BABAR-ERI: A tiny satellite with giant potential for unlocking cloud and energy flow mysteries

Beginning in the 1960’s, satellite instruments have been measuring 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 at space at infrare3d wavelengths.

The Laboratory for Atmospheric and Space Physics (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 PI of the project supported by NASA’s Earth Science Technology Office (ESTO).

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 the 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 3 instruments fit 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-thousandths of a second).

In a new paper published in Advances in Atmospheric Sciences, 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.