Abstract's details
Using corner reflectors for altimetry calibration
CoAuthors
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Regional and Global CAL/VAL for Assembling a Climate Data Record
Presentation type: Type Oral
Contribution: PDF file
Abstract:
This is a joint presentation from 2 submitted abstracts:
Abstract from: Radar Altimetry Calibration With Corner Reflectors: Current Status And Future Plans by Gibert et al. (see details in the corresponding poster)
The instrumental performance of radar altimeters in terms of range and datation measurements has traditionally been monitored by flying over ground-based active transponders, as these measurables are mandatory for an accurate sea level rise estimate. Indeed, currently operative altimeters including Sentinel-3 (S3A/B), CryoSat-2 (CS2) and Sentinel-6 Michael Freilich (S6-MF) rely on this kind of regular external calibration activities to assess the quality of their measurements. Alternative methods based on passive devices such as corner reflectors had been traditionally discarded as the minimum reflector size to achieve a Signal-to-Clutter Ratio (SNC) up to calibration standards was too high to allow for a practical implementation. The main reason was the large area of the clutter being integrated within the same range cell of the measured point target. Such area is limited by the pulse width in the across-track dimension and by the processing technique used in the along-track direction, around 8 – 10 km for Low Resolution mode processors and around 300 m for SAR mode processors. Both processing techniques include, either totally or partially, incoherent combination of received pulses.
However, recent development of full coherent processing techniques for radar altimeters, namely Fully-Focussed SAR (FFSAR, [1]) allow to coherently recombine all the pulses within the target illumination time, achieving theoretical along-track resolutions below the 1-m level. With such technique, the area of clutter is significantly reduced and the minimum reflector size is reduced to feasible dimensions, i.e. less than 2 m plate size, enabling now a realistic implementation of a passive device for radar altimetry external calibration.
Being aware of such an opportunity, isardSAT designed a square trihedral corner reflector and installed it in April 2021 in the summit of a prominent ridge near Barcelona, Catalonia, on a location compatible with S6-MF, S3B and CS-2 tracks. First observations after S6-MF satellite tracking window adjustments started in Summer 2021 and since then periodic analysis has been carried out with an isardSAT FFSAR processor, allowing at each pass to measure the along-track and across-track impulse response functions and the respective resolution in each dimension, the range and datation biases, and the radar cross section of the reflector (RCS) [2].
Now, after two years of successful data acquisition with S6-MF, the corner reflector has demonstrated its capability to perform external calibration activities by achieving similar performances in terms of range and datation as that achieved by transponders [3], together with a 97% of effectivity in terms of availability and very low maintenance costs. Furthermore, calibration operations for CS-2 and S3B have also been initiated, reporting promising results not just in SAR mode but also in interferometric mode for the CS-2 case, what confirms its feasibility in such operational mode as well.
The successful results from this experiment demonstrate the maturity of this experimental calibration technique and pave the way for the deployment of future corner reflector-based facilities for radar altimeter external calibration. Indeed, the ESA Sentinel-3 Mission Performance Center (S3MPC) has already included the corner reflector in its operational CALVAL activities, and the results are included in the cyclic reports.
Finally, due to their relatively low manufacturing cost and ease of installation and maintenance, corner reflectors may be deployed in a variety of areas where traditional transponders may face more difficulties, thus could be of interest to deploy a more complete network of calibration stations around the Earth to improve the yield from current radar external calibration operations. To showcase the remarkable versatility of the corner reflector, isardSAT has initiated a feasibility study for a compact and portable new corner design. This endeavor aims to explore the potential of a downsized corner reflector while emphasizing its adaptability. We have already tested it against S6-MF, S3A/B and CS-2 in a different set of locations with successful acquisitions.
In this presentation, we will present the main results of the campaigns, with special attention to the geophysical corrections applied. The next development steps of the facility will be presented and its utility for future missions will also be discussed.
Abstract from: Calibration of Sentinel-3 altimeter data using a corner reflector by Maraldi et al.(see details in the corresponding poster)
Corner reflectors (CR) are commonly used to calibrate and characterize the quality of SAR images. With the Fully Focused SAR (FFSAR) method, the along-track resolution is now sufficient for CR to be detected by altimeters, opening new possibility for altimetry calibration with CR.
CR are passive devices (by opposition to the transponder active devices historically used for altimetry calibration). In comparison to the transponders, this offers some advantages: the development is relatively inexpensive, it has a very broadband frequency response, maintenance is reduced, the deformation of the across-track impulse response, a known issue with the transponders, is not present on CR, there is no need in regular calibration as it is the case for transponders’ internal path delay… Also, because the signal is only backscattered and not amplified, CR theoretically allow for the calibration and the monitoring of the absolute sigma0 if the noise signal ratio of the site is sufficient.
For all these reasons, CNES has developed a CR in 2019. The 2 m squared side CR was deployed on 8th October 2020 in Toulouse under a Sentinel-3 track. The place has been chosen for its particular topography, and its sufficiently low clutter level. A GPS positioning campaign has been performed to locate precisely the CR. The on-board OLTC has been modified to update the tracking window to target the CR theoretical range distance.
In this study we present the analysis of Sentinel-3 CR acquisitions. Range bias, datation bias, and sigma0 have been estimated from FFSAR processing (2D PTR method). Range bias has been corrected from geophysical corrections extracted from the operational products, and from the distance between the GNSS acquisition and the CR phase center position. Sigma0 has been corrected from gain and attenuation correction taken from the level-2 operational products; the antenna pattern has also been compensated. Impulse response has also been used to monitor range and azimuth resolution. For sake of validation, the results have been compared with CDN1 transponder device at the same cycles and the same method has been used on both CR and transponder. Analysis show very good consistency between CR and transponder results in terms of bias and stability for range and datation bias. The sigma0 stability is also discussed.
Detections of the CR with other sensors working in different bands are also shown: detection in Sentinel-1 C-band SAR images during the campaign under the Sentinel-3 track, and, more recently, detection under the SWOT Ka-band swath.
Abstract from: Radar Altimetry Calibration With Corner Reflectors: Current Status And Future Plans by Gibert et al. (see details in the corresponding poster)
The instrumental performance of radar altimeters in terms of range and datation measurements has traditionally been monitored by flying over ground-based active transponders, as these measurables are mandatory for an accurate sea level rise estimate. Indeed, currently operative altimeters including Sentinel-3 (S3A/B), CryoSat-2 (CS2) and Sentinel-6 Michael Freilich (S6-MF) rely on this kind of regular external calibration activities to assess the quality of their measurements. Alternative methods based on passive devices such as corner reflectors had been traditionally discarded as the minimum reflector size to achieve a Signal-to-Clutter Ratio (SNC) up to calibration standards was too high to allow for a practical implementation. The main reason was the large area of the clutter being integrated within the same range cell of the measured point target. Such area is limited by the pulse width in the across-track dimension and by the processing technique used in the along-track direction, around 8 – 10 km for Low Resolution mode processors and around 300 m for SAR mode processors. Both processing techniques include, either totally or partially, incoherent combination of received pulses.
However, recent development of full coherent processing techniques for radar altimeters, namely Fully-Focussed SAR (FFSAR, [1]) allow to coherently recombine all the pulses within the target illumination time, achieving theoretical along-track resolutions below the 1-m level. With such technique, the area of clutter is significantly reduced and the minimum reflector size is reduced to feasible dimensions, i.e. less than 2 m plate size, enabling now a realistic implementation of a passive device for radar altimetry external calibration.
Being aware of such an opportunity, isardSAT designed a square trihedral corner reflector and installed it in April 2021 in the summit of a prominent ridge near Barcelona, Catalonia, on a location compatible with S6-MF, S3B and CS-2 tracks. First observations after S6-MF satellite tracking window adjustments started in Summer 2021 and since then periodic analysis has been carried out with an isardSAT FFSAR processor, allowing at each pass to measure the along-track and across-track impulse response functions and the respective resolution in each dimension, the range and datation biases, and the radar cross section of the reflector (RCS) [2].
Now, after two years of successful data acquisition with S6-MF, the corner reflector has demonstrated its capability to perform external calibration activities by achieving similar performances in terms of range and datation as that achieved by transponders [3], together with a 97% of effectivity in terms of availability and very low maintenance costs. Furthermore, calibration operations for CS-2 and S3B have also been initiated, reporting promising results not just in SAR mode but also in interferometric mode for the CS-2 case, what confirms its feasibility in such operational mode as well.
The successful results from this experiment demonstrate the maturity of this experimental calibration technique and pave the way for the deployment of future corner reflector-based facilities for radar altimeter external calibration. Indeed, the ESA Sentinel-3 Mission Performance Center (S3MPC) has already included the corner reflector in its operational CALVAL activities, and the results are included in the cyclic reports.
Finally, due to their relatively low manufacturing cost and ease of installation and maintenance, corner reflectors may be deployed in a variety of areas where traditional transponders may face more difficulties, thus could be of interest to deploy a more complete network of calibration stations around the Earth to improve the yield from current radar external calibration operations. To showcase the remarkable versatility of the corner reflector, isardSAT has initiated a feasibility study for a compact and portable new corner design. This endeavor aims to explore the potential of a downsized corner reflector while emphasizing its adaptability. We have already tested it against S6-MF, S3A/B and CS-2 in a different set of locations with successful acquisitions.
In this presentation, we will present the main results of the campaigns, with special attention to the geophysical corrections applied. The next development steps of the facility will be presented and its utility for future missions will also be discussed.
Abstract from: Calibration of Sentinel-3 altimeter data using a corner reflector by Maraldi et al.(see details in the corresponding poster)
Corner reflectors (CR) are commonly used to calibrate and characterize the quality of SAR images. With the Fully Focused SAR (FFSAR) method, the along-track resolution is now sufficient for CR to be detected by altimeters, opening new possibility for altimetry calibration with CR.
CR are passive devices (by opposition to the transponder active devices historically used for altimetry calibration). In comparison to the transponders, this offers some advantages: the development is relatively inexpensive, it has a very broadband frequency response, maintenance is reduced, the deformation of the across-track impulse response, a known issue with the transponders, is not present on CR, there is no need in regular calibration as it is the case for transponders’ internal path delay… Also, because the signal is only backscattered and not amplified, CR theoretically allow for the calibration and the monitoring of the absolute sigma0 if the noise signal ratio of the site is sufficient.
For all these reasons, CNES has developed a CR in 2019. The 2 m squared side CR was deployed on 8th October 2020 in Toulouse under a Sentinel-3 track. The place has been chosen for its particular topography, and its sufficiently low clutter level. A GPS positioning campaign has been performed to locate precisely the CR. The on-board OLTC has been modified to update the tracking window to target the CR theoretical range distance.
In this study we present the analysis of Sentinel-3 CR acquisitions. Range bias, datation bias, and sigma0 have been estimated from FFSAR processing (2D PTR method). Range bias has been corrected from geophysical corrections extracted from the operational products, and from the distance between the GNSS acquisition and the CR phase center position. Sigma0 has been corrected from gain and attenuation correction taken from the level-2 operational products; the antenna pattern has also been compensated. Impulse response has also been used to monitor range and azimuth resolution. For sake of validation, the results have been compared with CDN1 transponder device at the same cycles and the same method has been used on both CR and transponder. Analysis show very good consistency between CR and transponder results in terms of bias and stability for range and datation bias. The sigma0 stability is also discussed.
Detections of the CR with other sensors working in different bands are also shown: detection in Sentinel-1 C-band SAR images during the campaign under the Sentinel-3 track, and, more recently, detection under the SWOT Ka-band swath.