Search Results (4)

Satellite altimetry technology has been widely used for the observation of oceans and inland water bodies. At present, fully focused synthetic aperture radar (FF-SAR) data, which significantly enhances along-track resolution, has good prospects for river level estimation. However, FF-SAR data with large data volumes have more complex waveforms, which brings more challenges to waveform retracking. This study developed an improved adaptive multi-scale peak detection (ImpAMPD) retracker based on Sentinel-6 FF-SAR data. Initially, sub-waveforms are identified and extracted from each waveform. Subsequently, the data are segmented according to the number of gates and the minimum gate length. Finally, retracking calculations are performed on the segmented sub-waveforms to determine river levels. In this study, the in situ data from six river sections with different features in the middle and upper reaches of the Yangtze River were used to validate the accuracy of the ImpAMPD retracker and to perform a comparison of this developed retracker with three existing retrackers (OCOG, PTR, SAMOSA+). The results indicate that the ImpAMPD retracker can fully utilize the advantage of the high posting rate of FF-SAR data to process the complex multi-peak waveforms on the river surface, accurately extract the correct water surface signals, and achieve highly precise river level estimation. The best accuracy results were obtained in four river sections, namely, Zhicheng, Shashi, Hankou, and Huantan, with STDDs of 0.18 m, 0.26 m, 0.47 m, and 0.36 m, respectively. The ImpAMPD retracker is highly automated and adaptable to rivers of varying widths, providing robust support for river level monitoring and flood management. Full article

The observation of small to medium inland water targets with nadir radar altimeters is currently limited by the along-track resolution of UnFocused SAR (UFSAR) altimetry, which is approximately 300 $\mathrm{m}$ for Delay-Doppler processors. In this study, we analyze the benefits of the sub-meter along-track resolution provided by Fully Focused SAR (FFSAR) altimetry applied to Sentinel-6 Michael Freilich data over a collection of small to medium targets in the Ebre Basin, Spain. The obtained water level estimations over a 2-year period are compared to in situ data to evaluate the long-term accuracy of the algorithm. The proposed FFSAR altimetry methodology achieves an average MAD precision of roughly 4 $\mathrm{c}$$\mathrm{m}$, and allows for a full operational implementation as it can be processed in a totally unsupervised manner. The precision improvement with respect to Delay-Doppler products over the same targets is essentially attributed to the FFSAR capabilities to better filter out waveforms contaminated by off-nadir scatterers. Moreover, we evaluate the application of extended water masks, which exploit nadir–altimeter measurements where water is at nadir or up to 250 $\mathrm{m}$ across-track from nadir to increase the number of acquisitions while maintaining the same level of accuracy, increasing by an average of 48% the number of valid measurements per pass, while maintaining the same level of accuracy as nadir measurements over water. We thus demonstrate the potential of FFSAR altimetry to monitor the water level of small to medium inland water targets. Full article

Lake ice thickness (LIT) is a sensitive indicator of climate change, identified as a thematic variable of Lakes as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS). Here, we present a novel and efficient analytically based retracking approach for estimating LIT from high-resolution Ku-band (13.6 GHz) synthetic-aperture radar (SAR) altimetry data. The retracker method is based on the analytical modeling of the SAR radar echoes over ice-covered lakes that show a characteristic double-peak feature attributed to the reflection of the Ku-band radar waves at the snow–ice and ice–water interfaces. The method is applied to Sentinel-6 Unfocused SAR (UFSAR) and Fully Focused SAR (FFSAR) data, with their corresponding tailored waveform model, referred to as the SAR_LIT and FFSAR_LIT retracker, respectively. We found that LIT retrievals from Sentinel-6 high-resolution SAR data at different posting rates are fully consistent with the LIT estimations obtained from thermodynamic lake ice model simulations and from low-resolution mode (LRM) Sentinel-6 and Jason-3 data over two ice seasons during the tandem phase of the two satellites, demonstrating the continuity between LRM and SAR LIT retrievals. By comparing the Sentinel-6 SAR LIT estimates to optical/radar images, we found that the Sentinel-6 LIT measurements are fully consistent with the evolution of the lake surface conditions, accurately capturing the seasonal transitions of ice formation and melt. The uncertainty in the LIT estimates obtained with Sentinel-6 UFSAR data at 20 Hz is in the order of 5 cm, meeting the GCOS requirements for LIT measurements. This uncertainty is significantly smaller, by a factor of 2 to 3 times, than the uncertainty obtained with LRM data. The FFSAR processing at 140 Hz provides even better LIT estimates, with 20% smaller uncertainties. The LIT retracker analysis performed on data at the higher posting rate (140 Hz) shows increased performance in comparison to the 20 Hz data, especially during the melt transition period, due to the increased statistics. The LIT analysis has been performed over two representative lakes, Great Slave Lake and Baker Lake (Canada), demonstrating that the results are robust and hold for lake targets that differ in terms of size, bathymetry, snow/ice properties, and seasonal evolution of LIT. The SAR LIT retrackers presented are promising tools for monitoring the inter-annual variability and trends in LIT from current and future SAR altimetry missions. Full article

We address surface wave breaking caused by oceanic Internal Solitary Waves (ISWs) and how ISWs are manifested in the SAR altimeter onboard Sentinel-3A and -3B satellites by means of their effects in Significant Wave Height (SWH). Two different regions of the ocean are selected, namely the tropical Atlantic Ocean off the Amazon shelf and the Banda Sea in the Indian Ocean, where there are scenes of Sentinel-3 OLCI acquired simultaneously with an along-track SAR mode altimeter, which include signatures of large amplitude ISWs. New data of unfocused SAR (UF-SAR 20 Hz) and fully focused SAR (FF-SAR 160 Hz) modes are analyzed, which are retracked in full range and over a reduced range of bins (truncation carried out dynamically ten gates away from the estimated epoch position). At first order, in scales of 1–3 km, a strong decrease in the normalized radar cross section (NRCS) over the rough part of the ISWs is observed followed by a small increase in the smooth part relative to the unperturbed ocean background. A second order ISW signature, in scales of 20 km, is noted: the SWH is attenuated after the passage of an ISW, considering length scales of about 10 km before and after the ISW crest. The SWH signatures are unique in showing that the surface wave energy does not return to its unperturbed level after the passage of an ISW, admittedly because intense meter-scale wave breaking results in surface wave energy dissipation. Furthermore, Sentinel-2 MSI images are analyzed and provide insights into this same phenomenon: white-capping resulting in a radiance increase at all (visible) wavelengths. Modulation of breaking waves owing to ISWs is demonstrated by estimates of the fraction of breaking waves in the presence of internal waves. Full article