GNSS remote sensing

Editorial Global Navigation Satellite System (GNSS) remote sensing comprises GNSS reflectometry (GNSS-R) and GNSS refractometry (also known as GNSS radio occultation or GNSS-RO). GNSS-R involves analyzing measurements of GNSS signals reflected from the Earth's surface, while GNSS-RO utilizes measurements of GNSS signals refracted by the atmosphere, when the slanted propagation path is close to the Earth's limb. Investigations of GNSS remote sensing of the Earth's surface and atmosphere started more than two decades ago. Many ground-based, aircraft-based, and satellite-borne experiments have been conducted, and a huge amount of experimental data has been recorded using dedicated receivers. Various methods and techniques have been proposed to process the recorded data online or offline in order to retrieve the geophysical parameters of interest. It has been proven that GNSS-RO significantly improves weather forecasting, and it has become a key component of the operational observing system. On the other hand, GNSS-R is still an experimental technique, although significant efforts are being dedicated to advance this technology, especially through a number of missions funded or to be funded by space agencies. This special issue publishes several innovative ideas, methods, and applications in GNSS remote sensing, especially in GNSS-R. The issue comprises six research papers of both theoretical studies and practice and one review article. The topics investigated include sea level measurement, snow depth estimation, snow water equivalent estimation, soil moisture estimation, buried object detection, and simplified GNSS signal processing techniques. The review paper [1] states that the global availability of GNSS signals provides the ideal means of observing different parameters of the Earth's atmosphere, land, and ocean surface. The paper summarizes developments in theory and signal processing methodologies and mentions a number of satellite-borne, airborne, and ground-based experiments relating to GNSS reflectometry and refractometry. GNSS remote sensing is rapidly developing into a valuable