## 6 Discussion
In their research, the degree of agreement between the adjusted TOA reflectances of MODerate Resolution Imaging Spectroradiometer (MODIS) and Enhanced Thematic Mapper Plus (ETM+) increased by 1% once SCIAMACHY-based SBAFs were applied. Differences in the range of \(-\)3.01%-5.18% were reported between Hyperion-based and SCIAMACHY-based SBAFs, as well. In that case, the reported improvement was not only due to the 1-nm spectral resolution of SCIAMACHY, but also because of differences in the overall spectral shape of the reflectances measured by both sensors. The results of the preliminary evaluation of the cross-calibration of FASat-C are affected by the lack of knowledge of SBAF over some targets, since differences in GSD do restrict the evaluation,at least over Frenchman Flat. As a result, the TOA reflectance of some targets cannot be properly characterized by the Hyperion 30-m GSD, which did not allow the estimation of SBAF over those areas. The averaged factors applied for the evaluation process could be a possible cause of the higher biases and absolute errors, mainly for lower reflectance targets that do not present Lambertian behavior. In that sense, the results presented in this work suggest that per-coverage SBAF calculations are required for validation purposes; then, for future validation activities, this point should be considered in order to have representative and properly-adjusted samples. The previous idea supports the need for a validation over heterogeneous areas, where larger patches of different land covers are available. Besides, as mentioned by [PERSON] et al. [21], [PERSON] et al. [61] and [PERSON] et al. [62], TOA simulations, based on spectral libraries and in situ measurements, should be explored to evaluate improvements on the SBAF. However, despite the fact that spectral mismatches have been compensated through SBAF, this adjustment considers nominal conditions and does not account for the different sensitivities to the influence of atmospheric absorption features at specific wavelengths [17; 54; 61] or for any variation in the spectral performance of the sensor. According to several authors, other possible sources of uncertainty in the calibration of satellite sensors are the variations affecting the SRFs [91; 92; 93; 94; 95; 96; 97]. These factors, which generate biases in measured radiances and in data-derived parameters, are related to changes in the transmittance of filters or optical components and to shifts of the SRFs towards shorter or longer wavelengths. Therefore, the influence of SRF shifts on the results of the present work is rather likely, since any change in the instrumental spectral performance directly affects the accuracy of the radiometric calibration [94; 95]. In connection with the previous point, an example is that smile and keystone effects in Hyperion data have been reported to generate absolute radiometric differences in the order of 5%-10%, between airborne and in situ measurements [91; 92]. Furthermore, spectral and radiometric issues generate uncertainties, which propagate to SBAF and to the estimated calibration coefficients. [PERSON] et al. [19] emphasized the importance of having accurate values of SBAF, since any uncertainty in the cross-calibration is directly proportional to the uncertainty in SBAF. Different possible contributions to the observed errors have been mentioned and, according to the referred previous research [94; 95; 96; 97], some of them are more critical than others (e.g., SRF and BRDF related). Consequently, one should not exclude that the total contribution of these factors might be the cause of the resulting, although reduced, mismatches of the cross-calibration of FASat-C. Nevertheless, as mentioned before, MAPE, RMSE and MBE have substantially decreased after cross-calibration, meaning that the FASat-C radiances and reflectances are in good agreement with the RapidEye values.