# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 4 Calibrated Data Quality Analysis
There is therefore a possibility that the indicated sea ice type might actually be a mixture of different sea ice types and the GPS signals measures this variability, [18]. Figure 14: Arctic sea ice primary stage of development obtained from the NOAA National Ice Center website ([[https://www.natice.noaa.gov](https://www.natice.noaa.gov)]([https://www.natice.noaa.gov](https://www.natice.noaa.gov))) for February 2018. The brown color corresponds to old ice, and red, orange and blue color correspond to three types of first year ice. Pink color corresponds to new ice and purple color to young ice. The white dotted box defines the calibration target area selected for this study. Figure 13: Arctic sea ice concentration reported by the National Snow and Ice Data Center [43], University of Colorado Boulder on (**a**) December 2017 and (**b**) March 2018. Ice concentration remains constant since December to March. Images courtesy of the National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder. The images are derived from Sea Ice Index NSIDC data product, which relies on NASA-developed methods using passive microwave data from the Defense Meteorological Satellite Program (DMSP) F-18 Special Sensor Microwave Imager/Sounder (SSMIS). The Sea Ice Index [44] was developed by the NSIDC with financial support from NOAA NESDIS and in cooperation with NOAA NGDC. Color scale from 0% (dark blue) to 100% (white) on 10% increments. Calibration  
Equation (5) has therefore been applied to a large dataset of samples gathered in February 2018 within the selected area, accounting for the coherency of the reflected signals. The results of the calibration methodology are next evaluated. Figure 15 shows the differences observed on the received powers due to differences on the GPS EIRP. Data corresponds to February 2018 over the calibration selected area in the Arctic Chukchi-Beaufort Sea area. Figure 15 shows that different tracks measured from different GPS transmitters need corrections that can differ from \(-2\) dB to \(2\) dB from a reference GPS EIRP of 31.5 dB (mean of all GPS EIRP in Figure 6). We compared the dependence of the uncalibrated data and the calibrated data to the differences in the GPS EIRP. In order to verify the effect of calibrating the GPS transmitted EIRP (Ptx + Gtx) (dB), we need to select a pixel and observe its power return though time. We assumed that the surface conditions remained nearly constant and consequently a given pixel should have relatively constant signal power return (low variability) during consecutive measurements (in consecutive days). Because of the SMAP-R surface sampling configuration, observing the same pixels is unlikely. Therefore, to ensure multiple observations in time, instead of selecting a single pixel on the surface, we selected a small area (south-west area within our calibration target area). The caveat with this approach is that the variability within the scene is included in the signal, however, it ensures that enough measurements are available to analyze the time series variability of the GPS transmitter information. Figure 16 shows the effect of calibrating the differences in GPS transmitted EIRP (dB). We selected a reduced area within our calibration target area (south-west corner) that has been observed with a wide range of GPS satellite EIRP's over a number of days. The uncalibrated data (Figure 16a) shows a larger slope than the calibrated data (Figure 16b). A slope of 1.08 over a range of EIRP's (dB) of about 4 dB, causes differences of 4.32 dB in the mean. After calibration, those differences have been reduced to 0.8 dB in the mean (slope of \(-0.2\) dB) achieving an 81.5% reduction on the mean bias, with a residual error of 0.8 dB. The residual error possibly originates from the variability of the scene itself, CYGNSS information inaccuracies, and the actual azimuthal differences in the GPS antenna gain patterns not accounted for in this study.