# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 4 Calibrated Data Quality Analysis
Overall the calibration produces a flatter response, with a low dependence on the GPS transmitter information reduced from a mean value of 4.32 dB to a mean value of 0.8 dB. Figure 15: Impact of the direct signal power and gain differences between GPS satellites for February 2018 data. Up to 4 dB total difference correction is observed from minimum to maximum values, which applies to both H-pol and V-pol. 2. Figure 17 shows the result of applying a correction to the filtering effect of the SMAP high gain antenna to same data from February 2018. Figure 17 shows a power variability from 0 to \(-\)10 dB in the peak SNR. Most of the time the error is low, but there are some samples showing a sudden increase in variability. Those samples of higher variability correspond to consecutive measurements with incidence angles further away from the SMAP antenna pointing at 40\({}^{\circ}\). This reduction in SNR is therefore due to the SMAP high gain antenna filtering effect on the surface area corresponding to the DDM peak delay and Doppler bin. Similar information is plotted in Figure 18, with corrections applied to the peak SNR as a function of the incidence angle for February 2018. Figure 18 shows that the larger corrections correspond to reflections farther away from the SMAP antenna boresight. When considering only the delay-Doppler bin of the DDM peak, no correction is needed for incidence angles close to the antenna boresight (40\({}^{\circ}\)). For reflections with incidence angles below 38.4\({}^{\circ}\) and over 41.6\({}^{\circ}\) incidence angle, relevant corrections are needed. The correction shown in Figure 18 only applies to peak SNR. To calibrate the whole DDM, the corresponding scattering area normalization needs to be applied to each DDM delay and Doppler bin individually, as shown in  
Figure 16: February 2018 SMAP-R samples as a function of the GPS transmitter information (\(\mathrm{P_{tx}}+\mathrm{G_{tx}}\)) (**a**) uncalibrated data over a small area of the calibration target area (south-west corner) and (**b**) calibrated data over the same small area of the calibration target area. Note that calibrated values correspond to radar cross section values after applying calibration Equation (5), under coherent assumption. Figure 17: Impact of the SMAP antenna high gain filtering effect calibration for February 2018 data. Up to 10 dB total difference correction is observed. The larger errors correspond to reflections farther away from the SMAP antenna boresight. This correction applies to both H-pol and V-pol. Figure 12. The effect of this calibration is shown in Figure 19, where the dependence of the uncalibrated data and the calibrated data to the incidence angle is analyzed. The correction performed to account for the filtering effect of the SMAP high gain antenna is expected to reduce the dataset dependence on incidence angle. Therefore, in order to verify the effect of calibrating the SMAP high gain antenna effect, the whole dataset is plotted as a function of the incidence angle. Figure 19a shows the uncalibrated area following a curvature similar to what is expected from Figure 18. The curvature towards the incidence angles away from 40\({}^{\circ}\) is calibrated in Figure 19b, showing a non-incidence angle dependency for the whole dataset. The uncalibrated dataset shows a total variability of 7 dB, including mean angular dependency and data dispersion. It shows a curved signature dependency with the incidence angle. However, the calibrated data shows no angular dependency with 2.5 dB dispersion. Although the curvature angular dependency is removed, the calibrated data shows some degree of dispersion (2.5 dB) that is part of the intrinsic fluctuations of the scene itself together with the GPS transmitter inaccuracy corrections (-0.8 dB). Calibration Results:  
The calibration was applied to SMAP-R samples acquired during February 2018. Figure 20 shows the variability of the uncalibrated and calibrated measurements at both V-polarization (top) and the H-polarization (bottom) over old ice areas February 2018.