Editorial: MmWave technologies as opportunistic ISAC for environmental monitoring

Integrated Sensing and Communication (ISAC) is rapidly evolving from a forward-looking concept into a practical design principle for modern wireless systems. This Research Topic focuses on opportunistic ISAC, where sensing functionality is extracted from existing communication infrastructure, including commercial microwave links (CMLs), satellite channels, and automotive radars, to monitor the environment without deploying dedicated, costly hardware. By treating ambient radio signals as "opportunistic" sensors, one can retrieve high-resolution environmental information from networks that already exist. This capabilities expands the capabilities of the standard ISAC paradigm, as it uses an already deployed communication hardware, which was not originally designed for ISAC.Environmental information is often already embedded in radio measurements. For example, rainfall and water vapor affect microwave signlas, and can cause extra attenuation on microwave links channels. By processing the attenuation of these links, rainfall and water vapor monitoring can be realized. In addition, mmWave sensing also captures aspects of the physical environment, where robust processing of sparse radar point clouds can reveal structured features such as road boundaries. Turning these diverse sources of information into reliable weather and environmental products requires careful modeling, robust detection and estimation under non-stationary conditions, and principled fusion with complementary sensors and data sources.The five articles in this Topic reflect these capabilities from a few different angles. They show how opportunistic measurements can be turned into rainfall and humidity products, how algorithmic choices can be made with the end metric in mind, and how mmWave sensing methods can be advantageous for smart-city monitoring and transportation safety. In this short editorial, we summarize each article in turn and discuss its context and importance within the broader field.