## 5 Conclusions and Future Opportunities  
A GNSS-R experiment has been performed from the NASA's SMAP satellite after tuning the radar receiver to the GPS L2 frequency (1227.6 MHz). Delay Doppler Maps (DDMs) were computed on-ground using a clean-replica of the open-access L2C code. Upon investigation, it was determined that the optimum coherent integration time for this experiment was T\({}_{\text{coh}}\) = 5 ms. The high gain of the SMAP's antenna allowed it to receive strong signals from a variety of scenes. First results have been presented to investigate the characteristics of the reflected waveforms over land surfaces and cryosphere. The main observations from this study are: (a) the Polarimetric Ratio (PR) provided by this data set and the soil moisture content provided by the SMAP's radiometer show a Pearson linear correlation factor r \(\sim\)\(-\)0.6; (b) the Signal-to-Noise at H-Pol (SNR\({}_{\text{H}}\)) shows different mean levels for different terrain types: SNR\({}_{\text{H,Lakes}}\) \(\sim\)25 dB, SNR\({}_{\text{H,Sahara}}\) \(\sim\)18 dB, SNR\({}_{\text{H,Greenland}}\) = 17 dB, SNR\({}_{\text{H,Boreal Forests}}\) \(\sim\)13 dB, SNR\({}_{\text{H,Tropical Forests}}\) \(\sim\)12 dB; (c) the waveforms' parameters show sensitivity to seasonal fluctuations over regions with seasonal changes of geophysical parameters expressed by NDVI, and SWI; (d) the spreading of the waveforms' leading and trailing edge reduces for higher values of AGB (from ABG\({}_{\text{Boreal}}\) \(\sim\)100 ton/ha to AGB\({}_{\text{Tropical}}\) \(\sim\)350 ton/ha); (e) the AGB shows a more pronounced effect in GNSS-R signatures (PR, SNR, leading & trailing edges width) than NDVI; (f) the GNSS signals are depolarized upon scattering over areas with rough topography or high levels of AGB; and (g) the large spreading of the waveforms over the Greenland's ice cap suggests that the GNSS signals penetrate the subsurface significantly, which opens the possibility of cryosphere monitoring using GNSS-R sensors from space.  
While the GNSS-R experiment on SMAP was helpful in providing high SNR polarimetric data, the sampling characteristics are not optimal for process studies. On the other hand, the unprecedented improved spatio-temporal sampling provided by the NASA's CyGNSS suggests the possibility to use its data to study soil moisture and vegetation water content variability in the tropical latitudinal bands. A careful assessment should be performed of the potential synergies between CyGNSS and SMAP GNSS-R data sets to evaluate how the SMAP's high SNR polarimetric data could help elucidate some of the scattering mechanisms not observable with the CyGNSS lower antenna gain.  
This research was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech), under a contract with the National Aeronautics and Space Administration (NASA). The JPL Research and Technology Development (R&TD) program is gratefully acknowledged. The authors would like to thanks to [PERSON] and [PERSON] from JPL for their useful help and support. The authors would like to thank [PERSON] for doing available SMAP radiometer data. The authors would like to thanks the anonymous reviewers for their comments and suggestions. (c) 2017 California Institute of Technology. Government sponsorship acknowledged. All authors contributed significantly to this manuscript. [PERSON] processed and analyzed the data; [PERSON] designed the experiment; [PERSON] provided advice and support; [PERSON] and [PERSON] worked on signal processing during the experiment. The authors declare no conflict of interest.