Copernicus Sentinel-3 STM Thematic Products 2nd Test Data Set

ESA is pleased to announce the availability of Copernicus Sentinel-3 altimetry data set generated by the NEW ESA Sentinel-3 Thematic Instrument Processing Facilities (T-IPFs). Planned to be deployed in spring 2022 by the ESA Payload Data Ground Segment (PDGS), the LAND T-IPFs are dedicated and specialized delay-Doppler and Level-2 processing chains over Sea-Ice (SI), Land Ice (LI) and Hydrological (HY) surfaces.

In January 2022, a first Test Data Test (TDS) was produced for Hydrology and Sea Ice thematic products (details here: https://sentinel.esa.int/web/sentinel/-/s3-stm-thematic-products-1st-dataset).

Following this first TDS release, ESA is pleased to announce the availability of a new TDS, including Land Ice thematic products. The Land Ice T-IPF includes major processing evolutions, as detailed below. Compared to the current Sentinel-3 STM level-2 products, the format of the new level-2 thematic products has changed. The document describing the new Sentinel-3 STM land level-2 Product Format Specifications (PFS) is available in the ftp folder (see below for access information).

In the newly generated TDS,  the Sea-Ice and Hydrology products have been updated compared to the first TDS, to include some adjustments of the product geographical coverage. These TDS are released to the end users prior to the deployment of the T-IPFs, so they can become familiar with the new product format, and adapt their tools and processes in consequence. For any feedbacks or questions regarding the TDS, please contact: s3mpc-stm-user-request@groupcls.com.

New TDS available:

• 1 TDS for Land Ice Thematic Products, covering cycle 54 of the Sentinel-3A mission (from January 15^th, 2020, to February 11^st, 2020)
• 1 TDS for Sea Ice Thematic Products, covering cycle 53 and 54 of the Sentinel-3A mission (from December 19^th, 2019, to February 11^st, 2020)
• 1 TDS for Hydrological Thematic Products, covering cycle 60 of the Sentinel-3A mission (from June 25^th, 2020, to July 22^nd, 2020)

Data are available through the following IP address “91.188.4.53” (username: anonymous; no password required).

Major evolutions implemented for Land Ice thematic products

The Land Ice T-IPF first version includes the “extended-window processing” to optimize the generation of SAR mode waveforms over rough and/or steeply sloping surfaces. Over such surfaces, large tracker range variations generally occur within the ~2 seconds of integration time in SAR mode. Hence, after range migration operations, useful energy sampled in the Doppler looks can be moved out the 128-samples window used to stack the data. To compensate for this effect, the window analysis of the delay-Doppler stack is artificially extended, from 128 to 512 samples. This way, all potential useful energy remains kept in the delay-Doppler stack. After multi-looking operation, the SAR mode waveform is re-centred within a conventional 128-sample window analysis.

This processing is already implemented in the CryoSat-2 Payload Data Ground Segment, and also documented in Aublanc et al. (2018). The figure below illustrates the impacts in the SAR mode waveforms. In the top panel the waveforms as generated by the current LAND IPF drift in the window analysis when the satellite comes near to the coast. With the Land Ice T-IPF, the waveforms remain centred in the window analysis, with an improved coverage of the ice sheet margins.

Figure 1: Sentinel-3A SAR mode waveforms acquired along the track over the Antarctic ice sheet, as generated by the current LAND IPF (top panel) and the upcoming Land Ice T-IPF (bottom panel). Waveforms are normalized in amplitude. The track location is displayed in the right figure. Credits MSSL.

In addition, the slope models used by the Land Ice T-IPF to relocate the measurement at Point Of Closest Approach (POCA) are improved compared to the versions used by the current LAND IPF. These surface slope models are precomputed from digital elevation models published by Helm et al. (2014), for both Antarctica and Greenland. Thanks to this update, a better accuracy is expected in the measurement relocation at POCA.

Preliminary assessment of Sentinel-3 LAND ICE Thematic products

A first assessment of the “extended window processing” implemented in the Land Ice T-IPF was performed by the Mullard Space Science Laboratory (MSSL, UK) and CLS (F). As expected, results show that the processing is valuable over the ice sheet margins. Over these steep and/or rough topography a significant amount of SAR mode waveforms is recovered compared to the processing performed by the current LAND IPF. This is illustrated in the figures below, showing the measurement failures (red colour) along Sentinel-3A tracks located over the Mertz glacier (Antarctica). The failures are determined with the flag “waveform_qual_ice_20_ku” available in the products. This quality flag includes different tests performed to the SAR mode waveform, for: low power, leading edge position, waveform variance and waveform peakiness. With the upcoming Land Ice thematic products (right figure), there is approximately 11 % more valid measurements over the area compared to the current Land products (left figure).

Figure 2: Sentinel-3A POCA locations in SAR mode over the Mertz glacier (Antarctica), as obtained with the current Land products (left), and the Land Ice thematic products (right). Points in red colour indicate the measurement failures, i.e.: the 20 Hz measurements for which the waveforms are considered not valid based on the quality flag “waveform_qual_ice_20_ku” available in the products.

An assessment of the “extended window processing” was also performed at Antarctica global scale. In this assessment the measurement failure was determined based on the absence of clear waveform leading edge(s) in the window analysis. The figures below display the mean ratio of failures, mapped using a 25 km stereographic grid with a standard parallel of 71°S. The failure ratio is on average 9 % over the Antarctica continent with the current Land products (left figure), and 4.5 % with the Land Ice thematic products (central figure). The right figure displays the difference between both maps (current Land products – Land Ice thematic products), thus corresponding to the ratio of recovered data achieved by the Land Ice T-IPF. Hence, ~4.5% of the data are recovered on average over the continent with the T-IPF, especially over the ice sheet margins where this ratio can increase up to 20-30%.

Figure 3: Mean failure ratio mapped over the Antarctica continent using a 25 km stereographic grid, obtained with the current Land IPF (left) and the Land Ice T-IPF (centre). The difference between the two maps is displayed in the right figure (LAND IPF – Land Ice TIPF). A measurement is flagged as “failure” in case of absence of clear waveform leading edge in the window analysis.

Finally, thanks to the updated surface slope model used to relocate the measurement at POCA, better performances are also expected in the ice sheet elevations available in the products. This will be quantified very soon by the Sentinel-3 STM Mission Performance Cluster Team.

For any feedbacks or questions regarding the TDS, please contact s3mpc-stm-user-request@groupcls.com

References

Aublanc, J., Moreau, T., Thibaut, P., Boy, F., Rémy, F., & Picot, N. (2018). Evaluation of SAR altimetry over the Antarctic ice sheet from CryoSat-2 acquisitions. Advances in Space Research, 62(6), 1307–1323. https://doi.org/10.1016/j.asr.2018.06.043

Helm, V., Humbert, A., & Miller, H. (2014). Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2. The Cryosphere, 8(4), 1539–1559. https://doi.org/https://doi.org/10.5194/tc-8-1539-2014