Abstract
The AeroCube-11 spectral satellite (also known as AC-11 R3) is a visible and near infrared (VNIR) multispectral pushbroom imager integrated in a 3U CubeSat. AC-11 R3 utilizes six Landsat-8 Operational Land Imager (OLI) filters arranged in a butcher-block configuration overlaid onto the focal plane array (FPA). While CubeSats are designed to be small and relatively inexpensive, thorough ground calibrations were still performed with a short development time. We report on the calibration activities for AC-11 R3, which were performed in one of Aerospace’s TVac chambers fed by a large integrating sphere, a collimator, direct illumination by a lamp, and scattering of lamp light by a Lambertian screen. The calibration tests performed studied dark current (as a function of FPA temperature and gain), reciprocity (by varying illumination and integration time), electron conversion gain (by varying illumination), small source linearity (by toggling a weak source on and off in the presence of a brighter, varying illumination source), flat field, end-to-end spectral response, modulation transfer function (MTF, via a knife edge test), point response function (PRF, via a point source), and distortion (via imaging of a grid of dark points). Some of the lessons learned from the characterization process are recounted to improve speed and efficiency of characterization of similar systems.
A high level GUI was developed to operate the FPA in different modes and to test the effect of low-level FPA settings on data quality. This mitigates human error when manipulating FPA settings at a low programming level. For instance, determination of optimal bias for the FPA for different FPA temperatures and gains is crucial prior to calibration tests. It was necessary to increase bias such that the full +/- 3 sigma distribution of pixel values lay above zero to make accurate statistical assessment of images. Additionally, ensuring optimal clocking of data channels (skew correction) minimizes frame-to-frame variation in signal per pixel. However, as this was performed after the FPA was integrated into the payload, some basic functionality tests were performed on the critical path. Ideally, such testing would be performed prior to FPA integration so experienced users could minimize the time spent on calibration. Finally, rather than performing each calibration test sequentially, we exposed the FPA to a matrix of temperatures, illumination levels, integration times, and gains in order to perform many tests in parallel. For example, the same images may be used for both reciprocity and electron conversion gain.
Calibration of a Multi-Spectral CubeSat with LandSat Filters
The AeroCube-11 spectral satellite (also known as AC-11 R3) is a visible and near infrared (VNIR) multispectral pushbroom imager integrated in a 3U CubeSat. AC-11 R3 utilizes six Landsat-8 Operational Land Imager (OLI) filters arranged in a butcher-block configuration overlaid onto the focal plane array (FPA). While CubeSats are designed to be small and relatively inexpensive, thorough ground calibrations were still performed with a short development time. We report on the calibration activities for AC-11 R3, which were performed in one of Aerospace’s TVac chambers fed by a large integrating sphere, a collimator, direct illumination by a lamp, and scattering of lamp light by a Lambertian screen. The calibration tests performed studied dark current (as a function of FPA temperature and gain), reciprocity (by varying illumination and integration time), electron conversion gain (by varying illumination), small source linearity (by toggling a weak source on and off in the presence of a brighter, varying illumination source), flat field, end-to-end spectral response, modulation transfer function (MTF, via a knife edge test), point response function (PRF, via a point source), and distortion (via imaging of a grid of dark points). Some of the lessons learned from the characterization process are recounted to improve speed and efficiency of characterization of similar systems.
A high level GUI was developed to operate the FPA in different modes and to test the effect of low-level FPA settings on data quality. This mitigates human error when manipulating FPA settings at a low programming level. For instance, determination of optimal bias for the FPA for different FPA temperatures and gains is crucial prior to calibration tests. It was necessary to increase bias such that the full +/- 3 sigma distribution of pixel values lay above zero to make accurate statistical assessment of images. Additionally, ensuring optimal clocking of data channels (skew correction) minimizes frame-to-frame variation in signal per pixel. However, as this was performed after the FPA was integrated into the payload, some basic functionality tests were performed on the critical path. Ideally, such testing would be performed prior to FPA integration so experienced users could minimize the time spent on calibration. Finally, rather than performing each calibration test sequentially, we exposed the FPA to a matrix of temperatures, illumination levels, integration times, and gains in order to perform many tests in parallel. For example, the same images may be used for both reciprocity and electron conversion gain.