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Beginning in the 1960s, satellite instruments have measured Earth's reflected broadband shortwave radiation and emitted longwave radiation. These measurements have been used to estimate Earth's "energy balance," defined as the difference at the top of the atmosphere between the amount of incident solar irradiance absorbed by the Earth system and the amount of terrestrial radiation emitted in space at infrared wavelengths.

The Laboratory for Atmospheric and Space Â鶹ÒùÔºics (LASP) at the University of Colorado Boulder has recently designed and built a new CubeSat instrument.

"This is a stepping stone toward future energy balance observations with much sharper detail and greater accuracy than we've ever had before—helping us unlock new scientific discoveries faster," said Dr. Odele Coddington, the primary investigator of the project. The work is in the journal Advances in Atmospheric Sciences.

The team's motivation was the ambition to resolve cloud signatures in the outgoing radiation from a small platform that provides flexible observing and implementation strategies and easier access to space through more frequent launch opportunities.

What is so special about clouds?

Clouds are the predominant modulator of atmospheric radiation and significant uncertainties remain in understanding the relationships between clouds, radiation, and temperature. Clouds approximately double the reflection of shortwave radiation back to space (i.e. a cooling) and redirect about 20% more radiation back to Earth's surface (i.e. a warming) than would occur in their absence. A 1 km spatial scale is considered cloud-resolving.

The Black Array of Broadband Absolute Radiometers (BABAR) Earth Radiation Imager (ERI) is a downward-pointing pushbroom imager, and its suite of three instruments fits a 12U CubeSat form factor. The primary science channels are two co-registered telescopes that simultaneously measure the radiation leaving Earth in 1 km x 1 km footprints in a shortwave radiation channel and a total radiation channel. The longwave radiation is derived by subtracting the shortwave radiation from the total radiation.

The second instrument is a dual calibration monitor to track and correct any on-orbit degradation of the science channels. The third instrument is a visible wavelength camera for scene context and to facilitate accurate knowledge of the satellite position.

BABAR-ERI's key advances in measuring outgoing broadband radiation with low uncertainty and high spatial resolution come from novel detector arrays micro-fabricated at the Boulder laboratory of the National Institute of Standards and Technology (NIST) with near-perfect absorption properties from the incorporation of "blacker than black" vertically aligned carbon nanotubes. Each pixel of the 32-element detector arrays is approximately 125 µm on each side (a µm is one-millionth of a meter), operates independently from the other pixels in the array, and responds to incident radiation in 10 milliseconds or less (a millisecond is one-thousandth of a second).

In the paper, the team describes the science requirements used to design and build the BABAR-ERI instrument suite, detail instrument performance metrics, and provide a concept of operations for flight.

"Our key next step is securing a space flight to demonstrate BABAR-ERI's technology for enhancing the current science of clouds and radiation and for enabling new discoveries," said Odele.