William H. Swartz1
Lars P. Dyrud2
Warren J. Wiscombe3
Steven R. Lorentz4
Stergios J. Papadakis1
Dong L. Wu3
Robert A. Summers1
V. Edward Wells1
1Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland
2Charles Stark Draper Laboratory Inc., Cambridge, Massachusetts
3NASA Goddard Space Flight Center, Greenbelt, Maryland
4L-1 Standards and Technology Inc., New Windsor, Maryland
NASA’s Earth Science Technology Office recently funded a CubeSat mission to demonstrate technology needed to measure the absolute imbalance in the Earth’s radiation budget for the first time. This imbalance of less than 1 percent in the incoming and outgoing energy drives climate change. The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) will demonstrate an affordable, accurate radiometer that directly measures Earth-leaving fluxes of total and solar-reflected radiation. The RAVAN mission is led by (authors) Bill Swartz of the Johns Hopkins University Applied Physics Laboratory (APL) and Lars Dyrud of Draper Laboratory and their partners at L-1 Standards and Technology and NASA’s Goddard Space Flight Center.
Our ability to understand and predict future climate is limited by our ability to track energy within the Earth system. Virtually all the energy input into the Earth system comes from the Sun. The total solar irradiance (TSI) reaching the Earth is 1360.8 ± 0.5 W m−2, as measured from space during the most recent solar minimum by the Total Irradiance Monitor on board the Solar Radiation and Climate Experiment . Accounting for geometry, this means that the total incoming radiation (TSI/4) the Earth receives integrated over all wavelengths is 340.2 W m-2. Under equilibrium conditions, such as thought to have existed during the pre-industrial era, the total outgoing radiation (TOR), including both shortwave, solar-reflected and longwave, thermally emitted flux, is equal to the total incoming radiation: 340.2 W m-2. If there is an imbalance, however, the total energy of the Earth system will change, and sooner or later the climate will be impacted.
The Earth radiation imbalance (ERI) is the single most important number for predicting the course of climate change over the next century , . If ERI is negative, meaning the Earth radiates more than the input 340.2 W m-2, Earth will cool. If ERI is positive, Earth will warm as energy accumulates in the atmosphere and oceans. ERI is thought to be on the order of +0.5 to +1 W m-2 as a result of the net effect of anthropogenic emissions of greenhouse gases and aerosols . Accurately measuring ERI would help resolve the current ambiguity between aerosols and ocean down-mixing as the cause of the recent global warming slowdown and would improve the projection of future climate by climate models.
Two key goals lie at the frontier of climate observation from space: (1) measurement of ERI as a global synoptic constraint of the predictions of climate models and (2) measurement of the Earth radiation diurnal cycle at accuracies commensurate with the global imbalance. To achieve these challenging goals, a new approach to the Earth radiation budget is needed. ERI is too small to be measured definitively by previous and current space assets, due in part to temporal and spatial coverage that does not capture the system’s inherent and rapid variability; further, there has heretofore been a reliance on climate model calculations, making it difficult to come to closure on the Earth radiation budget. The maturation of small satellites, hosted payloads, and constellation technologies, however, provides a unique and timely opportunity for making the next great leap in Earth radiation budget measurement. What is needed is a space-based analog of the Argo ocean observation network: a constellation of compact, spaceborne radiometers that are absolutely accurate to NIST-traceable standards and that can be affordably built in quantities near 100 (see Fig. 1). Such a constellation would enable accurate, un-tuned measurements of ERI with the diurnal and multi-directional sampling needed to capture spatiotemporal variations in clouds, surfaces, natural and anthropogenic aerosols and gases, vegetation, and photochemical phenomena.
Before an Earth radiation budget constellation exploiting hosted payloads or inexpensive small satellites can be realized, it is necessary to build and fly a compact radiometer that captures all outgoing radiation from the ultraviolet (200 nm) to the far infrared (200 mm) with climate accuracy (better than 0.3 W m-2 absolute). Further, we have to show that the accuracy standard remains stable over time on orbit, and that such a radiometer is possible at low cost. These are the challenges RAVAN addresses.
RAVAN will demonstrate two key technologies that enable accurate, absolute Earth radiation measurements using a remarkably small instrument, developed at L‑1 (Fig. 2). The first is the use of vertically aligned carbon nanotubes (VACNTs; grown at APL) as the radiometer absorber. VACNT “forests” are some of the blackest materials known and have an extremely flat spectral response over a wide wavelength range. In addition to providing a very good approximation of a blackbody, they are ideal for space-based applications because they do not outgas, are mechanically robust, do not cause particulate contamination, and have very large thermal conductivity. The second key technology is the gallium calibration source. Embedded in RAVAN’s sensor head contamination cover is a gallium fixed-point blackbody that serves as an on-orbit calibration transfer standard. The blackbody consists of a high-purity gallium cell (99.9995 percent) located directly over the detector. The calibration source is used as a stable and repeatable reference to track the long-term degradation of the sensor. Additionally, design and manufacturing engineers from Draper Laboratory will support design review and validation to ensure that the RAVAN radiometer will be able to be economically manufactured in the quantities required for constellation measurement of ERI.
RAVAN is designed to fly on APL’s Multi-Mission Nanosatellite (MMN), an integrated space vehicle fitting in the standard 3U CubeSat volume of 10 × 10 × 34 cm3. The MMN bus has been designed for high reliability, radiation tolerance, mechanical integrity, and good thermal control in low Earth orbit. Two MMN prototypes are currently operating nominally on orbit. The RAVAN payload is accommodated in a module that integrates with the bus using a standard mechanical and electrical interface (Fig. 3). During the mission, RAVAN will observe the entire Earth disk with a wide field of view. The required pointing knowledge of less than one degree will be provided by a star tracker as the primary spacecraft attitude sensor. The spacecraft command and control computer also hosts a GPS receiver, which provides accurate position information for ground-based orbit determination processing as well as accurate time. A half-duplex UHF transceiver supports telemetry and command communication to APL’s nanosatellite ground station.
With a launch in fall 2015, RAVAN will demonstrate key technology advances that are needed to enable a new class of Earth science mission, one that targets measurement of Earth’s radiative diurnal cycle and absolute energy imbalance for the first time.
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