###### Abstract
Such sensor degradation is known to occur episodically, and thus, each smallest must be periodically recalibrated to maintain accuracy for both the top-of-atmosphere reflectance (TOAR) and SR retrieval calculated from it. Since orbiting smallsat radiance responses are unknown, a method has been offered for calibration of smallest radiance to standardize AC from RadTran workflows in a process of sensor cross-calibration with datasets of harmonized reflectance from S2 and L8/9 [5,9] and following a similar workflow to LaSRC SR retrieval [9]. This AC pathway is inconvenient because cloud-free and relatively clear atmospheric conditions are needed to match the necessary data between limited sets of satellite overpasses. This pathway may also add uncertainty, partially because bandwise sensor responses for these two satellite standards are not equivalent. More seriously, cross-calibration can introduce uncertainty through temporally mismatched images due to constantly changing atmospheric conditions, even when overpass times vary by minutes [7]. To assure accurate results, such discrepancies require that the workflow for harmonization be closely monitored and managed. Automation and ease of application are paramount for the operation of large flocks of smallsats. Rather than following a radiance-based workflow, CMAC was derived empirically based on the observed effect of atmospheric transmission upon reflectance. This reflectance pathway permits bypassing the consideration of sensor radiance for SR retrieval. Instead, each smallsat can use its pre-launch instrument calibration as the basis for conversion of the recorded raw radiance to an assumed reflectance following well-established processing methods [12]. The assumed reflectance can then be converted to SR estimates through bandwise relationships derived in an automated calibration procedure that customizes CMAC for each VNIR band of each smallsat. Every satellite must be calibrated to enable accurate CMAC processing. Once the bands of a smallsat are calibrated, CMAC SR retrieval can be performed from then on, subject to periodic recalibration to account for changing sensor response. CMAC research and development (R&D) is currently developing calibration software and design of calibration targets to be constructed. In the interim, sensors are calibrated vicariously using TOAR data that are paired with corrected images of AOIs with established SR. While far less precise than applying a calibration target, these temporary workarounds still yield excellent results for sensors aboard smallsats (tested in-house for Planet cubesats), for S2 (as presented in paper 1), and as tested here, for L8/9. CMAC is a new and radical departure from the existing AC technology. The analyses and results presented here can be cross-checked and verified. Available for download from the Supplemental Materials are an annotated image gallery that provides ancillary information related to CMAC and LaSRC AC, a python version of CMAC software v1.1, live spreadsheets used to develop the tables and figures, shapefiles for polygons that spatially define pixel data extractions, and a link to a cloud site where readers can browse, download and test correct images with current CMAC versions for SR retrieval of Landsat 8/9 or S2. Section 2, the Materials and Methods, consists of five subsections. Section 2.1 describes how CMAC derives a grayscale raster of atmospheric effect that scales the reversion to surface reflectance spatially to retrieve surface reflectance across each image as explained in Section 2.2. Section 2.3 introduces the application of quasi-invariant areas for procedures of image-to-image comparison to overcome the lack of surface reflectance at the scale and timing for calibration, evaluation and comparison of the methods. Section 2.4 describes a method for image-to-image calibration that enables the rapid translation of the method for other satellites, including smallsats. Finally, Section 2.5 describes four tests that were conducted to compare CMAC with LaSRC outputs.