Understanding spectrum use in cellular networks is important for efficient frequency allocation, base station deployment planning, and improving network performance through spectrum optimization and interference reduction. Gaining these insights requires systematic monitoring and analysis across multiple transmitters and carriers. Existing commercial spectrum monitoring tools have limitations, including high costs, delayed support for new technologies, closed architectures, and complex coordination requirements with equipment vendors for customized data collection. We present SyncScan, a spectrum monitoring system based on software-defined radios that provides a detailed view of cellular network deployments. SyncScan identifies active cells across frequency bands, decodes cell broadcast channels to extract configuration details correlates propagation measurements for transmitter localization, and tracks cells’ spectrum usage patterns. Evaluation of SyncScan demonstrates its ability to generate comprehensive datasets about cellular network deployments. To facilitate research and innovation in spectrum monitoring, we have released SyncScan as open-source, enabling researchers and developers to extend and improve system functionality freely.