local and regional calibration and validation efforts for satellite altimetry missions. Importantly, by providing high-frequency and regional and global estimates, this new system will have the capability to bridge the observational gap between the existing altimeter satellite system and the tide gauge observing system. Using the example of the historical trajectories of the drifters of the NOAA Global Drifter Program (GDP), we have recently demonstrated (see Elipot, 2020) that with the current configuration of the GDP array, global-mean sea-level decadal linear trend estimates with an uncertainty less than 0.3 mm per year could be achieved with daily random error of 1.6 m or less in the vertical direction for each individual drifter daily estimate. Can this target accuracy be reached with existing technologies?

In order to answer this question, we are conducting a pilot project that consists in deploying three drifters in a moored configuration, at two coastal locations, in La Jolla, CA and Virginia Key, FL, both collocated with NOAA tide gauges providing reference sea level measurements. The first drifter, acting as a control, is a standard (build to print) version of Surface Velocity Program (SVP) drifter from the Scripps Institute of Oceanography Lagrangian Drifter Laboratory. It is equipped with a standard precision single-frequency GPS chipset (u-blox) and antenna. The second drifter is also a standard SVP unit, but supports internal recording of the raw, high-rate (1-Hz) GPS tracking observations for post processing. Providing a gold standard of positioning data, the third drifter is additionally equipped with a geodetic-grade GPS chipset (Septentrio AsteRx) continuously recording dual-frequency tracking data from Global Navigation Satellite Systems (GNSS). This drifter is also equipped with an inertial measurement unit to continuously measure the platform attitude.

To characterize the performance, the raw full-rate (1-Hz) observations from the two enhanced SVP drifters will first be processed using the JPL GipsyX software in order to obtain the best achievable sea-level estimates for each case. These observations will be sequentially decimated and reprocessed in various solution strategies to understand the trade-offs between data sampling and battery capacity on one hand, and achievable sea-level estimate accuracy on the other hand. Ultimately, we will use the results of this pilot project to suggest, from a cost-analysis point of view, a pathway to deliver the envisioned new global mean sea-level observing system as an added value to the NOAA Global Drifter Program.