###### Abstract  
The absolute radiometric calibration of a satellite sensor is the critical factor that ensures the usefulness of the acquired data for quantitative applications on remote sensing. This work presents the results of the first cross-calibration of the sensor on board the Sistema Satellat de Observacion de la Tierra (SSOT) Chilean satellite or Air Force Satellite FASat-C. RapidEye-MSI was chosen as the reference sensor, and a simultaneous Nadir Overpass Approach (SNO) was applied. The biases caused by differences in the spectral responses of both instruments were compensated through an adjustment factor derived from EO-1 Hyperion data. Through this method, the variations affecting the radiometric response of New AstroSat Optical Modular Instrument (NAOMI-1), have been corrected based on collections over the Frenchman Flat calibration site. The results of a preliminary evaluation of the pre-flight and updated coefficients have shown a significant improvement in the accuracy of at-sensor radiances and TOA reflectances: an average agreement of 2.63% (RMSE) was achieved for the multispectral bands of both instruments. This research will provide a basis for the continuity of calibration and validation tasks of future Chilean space missions.  
F 1 2016 8612 10.3390/rs8070612 11 March 2016 11 July 2016 21 July 2016 11 March 2016 11 July 2016 11 July 2016 11 March 2016 11 July 201  
mission, is under consideration; in this context, Cal/Val activities and the exploration of potential sites in Chile are required for the compatibility of the data from the sensors on-board both FASat satellites.  
Every space platform is affected by degradation processes during its operational life [13; 14], one of the effects being the variation of the radiometric response of the instruments. In the case of satellites without an On-Board Calibrator (OBC), such as FASat-C, any variation must be monitored and compensated relying on vicarious targets, such as desert sites [5].  
In 2013, [PERSON] et al. [9] performed an in-flight absolute calibration activity for FASat-C in Santiago, Chile, in which the reliability of the pre-flight calibration coefficients was assessed. The objective of the present research is to update the radiometric calibration coefficients of the multispectral bands of NAOMI-1, through a cross-calibration using the simultaneous nadir overpass approach [15; 16; 17; 5; 18], based on the research of [PERSON] et al. [19; 20]. RapidEye-MSI was the reference sensor against which FASat-C was calibrated, and EO-1 Hyperion provided data for the calculation of a Spectral Band Adjustment Factor (SBAF) [21].  
This article is organized as follows: Section 2 presents a brief background on the absolute calibration of satellite sensors in general and the required compensation factors. Section 3 describes the study area and the satellite data acquisition. Section 4 provides the details of the method applied for the cross-calibration of FASat-C. Section 5 presents the obtained results and a preliminary evaluation of the pre-flight and updated calibration. Section 6 includes a discussion about the factors that have influenced the results, and in Section 7, the conclusions of the study and future remarks are outlined.