# An Assessment of CyGNSS v3.0 Level 1 Observables over the Ocean
## 5 Conclusions  
This work provides an assessment of CyGNSS L1 v3.0 products, which have been recently updated to feature a novel calibration approach that includes the capability to exploit data collected from the zenith front-end to account for the variability of available GPS power. Analysis of the CyGNSS L1 NBRCS shows good temporal stability and negligible inter-receiver and inter-antenna biases, which are constrained to \(\sim\)0.1 dB and \(<\)0.1 dB, respectively. However, prominent geographical biases are found in the global climatologies of NBRCS at given ocean surface conditions. Further examination of the dependence of such biases reveals a strong correlation with viewing geometry, including a marked drop-off at high incidence angles and some variability at lower ones, leading to the observed geographical pattern. Detailed examination of this effect, further segmented by SVN and GPS block, reveals that each block shows different relationships with geometry, with marked biases even between individual SVNs belonging to the same GPS block, indicating the persistence of issues linked to the power calibration of signals originating from different transmitters at different angles. As current CyGNSS L2 wind products are based on the use of two separate L1 observables (LES and NBRCS), the analysis conducted for NBRCS was extended to LES. The examination of the LES observable led to very similar results as found for NBRCS in terms of temporal stability, inter-FM bias, inter-antenna bias, and dependence on viewing geometry (shown in the Appendix A). However, spatial maps of composite average LES over all bistatic configurations revealed an additional geographical pattern that appears to be correlated with the contours of the geoid. This additional pattern is likely associated with inaccuracies in the current estimation process of the LES of the reflected waveform. In summary, a number of biases and residual dependencies were found in the latest CyGNSS v3.0 L1 NBRCS and LES products. All appear to be associated with issues either related to unresolved instrument and processing effects or to inaccuracies in the process of direct signal calibration. To avoid such issues in future GNSS-R missions and datasets, particular attention should be given to how bistatic geometry as well as individual GPS transmit powers are handled in the calibration process. Although only ocean-reflected data were analysed here, the calibration effects found in this study are likely to similarly affect land reflections. Given the growing interest in the community in the exploitation of GNSS-R also for land and ice applications, these results highlight the need for further improvements in our understanding of GNSS-R calibration ahead of future releases of the CyGNSS products, as well as in preparation for future missions. Methodology and investigations: M.L.H., G.F.; software: M.L.H. ; writing--original draft preparation: M.L.H. ; writing--review and editing: All; supervision: G.F., C.G., M.S. All authors have read and agreed to the published version of the manuscript. This work was funded by ESA contract 4000130560/20/NL/FF/gp. Not applicable. The ECMWF ERA-5 dataset analyzed in this study is publicly available. This dataset can be found at: [[https://cds.climate.opernicus.eu/cdsappff/dataset/reanalysis-era5](https://cds.climate.opernicus.eu/cdsappff/dataset/reanalysis-era5)]([https://cds.climate.opernicus.eu/cdsappff/dataset/reanalysis-era5](https://cds.climate.opernicus.eu/cdsappff/dataset/reanalysis-era5)) -single-levels?tab=overview (accessed on 20 August 2021). The CyGNSS v3.0 L1 dataset can be found at: [[https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V3.0](https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V3.0)]([https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V3.0](https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V3.0)) (accessed on 20 August 2021).