## 4 Discussion  
### Ocean Wind Speed  
In Figure 1, it is notable that in the current processing configuration the GPS DDM delivers the highest SNR (\(\sim\)1.5 dB greater than BeiDou-3 and Galileo), despite this surface having the highest wind speed conditions. This may result from differences in the signal processing including available signal bandwidth, front-end filtering effects, and coherent integration times. Transmitted GPS L1 C/A signals have a bandwidth of \(\sim\)2 MHz, whilst Galileo E1 OS and BeiDou-3 B1C services have a wider spectrum of \(\sim\)4 MHz (if only the BOC(1, 1) component is taken into account), a change necessitated in part by the different modulation type used. Figures S4 and S5 show the Galileo signal power spectral density and TDS-1 frequency response. In the context of remote-sensing applications, wider bandwidth of reflected signals (as filtered by the receiver front-end) are expected to be associated with increased horizontal surface resolution, but such reduced scattering area will also result in reduced reflected power, thereby lowering the available SNR [18]. Lower available signal-to-noise ratios have the potential to reduce the accuracy of scatterometric estimates of surface parameters; however, this effect could be at least partially mitigated by the use of longer integration times, also courtesy of the smaller Galileo footprint which potentially allows for longer integration at a given bandwidth. Further work to optimize signal processing of Galileo and BeiDou-3 returns for remote sensing applications by carefully balancing the above factors will likely help mitigate some of the current limitations.  
At first glance, the shape of the SNR response to wind-speed (Figure 3) seems to apply to all Galileo transmitters examined over \(\sim\)4 years of collections. However, there are some notable exceptions to this;  
Figure 5: (**a**) TechDemoSat-1 (TDS-1) tracks retrieved on 18 October 2018 (descending pass) including transition from open ocean to sea-ice from GPS (right) and Galileo (left) reflected signals. (**b**) Received SNR for the two tracks against distance from sea-ice edge. Along-track SNR shows a strong response as the specular point location transitions on to sea-ice, indicating high sensitivity of both Galileo and GPS to surface characteristics.  
for example, E11 appears to show no apparent response to wind speed, this is likely associated with contamination from noise variance as the average SNR level of this E11 subset appears to be particularly low (average of \(\sim\)0.18 dB). E11 is an in orbit validation (IOV) satellite with a lower transmit power (95 W) than the Full Operational Capability (FOC) satellites (254-273 W) [23]. Conversely, the subset including reflected data from Galileo E31 appears to achieve a higher-than-average SNR (average of \(\sim\)0.65 dB). This likely results from the relatively high antenna gain towards the specular point (AGSP) of the E31 collections (Figure S3--supporting information). In summary, as the observables considered so far are not radiometrically calibrated, some of these effects are most likely related to factors including different equivalent isotropic radiation power (EIRP) between Galileo satellites, viewing geometry, location of the specular point in the field of view of the instrument, and associated antenna gain.