Abstract
The CubeSat Multispectral Observing System (CUMULOS) was a three-camera secondary payload that flew on the Integrated Solar Array and Reflectarray Antenna (ISARA) 3U CubeSat mission, with the goals of researching the use of commercial cameras for Earth remote sensing, and demonstrating unique nighttime remote sensing capabilities. CUMULOS was deployed on 6 December, 2017 by the Cygnus CRS OA-8E mission into an approximately 450-km circular 52° inclination orbit. After the successful conclusion of the primary ISARA mission, the CUMULOS payload was activated and achieved first light on 11 June, 2018. The CUMULOS mission ended a bit over a year later, when battery charging and power systems limitations prevented new imaging experiments from being performed, after the last successful collect on 15 June, 2019. Three separate cameras comprised the CUMULOS payload: 1) a visible ON Semiconductor (VIS) Si CMOS camera, 2) a FLIR Tau SWIR thermoelectrically stabilized shortwave infrared (SWIR) InGaAs camera, and 3) a FLIR Tau 640 longwave infrared (LWIR) vanadium oxide microbolometer. A critical part of the CUMULOS mission was investigating how the three relativity inexpensive commercial, off-the-shelf (COTS) focal planes and associated cameras would perform in a variety of nighttime related settings to produce high quality imagery and radiometrically calibrated images if possible. The three sensors had nadir ground sample sizes of 133, 450, 306-m for VIS, SWIR and LWIR respectively. Ground-based calibration measurements were performed on the CUMULOS cameras, the first of our cubesat sensors on which we attempted radiometric calibration. Issues associated with this first-time effort led to a non-ideal state of the ground (pre-launch) calibration. This paper will briefly outline what was attempted on the ground, but will primarily focus on what calibrations were performed on orbit to allow radiometric calibration to be completed. The on-orbit activities included: 1) combining observations and models of stars α Tau and α Lyra to derive irradiance responsivity values, and to derive radiance responsivity calibration terms, 2) programming dithers into stellar calibration experiments to improve background subtraction to find dim objects, and 3) augmenting ground darks with very low exposure time images taken during collections, and/or dark deep space frames. It was typically found that the dark frames taken on orbit were superior to dark frames taken on the ground only for the SWIR sensor. The SWIR sensor suffered from a high and growing number of hot pixels (radiation damage) that caused problems identifying stellar objects, and degraded science quality images. The LWIR calibration is still being worked. In this paper we will: 1) address the stellar calibration of the VIS and SWIR sensors on-orbit and compare different datasets to assess the accuracy and repeatability of those calibrations, 2) show how the SWIR sensor’s hot pixels changed and increased over time and the methods used to suppress them, and 3) present images taken by the CUMULOS VIS and SWIR sensors and compare them to VIIRS images of the same region at the same time.
This research was funded by The Aerospace Corporation’s Independent Research and Development program.
A Radiometrically Calibrated CubeSat Sensor: CUMULOS
The CubeSat Multispectral Observing System (CUMULOS) was a three-camera secondary payload that flew on the Integrated Solar Array and Reflectarray Antenna (ISARA) 3U CubeSat mission, with the goals of researching the use of commercial cameras for Earth remote sensing, and demonstrating unique nighttime remote sensing capabilities. CUMULOS was deployed on 6 December, 2017 by the Cygnus CRS OA-8E mission into an approximately 450-km circular 52° inclination orbit. After the successful conclusion of the primary ISARA mission, the CUMULOS payload was activated and achieved first light on 11 June, 2018. The CUMULOS mission ended a bit over a year later, when battery charging and power systems limitations prevented new imaging experiments from being performed, after the last successful collect on 15 June, 2019. Three separate cameras comprised the CUMULOS payload: 1) a visible ON Semiconductor (VIS) Si CMOS camera, 2) a FLIR Tau SWIR thermoelectrically stabilized shortwave infrared (SWIR) InGaAs camera, and 3) a FLIR Tau 640 longwave infrared (LWIR) vanadium oxide microbolometer. A critical part of the CUMULOS mission was investigating how the three relativity inexpensive commercial, off-the-shelf (COTS) focal planes and associated cameras would perform in a variety of nighttime related settings to produce high quality imagery and radiometrically calibrated images if possible. The three sensors had nadir ground sample sizes of 133, 450, 306-m for VIS, SWIR and LWIR respectively. Ground-based calibration measurements were performed on the CUMULOS cameras, the first of our cubesat sensors on which we attempted radiometric calibration. Issues associated with this first-time effort led to a non-ideal state of the ground (pre-launch) calibration. This paper will briefly outline what was attempted on the ground, but will primarily focus on what calibrations were performed on orbit to allow radiometric calibration to be completed. The on-orbit activities included: 1) combining observations and models of stars α Tau and α Lyra to derive irradiance responsivity values, and to derive radiance responsivity calibration terms, 2) programming dithers into stellar calibration experiments to improve background subtraction to find dim objects, and 3) augmenting ground darks with very low exposure time images taken during collections, and/or dark deep space frames. It was typically found that the dark frames taken on orbit were superior to dark frames taken on the ground only for the SWIR sensor. The SWIR sensor suffered from a high and growing number of hot pixels (radiation damage) that caused problems identifying stellar objects, and degraded science quality images. The LWIR calibration is still being worked. In this paper we will: 1) address the stellar calibration of the VIS and SWIR sensors on-orbit and compare different datasets to assess the accuracy and repeatability of those calibrations, 2) show how the SWIR sensor’s hot pixels changed and increased over time and the methods used to suppress them, and 3) present images taken by the CUMULOS VIS and SWIR sensors and compare them to VIIRS images of the same region at the same time.
This research was funded by The Aerospace Corporation’s Independent Research and Development program.