Abstract's details
Sea-ice freeboard and sea level from altimetry using fast and robust 2 dimensional retracker
CoAuthors
Event: 2019 Ocean Surface Topography Science Team Meeting
Session: Instrument Processing: Measurement and Retracking
Presentation type: Type Oral
Contribution: PDF file
Abstract:
The Arctic sea-ice is a fundamental actor and witness to the on-going climate changes. The SIRAL instrument aboard the polar mission CryoSat-2, endowed with the delay-Doppler mode, has enabled to produce several truthful estimations of the ice freeboard and consequently the Arctic ice volume. However, the ice thickness estimation using altimetry still holds several sources of uncertainties.
One major source of errors is the result of the misinterpretation of complex multi-return echoes by the retracker. These parasitic peaks occur when high backscatter surfaces adjoin low backscatter surfaces because of two well-known effects in SAR altimetry: the side-lobe effect and the off-nadir effect. This configuration is very frequently encountered over sea-ice where high backscatter fractures travel through the ice pack.
Even if the Hamming filtering attenuates the side-lobe effect, these phenomenon induce several centimeters of error. For instance about 15% of the measurements obtained with SAMOSA+ (Dinardo) are impacted.
The solution presented here allows identifying the peak of the waveform that corresponds to the nadir, even in very complex configurations. It relies on the flatness characteristic of the sea-ice (+-50cm) to detect and extract the ground line from the whole echogram one satellite track.
Several options can be considered to exploit the output of this 2D retracker: it can be used as a post-processing to remove the outliers find out by the retracker or it can be used as a first guess for a heuristic or a physical retracker.
We will show the positive impacts of this 2D retracker to retrieve the sea ice freeboard and the sea level anomaly among the ice in conjunction with heuristic retracker and with the physical retracker SAMOSA+.
One major source of errors is the result of the misinterpretation of complex multi-return echoes by the retracker. These parasitic peaks occur when high backscatter surfaces adjoin low backscatter surfaces because of two well-known effects in SAR altimetry: the side-lobe effect and the off-nadir effect. This configuration is very frequently encountered over sea-ice where high backscatter fractures travel through the ice pack.
Even if the Hamming filtering attenuates the side-lobe effect, these phenomenon induce several centimeters of error. For instance about 15% of the measurements obtained with SAMOSA+ (Dinardo) are impacted.
The solution presented here allows identifying the peak of the waveform that corresponds to the nadir, even in very complex configurations. It relies on the flatness characteristic of the sea-ice (+-50cm) to detect and extract the ground line from the whole echogram one satellite track.
Several options can be considered to exploit the output of this 2D retracker: it can be used as a post-processing to remove the outliers find out by the retracker or it can be used as a first guess for a heuristic or a physical retracker.
We will show the positive impacts of this 2D retracker to retrieve the sea ice freeboard and the sea level anomaly among the ice in conjunction with heuristic retracker and with the physical retracker SAMOSA+.