arXiv:2610.37400v1 Announce Type: new Abstract: Monitoring the relative radiometric stability of the European Space Agency's BIOMASS P-band synthetic aperture radar (SAR) requires separating instrumental and processing effects from temporal changes in the scene backscatter. This study investigates the use of permanent scatterers (PSs) as opportunistic references for estimating residual radiometric gains without dedicated calibration targets. The PS-based calibration (PSCal) framework combines amplitude-based scatterer selection with iterative maximum-likelihood gain estimation. Simulations assess detection and estimation performance under the limited number of acquisitions available in BIOMASS stacks, highlighting the importance of stringent selection thresholds and the dependence of estimation accuracy on scatterer abundance and signal-to-noise ratio. The approach is applied to datasets over Nairobi, Kenya, and Cordoba, Argentina. Over Nairobi, PSCal identifies an approximately 1 dB radiometric anomaly subsequently traced to a defect in radio-frequency interference mitigation, while the remaining acquisitions exhibit estimated gain variations generally within 0.1 dB. Over Cordoba, twelve acquisitions spanning two mission phases separated by approximately eight months yield residual gain variations on the order of 0.1 dB despite pronounced changes in the scene backscatter and acquisition geometry. Results were observed to be consistent with measurements from the BIOMASS Calibration Transponder (BCT) installed in New Norcia (Australia). These results demonstrate the potential of carefully selected P-band PSs to complement dedicated calibration infrastructure for relative radiometric monitoring and the detection of processing anomalies.