Abstract's details
Radar Altimetry Calibration With Corner Reflectors: Current Status And Future Plans
CoAuthors
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Regional and Global CAL/VAL for Assembling a Climate Data Record
Presentation type: Type Poster
Contribution: PDF file
Abstract:
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.
REFERENCES
[1] A. Egido and W. Smith, “Fully Focused SAR Altimetry: Theory and Applications,” IEEE Trans. Geosci. Remote Sens. 2017, pp. 1–15.
[2] F. Gibert et al., "A Trihedral Corner Reflector for Radar Altimeter Calibration," in IEEE Transactions on Geoscience and Remote Sensing, vol. 61, pp. 1-8, 2023, Art no. 5101408.
[3] S. P. Mertikas et al, “Performance evaluation of the CDN1 altimetry Cal/Val transponder to internal and external constituents of uncertainty”, Advances in Space Research, 2022, vol. 70, no. 8,
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.
REFERENCES
[1] A. Egido and W. Smith, “Fully Focused SAR Altimetry: Theory and Applications,” IEEE Trans. Geosci. Remote Sens. 2017, pp. 1–15.
[2] F. Gibert et al., "A Trihedral Corner Reflector for Radar Altimeter Calibration," in IEEE Transactions on Geoscience and Remote Sensing, vol. 61, pp. 1-8, 2023, Art no. 5101408.
[3] S. P. Mertikas et al, “Performance evaluation of the CDN1 altimetry Cal/Val transponder to internal and external constituents of uncertainty”, Advances in Space Research, 2022, vol. 70, no. 8,