SkyCap: Bitemporal VHR Optical-SAR Quartets for Amplitude Change Detection and Foundation-Model Evaluation

TL;DR

SkyCap builds a bitemporal optical–SAR dataset by transferring optical change labels to co-registered SAR, enabling SAR amplitude change detection (ACD) without SAR-expert annotations. The study compares SAR-specific pretraining against transferring optical foundation models with various SAR preprocessing, finding that optical FMs with dB+Z-score preprocessing achieve the best ACD performance on Capella X-band data. Results show a strong dependence on preprocessing alignment and that optical FM rankings in optical change detection do not always translate to SAR ACD. The work demonstrates a scalable path for evaluating foundation models on VHR SAR ACD and highlights remaining modality gaps for all-weather infrastructure monitoring.

Abstract

Change detection for linear infrastructure monitoring requires reliable high-resolution data and regular acquisition cadence. Optical very-high-resolution (VHR) imagery is interpretable and straightforward to label, but clouds break this cadence. Synthetic Aperture Radar (SAR) enables all-weather acquisitions, yet is difficult to annotate. We introduce SkyCap, a bitemporal VHR optical-SAR dataset constructed by archive matching and co-registration of (optical) SkySat and Capella Space (SAR) scenes. We utilize optical-to-SAR label transfer to obtain SAR amplitude change detection (ACD) labels without requiring SAR-expert annotations. We perform continued pretraining of SARATR-X on our SAR data and benchmark the resulting SAR-specific foundation models (FMs) together with SARATR-X against optical FMs on SkyCap under different preprocessing choices. Among evaluated models, MTP(ViT-B+RVSA), an optical FM, with dB+Z-score preprocessing attains the best result (F1$_c$ = 45.06), outperforming SAR-specific FMs further pretrained directly on Capella data. We observe strong sensitivity to preprocessing alignment with pretraining statistics, and the ranking of optical models on optical change detection does not transfer one-to-one to SAR ACD. To our knowledge, this is the first evaluation of foundation models on VHR SAR ACD.