Development of the Bass Strait GNSS/INS buoy capability in preparation for wide‑swath altimetry validation
Creators: name: Zhou, Boye
Publisher: name: University of Tasmania
Year: 2024

description: Satellite altimetry has revolutionised our understanding of large-scale ocean processes and sea level change. Over decades of development, altimeters have progressed from pulse limited radar with a ~7 km footprint to synthetic aperture radar (SAR) yielding an enhanced resolution of ~300 m along track. However, a key limitation that has not yet been addressed is the spatial sampling governed by the ground track separation (nominally 315 km at the equator for the Jason-series reference missions), preventing the resolution of (sub-)mesoscale ocean processes. To counter this, the ambitious Surface Water and Ocean Topography (SWOT) mission is set to launch by the end of 2022. SWOT will be the first altimeter mission to generate swath-based estimates of sea surface height using an interferometric mode of SAR. Two ~50 km wide swaths will enable the observation of finer scale ocean processes with wavelengths as small as 15 km. The unprecedented capability of this mission inevitably poses challenges to validation teams due to its stringent requirements defined in continuous wavenumber space. Consequently, most traditional point-based validation approaches are no longer sufficient for SWOT – this includes the long-standing approach adopted at the Bass Strait validation facility operated by the University of Tasmania (UTas) as part of the Integrated Marine Observing System (IMOS) – the only such facility in the Southern Hemisphere. The primary aim of this thesis is to further develop the Bass Strait validation approach in preparation for its contribution to the SWOT mission. Specifically, this work focuses on advancing buoy technology and the formation of a buoy array for swath-based geometric validation. To start with, a precision assessment was performed on the existing UTas/IMOS Mk-IV buoys. The buoy solutions were benchmarked against an in situ mooring with results indicating a ~1.5 cm overarching precision as the upper bound for the buoy-mooring system. While this metric includes a contribution from the in situ mooring system, it shows an immediate discrepancy with the SWOT mission requirement of 1.2 cm at 15 km wavelength. To address this shortcoming, several enhancements to the buoy system were explored, specifically, the inclusion of further Global Navigation Satellite Systems (GNSS) and an Inertial Navigation System (INS). The work is based on the TRACK processing suite, which is officially developed and maintained by Massachusetts Institute of Technology (MIT). Modification to TRACK enhanced its functionality to handle data from the Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONASS) constellation and extended its capability to incorporate INS. Addition of GLOANSS removed the sole dependence on the Global Positioning System (GPS) constellation in existing solutions. However, this introduced inter-system/inter-frequency biases into the system. To reduce the impact, separated bias fixing strategies for different constellations were implemented. This allowed a hybrid ambiguity resolution (AR) combination – integer for GPS, float for GLONASS. The resultant enhancement in both geometry and AR of the tracked satellites led to improvement in buoy SSH solutions with up to 3 cm root mean square (RMS) reduction when benchmarked against the in situ mooring. The integration of an INS unit into the buoy system enabled the rigorous treatment of the dynamic platform orientation, notably the antenna phase centre variations with buoy tilt and rotation. The observational model in TRACK was in turn updated with adaptive range corrections using modelled orientations. The results showed up to centimetre observation-level biases could result from orientation variations in high sea states. In parallel with the GNSS/INS integration, the dynamics of the tethered buoy was investigated. An empirical model was initially built for historic deployments, in which spurious semi-diurnal signals of up to 2 cm amplitude were identified in the buoy minus mooring (B-M) residual for specific deployments. Using the model, a 51% decrease in the variance over low frequency bands of the B-M residual was achieved. To improve on this empirical model, the characteristics of the buoy motion was further examined. Using metrics derived from four recent deployments with INS and GNSS observations, the relationship between B-M residuals and external forcings (i.e., current, waves and wind) was investigated. The results showed no significant relationships were evident in conditions with an average sea state of 1.2 m (max 2.3 m) and current up to 0.32 m/s. This suggested the Mk-IV design functioned well in typical Bass Strait oceanic conditions, with a few older deployments potentially influenced by the type of tether system used. To extend the use of the buoy approach to the geometric validation of SWOT, sustained deployment of multiple buoys is required over the proposed 3-month validation phase. To cope with a range of sea states, the buoy was redesigned, enlarging the Mk-IV design to slightly elevate the antenna height. Solar power and data telemetry was also introduced. The resultant Mk-VI buoy aims to extend the deployment duration, improve platform stability meanwhile maintain data quality. A buoy array consisting of six Mk-VI buoys and one old Mk-IV buoy was then tested via a proof-of-concept trial deployment over 8 days. Buoys were deployed at five locations spaced over 40 km along pass 088 of Sentinel-6 Michael Freilich (S6MF). The array was centred on the historical Jason-series comparison point at the Bass Strait facility. Results revealed the buoy array was successful at resolving spatial and temporal changes in ocean and atmospheric state, in particular significant changes in ocean tide (up to 0.2 m over 20 km), as well as changes in water vapour and sea state (as exposure increased offshore). Under specific assumptions, the spatial scale of the errors in the buoy array was investigated. Results provided some preliminary insights into possible buoy array configurations to optimise the contribution to SWOT validation. Finally, for illustrative purposes, a comparison was made between buoy array solutions and high-resolution SAR outputs from a single overpass of S6MF. Standard deviation of the wave height difference between the altimeter and the buoy was ~4 cm, while standard deviation of the SSH difference was &lt; 1 cm, both showing good consistency as expected. Potential land contamination in the radiometer measurements from S6MF was observed when comparing wet tropospheric delay with buoy estimates. Additionally, some spatial variation in the sea level anomaly was observed by the buoy array warranting further studies with additional passes. These findings support the benefits of the buoy-array concept to provide additional diversity in approaches that can contribute to SWOT validation as well as in other research areas (such as regional weather forecasting). This work has provided an important set of developments in the lead up to the launch of SWOT. Further areas for improvement were also identified. These include further refining the understanding of possible biases in buoy solutions as a function of higher sea states. Longer deployments of the buoy array will enable an improved characterisation of the spatial scale of the buoy error, and the contribution of ageostrophic signals in Bass Strait. Spatial constraints of the tropospheric delay on the buoy network and common mode error analysis of the land-based reference stations in the buoy deployment are likely to further reduce these errors. To conclude, the buoy array is a critical step forward for in situ approaches in the context of SWOT validation. As an independent geometric technique, the buoy array will also help understand the complex characteristics of shallow water shelf areas such as Bass Strait. Both will ultimately contribute to the improved knowledge of the ocean and its diverse role in the Earth’s changing climate.