# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 4 Calibrated Data Quality Analysis  
To validate this calibration approach, we have used a vicarious surface calibration target. In order to reduce the uncertainty within a scene, i.e., the actual variability due to geophysical properties of the surfaces, we required surfaces with large areas of constant observational conditions for a prolonged time. Observations dominated by coherent scattering, with smaller scattering areas (\(\sim\)1 km \(\times\) 1 km), are useful because they have less spatial variability. The expected scene variability varies with the type of scattering surface. For example:  
* Land surfaces have a large scene composition variability with convoluted geophysical parameters characterization that impact the power reflected from those areas. For example, two specular points side by side will not have the same power;
* Ocean surfaces are generally rough. The roughness varies with the wind speed intensity and the wind direction (which can change quickly). Measurements from the ocean are therefore not constant over long periods of time;
* Sea ice surfaces are highly coherent and are usually characterized by low variability characteristics over large areas. Particularly in the Arctic, large areas of old sea ice are formed and remain stable during winter months;
* A global surface coherency mapping was developed by authors in [35; 36] using TDS-1. Results showed that between June and August, the most coherent results come from the sea ice surrounding the Antarctica, the Sahara Dessert, and Australia, while between November and March, the most coherent results come from Arctic sea ice, the Sahara Dessert, and Australia;  
Expected calibration effect:  
The calibration surface target selected are extensive areas of old sea ice in the Arctic, with minimal presence of first year ice towards the Alaskan coast, during the month of February in 2018. At this time, the sea ice was well established with constant 100% concentration. A research study [18] indicated that old sea ice is the least reflective type of sea ice when compared to thinner ice such as pancake ice or new ice. Ice type has an impact on the reflectivity and therefore the DDM peak SNR, as it was shown in [18]. That study also showed that young ice has higher reflectivity since it tends to be smoother. Old ice is rougher and shows lower reflectivity as compared to new ice. However, old ice is usually more stable over long periods of time and covers a larger portion of the sea ice. The expected DDM power will be higher than ocean areas (\(>\)15 dB--uncalibrated levels) and lower than the more reflective young sea ice (\(<\)30 dB--uncalibrated levels). Power levels should therefore range between 10-12 dB for the more reflective ice (i.e., \(>\)18 dB--uncalibrated levels). The calibration performed in this study is expected to preserve the variability of the scene due to differences in the sea ice, while correcting for the GPS EIRP dependence as well as the incidence angle dependence derived from the spatial filtering effect of the high gain SMAP antenna. Figures 13 and 14 illustrate the two described characteristics of the selected calibration surface target: 100% sea ice concentration of old ice and first year ice types. These characteristics ensure coherent reflections, i.e., a limited scattering area size to something close to \(\sim\) 1 km \(\times\) 1 km, and no presence of open water. Figure 13 shows the Arctic sea ice coverage during the two extremes of a stable time period. Figure 13 shows that the sea ice coverage remains nearly constant from December to March--2017/2018 winter season, and the ice concentration is 100% for the entire period for most of the area. Figure 14 shows the dominant ice type of the area selected as our calibration target (white dotted box). Figure 14 clearly shows that the Alaskan coast is covered by first year ice (orange), while the rest of the area is covered by old ice (brown). Figure 14 also identifies the dominant ice type, which indicates that over 50% of the area was covered by the indicated particular ice.