# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 6 Conclusions  
This manuscript describes the specific characteristics of SMAP-R dataset and how it compares to CYGNSS and TDS-1 missions' measurements. The calibration of the SMAP-R dataset is shown more challenging than the calibration required by other GNSS-R missions specifically designed for the technique. In particular, this manuscript addresses both the lack of direct power information and the filtering effect of the high gain antenna. While consideration for the SMAP antenna pattern is important for SMAP-R, the antenna filtering effect minimally impacts other missions. For example, CYGNSS may observe a similar effect at high incidence angles, as the scattering area extends outside of the CYGNSS antenna beam pattern. In general, only a few delay-Doppler bins around the specular point are considered as input for estimates of geophysical phenomena; near the specular point, antenna filtering effects are of less concern. This manuscript describes the calibration methodology applied to SMAP-R measurements and provides an analysis of the quality of the calibration for a small region characterized by constant old ice. We selected this area to ensure coherent returns (reduced scattering areas) with lower scene variability, which allowed us to better explore the particular corrections performed in our study. Other targets could had been considered, specifically other surfaces characterized by large extensions of coherent scattering. The Sahara Desert, big lakes around the globe, or areas of Australia are some examples of those areas. A standardized methodology can therefore employ the calibration presented in this manuscript performing quality analysis over those surfaces on a regular basis. Our results show that the calibration methodology applied to correct the dependence on GPS transmitter information achieves an 81.5% reduction on the mean bias. The methods presented here reduce the variability observed in the uncalibrated dataset (4.2 dB) to a variability of 0.8 dB in the calibrated dataset. Also, the calibration methodology presented corrects the dependence of the SMAP-R dataset with incidence angle, reducing the mean dispersion from 7 dB to 2.5 dB. This correction is obtained by compensating the filtering effect of the SMAP high gain antenna. Finally, the calibrated data shows an overall reduction in the mean spatial variability of 2-3 dB and a reduction in the areas of high spatial variability that were observed with the uncalibrated data. Future work will include the calibration of the entire DDMs. The calibration will be done applying corrections similar to the one shown in Figure 11 to each bin of the DDM. Or equivalently, applying similar corrections to the ones shown in Figure 17 at each delay-Doppler bin of the DDM. In addition, to eliminate sources of error in the calibration, future work should employ CYGNSS version 3.0 data to obtain more accurate EIRP values as well as perform a subday collocation between SMAP measurements and CYGNSS-reported transmitted powers to avoid averaging out relevant variationsdue to GPS flex mode applied to the different blocks. Future work will also include the development of new science products from the calibrated dataset. Conceptualization, N.R.-A., S.M., E.P., M.M. and X.B.-L.; methodology, N.R.-A. ; software, N.R.-A. ; validation, N.R.-A. ; formal analysis, N.R.-A. ; investigation, N.R.-A. ; writing--original draft preparation, N.R.-A. ; writing--review and editing, N.R.-A., S.M., E.P., M.M. and X.B.-L.; visualization, N.R.-A. and M.M.-; funding acquisition, E.P. and S.M. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The research was supported by a JPL fund on R&A Hydrology & Weather from the Soil Moisture Active Passive (SMAP) project and by a JPL Research and Technology Development (R&TD) Grant (PI-E. Podest). (c) 2019. California Institute of Technology. Government sponsorship acknowledged.