Inter-comparison of optical and SAR-based forest disturbance warning systems in the Amazon shows the potential of combined SAR-optical monitoring.

More than half a decade after the launch of the Sentinel-1A C-band SAR satellite, several near real-time forest disturbances detection systems based on backscattering time series analysis have been developed and made operational. Every system has its own particular approach to change detection. Here, we have compared the performance of the main SAR-based near real-time operational forest disturbance detection systems produced by research agencies (INPE, in Brazil, CESBIO, in France, JAXA, in Japan, and Wageningen University, in the Netherlands), and compared them to the state-of-the-art optical algorithm, University of Maryland's GLAD-S2. We implemented an innovative validation protocol, specially conceived to encompass all the analysed systems, which measured every system's accuracy and detection speed in four different areas of the Amazon basin. The results indicated that, when parametrized equally, all the Sentinel-1 SAR methods outperformed the reference optical method in terms of sample-count F1-Score, having comparable results among them. The GLAD-S2 optical method showed superior results in terms of user's accuracy (UA), issuing no false detections, but had a lower producer accuracy (PA, 84.88%) when compared to the Sentinel-1 SAR-based systems (PA ∼ 90%). Wageningen University's system, RADD, proved to be relatively faster, especially in heavily clouded regions, where RADD warnings were issued 41 days before optical ones, and the one that better performs on small disturbed patches ( < 0.25 ha) with a UA of 70.11%. Of all the high-resolution SAR methods, CESBIO's had the best results regarding UA (99.0%). Finally, we tested the potential of three hypothetical combined optical-SAR systems. The results show that these combined systems would have excellent detection capabilities, exceeding largely the producer's accuracy of all the tested methods at the cost of a slightly diminished user's accuracy, and constitute a promising and feasible approach for the forthcoming forest monitoring systems. Recently developed automated SAR-based tropical forest disturbance detection systems showed excellent detection accuracies, even in small, difficult-to-spot deforested patches. SAR detections can be as precise and as fast as optical ones, being more precise and faster in very cloudy areas or in areas subjected to illegal mining. The combination of recently developed SAR and optical warnings systems can yield optimized results, in terms of overall accuracy and producer's accuracy. [ABSTRACT FROM AUTHOR]