# The Use of SMAP-Reflectometry in Science Applications: Calibration and Capabilities
## 3 Calibration of SMAP-R Signals
### Direct Power Information  
Regardless of the scattering regime--incoherent or coherent--it is important to have knowledge of the transmit power. In both Equations (1) and (2), the direct power information corresponds to both GPS transmitted power \(P_{tx}\) and GPS antenna gain in the direction of the specular reflection \(G_{tx}\). In previous publications [30], two assumptions were made:  
* \(G_{tx}\) is constant due to the fixed observing geometry;
* \(P_{tx}\) biases between GPS satellites are small and no greater than 1.8 dB. In [30], a first assessment of the variability of the freeze/thaw observations used these rough assumptions were considered to have a low impact in the results. To validate the assumptions, we looked into the GPS transmitted power and GPS antenna gain information contained in the metadata files of CYGNSS dataset, shown in Figure 5.  
Figure 5 shows that the transmitted power \(P_{tx}\) variation between GPS satellites can be up to 3.6 dB, twice as large as the maximum estimated in the first studies [30]. Different GPS blocks cause different  
Figure 5: GPS transmitted power \(P_{tx}\) for the 32 satellites of the constellation. The information is in the variation range of those transmitted power. Each color corresponds to a different GPS satellite. (**a**) GPS block IIF satellites are represented by circles, (**b**) GPS block IIRM satellites are represented by squares, and (**c**) GPS block IIR are represented by crosses. transmitter power levels. In Figure 5, we have included the legend with information on the type of block: Figure 5a, GPS block IIF satellites; Figure 5b, GPS block IIRM satellites; and Figure 5c, GPS block IIR. Overall, GPS Block IIRM satellites appear to transmit more power as compared to GPS block IIR satellites. GPS block IIF satellites present powers that vary between 13.8 and 16.3 dB. GPS block IIRM and GPS block IIF satellites have the capability to redistribute transmit power between individual signal components (flex power) which is used for protection against jamming [41]. In addition, it has been reported that the transmitter power has a geographical dependence [41] with an impact up to 2.5 dB on the transmitted power. We use a daily mean of CYGNSS-reported transmitted values, if the power is increased in the middle of the day up to 2.5 dB, we could be introducing a 1.25 dB error to our calibration. As we rely on CYGNSS, all transmitted power variations included in the dataset would be accounted for in our calibration as daily means, but to increase the accuracy, future work should include a collocation of the transmitted power for the same GPS satellite within minutes. On top of the transmitted power, the GPS antenna gain \(G_{tx}\), which is variable and dependent on the particular GPS space vehicle, should also be considered. Figure 5 shows the antenna gain \(G_{tx}\) variability for several GPS satellites for an incidence angle range from \(0^{\circ}\) to \(72^{\circ}\). For SMAP-R, only the range of incidence angles of the specular reflections in the SMAP antenna pattern range \(37.3^{\circ}\) to \(42.7^{\circ}\) needs to be considered. The antenna gain difference observed for all GPS satellites within this range of incidence angles varies from 0.4 to 1 dB. Therefore, by limiting the GPS transmitter values within the range of incidence angles captured by the SMAP antenna, and combining both \(P_{tx}\) and \(G_{tx}\), i.e., Figures 5 and 6, we calculate the total impact that these variations can have on the received power, shown in Figure 7.  
Figure 7 quantifies a total observed difference of 4.2 dB difference is observed for the combined effect of the GPS EIRP \(P_{tx}G_{tx}\) at the specular incidence angle for the range of incidence angles measured by SMAP, i.e., 37.3\({}^{\circ}\) to 42.7\({}^{\circ}\). A difference of 4.2 dB is not negligible and requires standardization. GPS transmitter differences must be accounted for using the data for scientific research. Often, the sensitivity of forward scattered signals to geophysical parameters is small.