Conference Agenda

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Launched April 29, 2025, BIOMASS is ESA's seventh Earth Explorer, carrying the first spaceborne P-Band Synthetic Aperture Radar (SAR), fully polarimetric and with across-track interferometric capabilities. Its primary objective is to map biomass and forest height, as well as their changes over time. The onboard antenna is based on a large deployable 12 m reflector with an offset feed array. BIOMASS operates in a Stripmap mode with a swath illuminated by a single antenna beam. Global coverage is obtained by the interleaved acquisition of three complementary swaths. The mission will also feature an experimental tomographic phase during its first year of operations, to provide 3D mapping of forests.

In-Orbit Commissioning (IOC) Phase activities have been mostly dedicated to instrument calibration and validation, to meet required data quality. The peculiarity of the mission is such that presents several challenges. Examples are: the use of a dedicated transponder instead of passive corner reflectors, due to the medium resolution and low expected RCS; difficulty of relying on rainforest as a natural calibration site, due to high penetration into the vegetation; strong ionospheric effects and relevant RFI contamination.

In this presentation we discuss the activities performed by Aresys and Politecnico di Milano in the IOC framework, with perspective on their continuation during the rest of the mission:

- Radiometric stability based on the PSCAL technique [2]

- RFI characterization and mitigation [1]

- Antenna pattern and pointing verification using natural distributed targets

- Noise Equivalent Sigma Nought (NESZ) estimation from very low back-scatter areas

- Noise characterization from transition scenes

Preliminary to these activities, a dedicated assessment of Targets of Opportunity (ToO) was carried out to enable accurate Antenna pattern and pointing verification, as well as NESZ estimation. Different candidates, including deserts, oceans, Antarctica and rainforest were assessed in terms of In/Out Band Power Ratio, radiometric homogeneity, and temporal stability. Amazon rain forest was identified as the most reliable ToO for Antenna pattern and pointing verification due to its high spatial uniformity and stability, while Antarctica and Desert were identified as optimal for NESZ estimation. RFI activities have been mostly dedicated to tune the mitigation algorithm featured by the operational processor, as well as assessing RFI global presence and effects on polarimetry and interferometry. Rx-Only acquisitions over transition areas have been analyzed to characterize the noise level of sea/ice/land scenes. PSCAL activities are discussed in a companion presentation [3].

Results so fairly confirm the robustness of BIOMASS calibration strategies despite the challenges of P-band and the huge potential of the mission to deliver consistent global forest monitoring

References

[1] Franceschi, Niccolo, et al. "Operational RFI mitigation approach in Sentinel-1 IPF." EUSAR 2022; 14th European Conference on Synthetic Aperture Radar. VDE, 2022.

[2] D. D'Aria, A. Ferretti, A. Monti Guarnieri and S. Tebaldini, "SAR Calibration Aided by Permanent Scatterers," in IEEE Transactions on Geoscience and Remote Sensing, April 2010

[3] Manzoni, Marco and Banda, Francesco “Evaluation of BIOMASS Radiometric Stability using Permanent Scatterers: first results from the commissioning phase”, submitted to POLINSAR 2026

This work presents the results of the assessment of the radiometric stability of the BIOMASS mission, a key step in validating the performance of its P-band radar instrument. The evaluation of stability relies on the exploitation of Permanent Scatterers (PSs), which are assumed to be radiometrically stable over time, though their reflectivity varies spatially from scatterer to scatterer. In contrast, any residual radiometric gains are expected to remain spatially constant but vary temporally, making PSs an ideal reference for disentangling temporal instrument variations from scene-dependent effects.

The proposed approach involves a two-stage processing framework. In the first stage, PSs are detected using a stack of radar images, where the reliability of detection increases with the number of images. During the BIOMASS Commissioning Phase, however, the available acquisitions per site were limited to between 7 and 11 images, depending on location. Consequently, the initial part of this study focused on evaluating the probability of false alarms and missed detections as a function of the number of images and the Signal-to-Noise Ratio (SNR) of the PSs. Simulation results demonstrated that, even under these constraints, PSs can be detected with satisfactory reliability.

The second part of the analysis focused on evaluating the performance of the estimator for the residual radiometric gains. Since BIOMASS is the first mission operating at P-band, no prior information was available on the expected density of PSs within a scene. To address this uncertainty, extensive simulations were conducted by varying the number of PSs from as few as 10 to several hundred. The results indicated that the estimator remains robust even with a relatively small number of PSs, provided that the average SNR is sufficiently high to ensure stable estimates.

Following the satellite launch, the algorithm was tested on multiple sites worldwide, encompassing a wide range of topographies, vegetation types, levels of Radio Frequency Interference (RFI), and the presence or absence of urban areas. The analysis revealed that PSs are more abundant at P-band than initially expected, disproving earlier assumptions of their scarcity. The results on radiometric stability consistently confirmed excellent instrument performance, with observed variations remaining below 0.2 dB across all test sites. These findings validate both the robustness of the proposed PS-based method and the outstanding radiometric stability of the BIOMASS sensor.