For over 25 years, satellite altimetry observations have provided invaluable information about sea-level variations, from Global Mean Sea-Level to regional meso-scale variability. However, this information remains difficult to extract in coastal areas, where the proximity to land and complex dynamics create complications that are not sufficiently accounted for in current models. Detailed knowledge of local hydrodynamics, as well as reliable sea-surface height measurements, is required to improve and validate altimetry measurements. New kinematic systems based on Global Navigation Satellite Systems (GNSS) have been developed to map the sea surface height in motion. We demonstrate the capacity of two of these systems, designed to measure the height at a centimetric level: (1) A GNSS floating carpet towed by boat (named CalNaGeo); and (2) a combination of GNSS antenna and acoustic altimeter (named Cyclopée) mounted on an unmanned surface vehicle (USV). We show that, at a fixed point, these instruments provide comparable accuracy to the best available tide gauge systems. When moving at up to 7 knots, the instrument velocity does not affect the sea surface height accuracy, and the two instruments agree at a cm-level.
Precise underwater geodetic positioning remains a challenge. Measurements combining surface positioning (GNSS) with underwater acoustic positioning are generally performed from research vessels. Here we tested an alternative approach using a small Unmanned Surface Vehicle (USV) with a compact GNSS/Acoustic experimental set-up, easier to deploy, and more cost-effective. The positioning system included a GNSS receiver directly mounted above an Ultra Short Baseline (USBL) module integrated with an inertial system (INS) to correct for the USV motions. Different acquisition protocols, including box-in circles around transponders and two static positions of the USV, were tested. The experiment conducted in the shallow waters (40 m) of the Bay of Brest, France, provided a data set to derive the coordinates of individual transponders from two-way-travel times, and direction of arrival (DOA) of acoustic rays from the transponders to the USV. Using a least-squares inversion, we show that DOAs improve single transponder positioning both in box-in and static acquisitions. From a series of short positioning sessions (20 min) over 2 days, we achieved a repeatability of ~5 cm in the locations of the transponders. Post-processing of the GNSS data also significantly improved the two-way-travel times residuals compared to the real-time solution.
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Abstract. To better understand sea level evolution in coastal areas, one needs to link and combine global observations from altimetry satellites with the scattered but long-term tide gauges measurements. In New-Caledonia, the Noumea lagoon is an example of this challenge as altimetry, coastal tide gauge and vertical land movements from Global Navigation Satellite Systems (GNSS) do not provide consistent information. The GEOCEAN-NC 2019 field campaign tries to address this question with the deployments of in situ instruments in the lagoon, with a particular interest for the crossing point of three different altimetry tracks (Jason/Sentinel-3a). Thanks to GNSS buoy and pressure gauge observations, we propose a method to virtually transfer the Noumea tide gauge offshore, to obtain a long-term sea surface height (SSH) time series at the altimetry crossover point. We also reprocess the 20 Hz along-track data from Jason and Sentinel-3a Geophysical Data Records (GDR) with the best correction parameters in the area. These two SSH time series (i.e. in situ and altimetry) allow us to compute the altimeter biases time series over the entire Jason and Sentinel-3a period. With our 3 weeks field campaign, we reanalyse about 20 years of altimetry observations and find inter-mission biases consistent with historical calibration sites, thus further increasing our knowledge of the local sea level rise in this region. This offers many opportunities to develop Cal/Val activities in the lagoon, which is also the subject of several experiments for the scientific calibration phase of the future large-swath altimetry mission SWOT.
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