Minimize Sentinel-3 SRAL Open-Loop Tracking Command (OLTC)

Over the past decade, there has been a growing interest for altimetry measurements over inland waters. Studying lakes, reservoirs and rivers water level is of prime importance for the hydrology community to assess the Earth's global resources of fresh water. In the context of the European Union's Copernicus Program, Sentinel-3 altimeters are key to provide such global and continuous datasets of water surface height. Indeed, much progress has been made in altimeters capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC) represents a major evolution of the tracking function. This tracking mode's efficiency has been proven on past missions and it is now the operational mode on Sentinel-3 altimeters.

SRAL Mode Mask

On Sentinel-3, areas of interest (ocean and several land surfaces corresponding to the grey surfaces hereafter) define where SRAL is operated in OLTC mode. Outside these areas, SRAL altimeters operate in Close-Loop mode (a.k.a. autonomous tracking mode), with no a priori elevation information used to set the echo reception window.

SRAL mode masks

Figure 1: SRAL mode masks. The bottom panel corresponds to the acquisition mode mask used on Sentinel-3A from launch until 9 March 2019. After this date, the same mask used on Sentinel-3B (bottom panel) is also used on Sentinel-3A. The bottom panel shows the acquisition mode mask used on Sentinel-3B (whole mission life) and on Sentinel-3A since 9 March 2019.

OLTC Tables

When operated in OLTC mode, the SRAL altimeter computes the distance between its antenna and the on-ground surface by combining the satellite altitude (Hs) provided by the DIODE navigator on the DORIS instrument with the on-ground elevation information (H0) available in the OLTC tables (see Figure 2). The water surface height (Hw) can then be derived through on-ground processing. The information coded in the OLTC is tuned over specific identified targets such as lakes, rivers, glaciers etc. Elsewhere the information used might not be meaningful.

OLTC onboard tables are generated using a software designed by CNES (F) and operated by NOVELTIS (F). This software merges elevation information from multiple sources (hydrological targets defined by users, global mean sea surface, local Digital Elevation Models) with satellite orbit reference file. The elevation information H0 is referenced with respect to the altimeter height reference (e.g. ellipsoid for Sentinel-3) and sampled every 1 km along the altimeter on-orbit position. For Sentinel-3A and Sentinel-3B, a supplementary optimisation step is applied to reduce the size of OLTC tables due to limited on-board memory size: a limited number of segments of constant elevation is generated for each orbit. These OLTC tables are then uploaded in the dedicated on-board memory.

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Figure 2: Schematic view of water surface height measurement from altimetry in Open-Loop mode. Credits: Biancamaria & Blumstein (LEGOS)

OLTC Tables and users' contributions

OLTC tables are uploaded onboard the altimeter memory by telecommand operations. That is why it is of high importance to define the most accurate and enriched contents of the OLTC onboard tables. For that matter ESA, together with its partners CNES and NOVELTIS, has developed a website for users to display and contribute to the OLTC contents.

An interactive map allows visitors to view elevations defined onboard Sentinel-3 altimeters and navigate over inland water targets worldwide. Several visualization tools have been added to enhance the OLTC website experience: choice of map layout, display of satellite ground tracks and areas of interest. Users wishing to further contribute to OLTC contents may sign up and have access to additional functionalities: once logged in, they have the possibility to submit modifications of existing elevations as well as requesting targets that may not yet be defined in the onboard OLTC tables. For example, adding a virtual station over a river or a lake, in order to enhance the OLTC contents and make sure that the altimeter will acquire quality echoes over the defined targets. Requests may be submitted directly using the "Contribute" tool or by sending a list of targets to the OLTC database management committee.

Scientists and Sentinel-3 users may visit the website and contribute to the S3 OLTC contents in order to enhance the altimeter's capability over inland waters.

OLTC performance over Inland waters

Sentinel-3A OLTC tables were updated (to v5) in March 2019. It consisted of:

  • Increasing the number of targets over which an appropriate consign of surface elevation is provided to the altimeter
  • Updating the previous consign values (defined in the previous OLTC version). As presented in Figure 3, OLTC v5 presents an improvement over v4 as the number of observations with low backscatter coefficients have been drastically reduced.

The improved distribution of the reflectivity (illustrated in Figure 3) is an indication that the v5 consign allows the altimeter to track water rather than other surfaces with lower reflectivity.

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Figure 3: Distribution of backscatter coefficient for water targets over rivers

Figure 4 presents the time series of the backscatter coefficient values over one Sentinel-3A intersection with the Mkuju River. Vertical lines consist of high frequency (20Hz) points over a same transect. Before the OLTC v5 update the coefficients were lower than 40 dB: the consign did not allowing the tracking of the water. Since March 2019, with the upload of OLTC v5, the coefficients present a drastic improvement and values are consistent with a signal related with water reflectivity.

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Figure 4: Time series of backscatter coefficients over a station on Mkuju River (Tanzania)

OLTC performance over coastal areas

The following illustration (Figure 5) shows the difference of radargram between a SRAL acquisition in Open-Loop and an acquisition in Close-Loop. Indeed, during the tandem phase, Sentinel-3B (right panel), during this period operated in Close-Loop mode at global scales. Between 48.90 and 48.95° of latitude, one can observe the impact of the land contamination, the leading edge has clearly moved on the right part of the tracking window, meaning that the instrument is tracking the wrong surface. On the contrary, for Sentinel-3A (left panel) the measurements were acquired in Open-Loop, a better stability of the energy in the tracking window is observed, especially for measurements located between the island and the continent.

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Figure 5: Sentinel-3A (left panel) and Sentinel-3B (right panel) radargrams (SARM waveforms) plotted over a coastal area over France and Britain. Measurements were acquired during the Sentinel-3 tandem flight period, when Sentinel-3B flew 30 seconds ahead of Sentinel-3A. Sentinel-3A SRAL operated in Open-Loop mode over this region, whereas the Sentinel-3B measurements were acquired in Closed-Loop mode.