# The CYGNSS Mission: On-Going Science Team Investigations
## 2 Methodology, Results, and Discussions
### Land Surfaces
Reflectometry has been used in various studies for land applications especially for retrieving SMC [53; 54; 55; 56]. In this session, an update on the existing and potential calibration and validation sites for soil moisture applications is provided. The first three presentations demonstrated the potential of the use of CYGNSS over land surfaces to detect and monitor water extent over short time scales [57]. The first method analyzed the distribution of time series of SNR for a given pixel to define dry and wet areas. With this method, a dynamic water extend mask can therefore be derived. Several case studies showed good correlation with rainfall events and the Pekel mask considered as the truth. Sensitivity of CYGNSS to water extent changes over short time scales was demonstrated by analyzing the spatial SNR variations with the seasonality of the Pascagoula River. Better detection results were obtained with a higher sampling rate. Finally, an investigation on the impact of land characteristics, such as SMC, surface water, topography and Vegetation Water Content (VWC) on the coherence of CYGNSS reflected signals was presented. The coherence was quantified using the tracked carrier phase from CYGNSS raw IF data. It was concluded that the coherent scattering term is most often present over inland water bodies. The next topic of this session was assessing the impact of surface roughness, which is a critical parameter for correctly assessing the scattering of GNSS signals over land surfaces. A theoretical model was developed to decompose the surface into topography elevation and slopes, small-scale surface roughness, and surface correlation length. This new parametrization of the surface was tested using several classical forward-scattering models, including Geometrical Optics, Physical Optics, and Numerical Maxwell Model 3D (NMM3D). The modeled DDMs were then compared to CYGNSS-derived DDMs, showing a good agreement. In May 2020, experimental activities were performed along two cal/val sites over the San Luis Valley to characterize the small-scale surface roughness using lidar. Lidar-derived DEMs were generated at two different spatial resolutions, \(\sim\)10 cm and \(\sim\)30 cm. The last three presentations dealt with actual and potential SMC cal/val sites for CYGNSS. First, an update was provided about the status of the SOILSCAPE in situ SMC  
Figure 7: Example of CYGNSS-derived VOD by [PERSON] et al. sensor networks, that were installed in two cal/val sites in the San Luis Valley. All sensors were shown to be working properly. Calibration of the SMC measurements is in process. New potential cal/val sites have been identified in the USA (Walnut Glutch, AZ, White Sands, NM) and in New Zealand. New Zealand cal/val sites are particularly of interest because the next generation GNSS-R receiver [58] is going to be installed on a regional Air New Zealand commercial aircraft to complement CYGNSS data, and to test this new receiver in preparation of a potential CYGNSS follow-on mission. An extensive analysis of the coverage provided by one Air New Zealand commercial aircraft was performed to identify the best cal/val site locations for SMC and wetlands studies. These locations were cross-compared with the CYGNSS coverage, showing several overlapping sites. This project will advance terrestrial and coastal retrievals, by generating long-term datasets with high spatial resolution and high spatiotemporal sampling. In addition, in support of this project, the deployment of the next-generation GNSS-R receiver on one Air New Zealand aircraft was simulated to get a better understanding of the differences of the reflected signal acquired on an aircraft as compared to a satellite. Flight paths, flight frequency, and GNSS-R coverage were analyzed. It was found to have promising coverage, except for some mountain regions in the South Island (Figure 8). Installing this new receiver on one aircraft will provide a large amount of information over both ocean and several land cover types.