RAVAN (Radiometer Assessment using Vertically Aligned Nanotubes) Pathfinder Mission
RAVAN is an instrument development project to be flown on a 3U CubeSat within NASA's InVEST (In-Space Validation of Earth Science Technologies) program. The RAVAN CubeSat mission is funded by NASA's ESTO (Earth Science Technology Office). The objective of RAVAN is to demonstrate a radiometer that is compact, low cost, and absolutely accurate to traceable standards. RAVAN and CubeSats allow for constellations that are affordable in sufficient numbers to measure Earth's radiative diurnal cycle and absolute energy imbalance to climate accuracies (globally at 0.3 W/m2) for the first time. The key technologies that enable a radiometer with all of these attributes are: a gallium fixed-point blackbody as a built-in calibration source, and a VACNT (Vertically Aligned Carbon Nanotube) absorber. 1) 2) 3) 4) 5) 6) 7)
VACNTs are the blackest known substance, making them ideal radiometer absorbers with order-of-magnitude improvements in spectral flatness and stability over the existing art. Neither the VACNT, nor gallium blackbody has ever been used in an orbiting instrument, and successful demonstration will raise these technologies and RAVAN from TRL from 5 to 7, paving the way for a constellation Earth radiation budget mission that can provide the measurement that is needed to enable vastly superior predictions of future climate change, serving the goals outlined in NASA's "Climate-Centric Architecture."
The RAVAN mission is led by William H. Swartz of JHU/APL (Johns Hopkins University/ Applied Physics Laboratory), Laurel, MD, and by Lars Dyrud of Draper Laboratory, Cambridge, MA and their partners at L-1 Standards and Technology, and NASA/GSFC (Goddard Space Flight Center).
Some background on ERI (Earth Radiation Imbalance) and ERB (Earth Radiation Budget):
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. ERI is the difference between TSI (Total Solar Irradiance) divided by 4 (TSI/4) and TOR (Total Outgoing Radiation). 8) 9) 10)
Using the TSI of the SORCE (Solar Radiation and Climate Experiment) mission, TSI= 1360.8 ± 0.5 W m-2, as measured from space during the most recent solar minimum by TIM (Total Irradiance Monitor) on board SORCE. 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 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.
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.
Figure 1: ERI (Earth Radiation Imbalance) is most important quantity for climate change (image credit: JHU/APL)
Figure 2: The problem is the absolute value of ERI (image credit: JHU/APL)
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
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 ERB 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 spaceborne 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 (Figure 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.
Figure 3: ERB constellation enabling the definitive measurement of the ERI and diurnal variation (image credit: JHU/APL)
But prior to an ERB 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 UV (200 nm) to the far infrared (200 µm) with a climate accuracy (better than 0.3 W m-2 absolute). Further, the project has 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.
The objective of the RAVAN mission is to demonstrate two key technologies that enable accurate, absolute Earth radiation measurements using a remarkably small instrument, developed at L-1 Standards and Technology Inc., New Windsor, MD (Figure 2).
• The first is the use of VACNTs (Vertically Aligned Carbon Nanotubes, 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 spaceborne 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%) 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. - Two gallium fixed-point black bodies serve as on-orbit infrared sources that, when coupled with deep space looks, provide an additional means to determine the offset for the total channels. The project uses the gallium solid–liquid phase transition (29.76°C) as a stable, repeatable reference for the black body emission to track the long-term degradation of the radiometer sensors.
Additionally, the design and manufacturing engineers from Draper Laboratory will support the design review and validation, to ensure that the RAVAN radiometer will be able to be economically manufactured in the quantities required for a later constellation measurement of ERI.
VACANT (Vertically Aligned Carbon Nanotubes) as Radiometer Absorbers
Carbon nanotubes are an allotrope of carbon that, at a microscopic level, are essentially long, hollow graphene cylinders. These nanostructures have a number of unusual properties that make them ideal for certain applications. Vertically aligned carbon nanotube "forests" are some of the blackest materials known and have an extremely flat spectral response over a wide wavelength range. VACNTs, as shown in Figure 4, are actually mostly empty space and are highly efficient photon traps. In addition to providing a very good approximation of a black body, their high thermal conductivity suits them well as radiometer absorbers. Further, they are ideal for space-based applications because they are compact, do not outgas, and are mechanically robust.
Figure 4: VACNT forest viewed edge-on under a scanning electron microscope. The height of the forest is roughly 300 µm (image credit: JHU/APL)
The VACNT forests used in RAVAN were grown at JHU/APL using water-assisted chemical vapor deposition with ethylene as the carbon feedstock on silicon wafers covered with an iron catalyst layer. Post-growth vapor modifications and plasma etching were then performed to decrease the material's reflectivity further. We experimented with a number of processes with varying VACNT forest thickness, single/multiple growths, and a range of post-growth modification severity, in order to optimize the performance for RAVAN. The infrared reflectivity indicative of early experiments and the final RAVAN flight VACNT radiometer absorbers are shown in Figure 5. The RAVAN VACNTs stay below a target 0.1% reflectivity out to about 13 µm. The project found agreement with literature techniques that increasing the forest thickness to 1 mm and a more aggressive O2 plasma etching post-growth each improved the infrared performance compared to our early experiments.
Figure 5: Spectral reflectivity of VACNT forests produced with two different processes. Both are single growths, but the latter (b) was grown to a greater thickness (1 mm) and with more aggressive post-growth modification (image credit: JHU/APL)
The RAVAN instrument will fly on a 3U CubeSat bus of BCT (Blue Canyon Technologies), Boulder, CO, based on the XB3 model, providing a complete CubeSat bus solution in a highly integrated, precision spacecraft platform. 11) BCT's XB3 has been manufactured to be the highest precision-pointing nanosatellite ever flown. 12)
The BCT bus has 3-axis attitude control afforded by three reaction wheels, three magnetic torque rods, and two star trackers, with a GPS receiver for position. Power is provided by four deployable solar arrays and enough battery capacity to accommodate eclipse and RAVAN's various attitude orientations (Table 1). Communications will use a redundant system including both a UHF radio and the Globalstar network (each can be used for command and telemetry communications). The RAVAN payload will produce about 2.5 MB of science and housekeeping data per day. The RAVAN nanosatellite has a mass of < 5kg.
Table 1: Spacecraft modes of operation
Figure 6: Artist's rendering of the deployed RAVAN 3U CubeSat (image credit: BCT, JHU/APL)
Legend to Figure 6: The RAVAN payload occupies the 1U section at the bottom of the figure, shown with its doors open. The four deployable solar arrays are right and left (the shadowed face of the space vehicle is shown—the solar panels are mounted on the opposite side). The UHF antenna extends from the front edge of the bus, and openings for the two star trackers are visible in the upper 1U section.
Payload integration and testing will be performed at BCT in Boulder, Colorado, including complete RAVAN spacecraft vibration, thermal vacuum, and launch acceptance testing, such as day-in-the-life testing. RAVAN will be delivered to Cal Poly for launch vehicle integration in July 2016. The flight model payload was delivered in June 2016 for integration with the spacecraft bus (Ref. 7).
Launch: The RAVAN nanosatellite was launched as a secondary payload on November 11, 2016 (18:30 UTC) on an Atlas-V 401 vehicle of ULA (United Launch Alliance) from VAFB, CA, SLC-3E. The primary payload on this flight was the WorldView-4 spacecraft of DigitalGlobe. 13) 14)
RAVAN was selected by the NASA CubeSat Launch Initiative for flight, with an anticipated launch in late 2016. After a one-month commissioning and check-out phase, RAVAN will operate for a minimum of five months. With the exception of the various calibration maneuvers, RAVAN will view the Earth continuously.
Orbit of WorldView-4: Sun-synchronous near-circular orbit, altitude of 617 km, inclination = 98º, period = 97 minutes, LTDN (Local equatorial crossing Time on Descending Node) at 10:30 hours, effective revisit time capability ≤ 3 days.
Secondary payloads: 15)
DigitalGlobe has included a CubeSat rideshare program. The CubeSats will be launched by use of ULA's Centaur Aft Bulkhead Carrier that has flown successfully on four previous Atlas V missions. All of the CubeSats manifested for the WorldView-4 mission are sponsored by the U.S. NRO (National Reconnaissance Office) and are unclassified technology demonstration programs. DigitalGlobe is also partnering with California Polytechnic State University, Tyvak Nanosatellite Systems Inc., Lockheed Martin and United Launch Alliance to bring this rideshare program to fruition.
Tyvak Nanosatellite Systems (Irvine, CA) served as the integrator for all seven CubeSats. ENTERPRISE is the fifthNRO mission to utilize ridesharing, but the first in which the organization has partnered with a commercial company to do so. 16)
• CELTEE-1 (CubeSat Enhanced Locator Transponder Evaluation Experiment-1), a 1U CubeSat built by M42 Technologies (Seattle,WA) for AFRL (Air Force Research Laboratory). The goal is to test the performance of an ELT (Enhanced Location Transponder) in support of SSA (Space Situational Awareness).
• Prometheus-2 x 2, two 1.5U technology demonstration CubeSats (Block 2) of LANL (Los Alamos National Laboratory). Test of communications between remote field sites and ground station terminals in a store-and-forward environment.
• AeroCube-8C and -8D, two 1.5U technology demonstration CubeSats of the Aerospace Corporation (El Segundo, CA) to test electric propulsion, CNT (Carbon Nanotubes) and solar cell technology.
• U2U (Untitled 2U), a 2U CubeSat of AFRL to demonstrate the EGM (Electron and Globalstar Mapping) experiment.
• RAVAN (Radiometer Assessment using Vertically Aligned Nanotubes), a 3U CubeSat mission funded by the NASA and developed and operated by JHU/APL.
The CubeSats will be deployed after WorldView-4 separation as part of the NRO-sponsored ENTERPRISE mission.
• April 2017: RAVAN made routine Earth observations from mid-January 2017 through early March as a part of payload commissioning. The spacecraft's SD Card (Secure Digital Card)— the project's planned means of telemetry storage between UHF downlinks—failed in early March. Fortunately, it was possible to reprogram the payload to store its data in payload RAM. As of mid-April 2017, spacecraft operations have resumed, starting with a series of solar calibration sequences. Analysis of all the data collected to date is still ongoing. 17)
- The gallium black bodies have been undergoing phase transitions ever since launch, as the normal orbital temperature cycling of the spacecraft drives the melting and freezing of the cells each orbit. The temperatures of the melt cells have been monitored since the payload was powered on mid-December 2016. Calibration of the radiometer Total channels is possible while actively controlling the cells at their melting points. Several black body calibrations have been performed.
- The radiometer doors were opened for the first time on January 25, 2017, confirming that the door mechanisms work as designed. Further, this provided the project with the first light on the sensors, as shown in Figure 7. Although the values shown have not been absolutely calibrated yet, the receiver signals have been scaled to emphasize how well they track each other. There is excellent agreement between the individual Total channels and the individual shortwave channels. Further, there is qualitative agreement between the Total and SW channel pairs, as the Earth outgoing radiation varies with scene below the spacecraft as it flies in its orbit. Absolute calibration will be possible after recent solar calibrations have been analyzed.
Figure 7: RAVAN "first light" on January 25, 2017 for the (Top) Total channels and (Bottom) SW channels for both VACNT and cavity-type radiometers. The radiometer doors were opened close to 17:30, as indicated. Two eclipse periods are also indicated, in the SW channels. The large vertical spikes going into and out of eclipse are presumably due to glint off the instrument and are now being avoided with a spacecraft attitude maneuver at the terminator crossing (image credit: JHU/APL, L-1, BCT, NASA)
• January 2017: During the bus commissioning and check-out phase the radiometer doors remained closed, to reduce the risk of contamination. The RAVAN project started the payload commissioning, with "first light" operations in January 2017. The principal calibration of RAVAN occurs on orbit, with the Sun as the primary, absolute standard and deep (cold) space characterizing the offset. The CubeSat slews for the solar and deep space views, using the Sun to provide both absolute calibration and on orbit characterization of the radiometer angular responsivity. The integral gallium black body emitters monitor degradation of both the primary and secondary Total channels (Ref. 17).
• The RAVAN nanosatellite was deployed in December 2016.
The RAVAN project will demonstrate a bolometer radiometer that is compact, low cost, and absolutely accurate to NIST (National Institute of Standards and Technology) traceable standards. RAVAN could lead to affordable CubeSat constellations that, in sufficient numbers, might measure Earth's radiative diurnal cycle and absolute energy imbalance to accuracies needed for climate science (globally at 0.3 W/m2) for the first time.
The idea for the RAVAN instrument stems from the NISTAR (National Institute of Standards and Technology Advanced Radiometer), built between 1999 and 2001 for the DSCOVR (Deep Space Climate Observatory) mission, formerly known as Triana, a NASA mission proposed in 1998, then stalled until NOAA revived the initiative in 2012. NISTAR is a three-channel cavity radiometer with a mass of ~25.5 kg and uses 43 W of power. After producing the radiometer, L-1 began exploring other missions that would benefit from similar technology. 18)
The RAVAN radiometer is developed at L-1 Standards and Technology Inc. of New Windsor, MD, USA. The radiometer measures the total outgoing radiation, from 200 nm to 200 µm. The radiometer uses two key technologies: a VACNT (Vertically Aligned Carbon Nanotube) forest absorber (grown at APL) and a gallium fixed-point blackbody source as a calibration transfer standard.
Figure 8: Illustration of the RAVAN radiometer developed at L-1 (image credit: L-1, JHU/APL)
The RAVAN instrument comprises four independent radiometers in two pairs, as shown in Figure 8. The primary pair uses VACNT absorbers; the secondary pair uses a traditional, conical cavity design, for intercomparison, redundancy, and degradation monitoring. Each pair has a total (Total) channel, measuring all radiation from the ultraviolet (200 nm) to the far infrared (200 µm), and a shortwave (SW) channel, which is limited to wavelengths less than about 5.5 µm by a sapphire dome. The SW channels will allow RAVAN to distinguish between solar-reflected sunlight and the Earth's total emission. The radiometers have a wide FOV (Field of View), close to 130º. This is needed so that the entire Earth disk can be viewed from LEO (Low Earth Orbit). Apart from the sapphire domes of the SW channels, there are no optics between the light source and the radiometer absorbers.
The RAVAN instrument has two re-closeable doors, actuated by stepper motors, that cover the primary and secondary radiometer pairs. The doors protect the radiometers before launch and during commissioning, and they will be closed as needed during its mission. The gallium blackbodies are contained in the doors such that they lie directly over the Total channels when the doors are closed. Incidentally, VACNTs also cover the gallium sources, desirable because of their high emissivity. We used the same growth procedure for these as the radiometer VACNT absorbers.
The radiation sensors themselves are electrical substitution radiometers. In each, thermistors monitor the temperatures of the absorber and heat sink. A bridge circuit senses temperature changes due to light absorption. Electrical heaters in the absorber remove the thermal link, thermistors, and bridge circuit from calibration. Heaters in the heat sink control the temperature of the detectors versus the spacecraft bus. The radiometers are then calibrated for power responsivity, noise floor, aperture area, spectral bandpass, and field of view.
According to Steven Lorentz of L-1 and also the PI of the RAVAN instrument, the radiometer has a mass of < 1 kg, uses about 1.9 W of power (orbit average), and fits in a 1U volume. L-1 was able to produce RAVAN's tiny radiometer by employing passive temperature control techniques instead of the active systems on NISTAR and equipping the new instrument with sophisticated analog-to-digital converters. Although the two instruments employ different technology and measurement techniques, both are designed to offer detailed data on irradiance reflected and emitted from the sunlit face of Earth. The RAVAN instrument will produce approximately 2.5 MB of science and housekeeping data per day.
Currently, NASA obtains data on solar radiation reaching Earth's atmosphere with a variety of instruments, including TIM (Total Irradiance Monitor) on NASA's SORCE (Solar Radiation and Climate Experiment)mission launched on January 25, 2005 and the TCTE [TSI (Total Solar Irradiance) Calibration Transfer Experiment] launched on November 20, 2013 on the STPSat-3 of the ORS-3 (Operationally Responsive Space-3) mission of the USAF STP (Space Test Program). The CERES (Clouds and Earth's Radiant Energy System) instruments of NASA/LaRC are being flown on NASA's Aqua (launch Dec. 18, 1999) and Terra (May 4, 2002) missions as well as on the NOAA/NASA Suomi-NPP partnership mission (launch Oct. 28, 2011).
While the CERES instruments have proven adept at highlighting long-term trends in outgoing radiation, a RAVAN constellation would be designed to reveal both long-term and short-term variations. For example, a RAVAN constellation would be able to detect changes in the levels of energy radiated into space from any location on Earth and during any time of the day or night. Researchers could use that information to improve climate models.
The RAVAN spacecraft includes its own black body calibration source and vertically aligned carbon nanotubes to absorb incoming light. Neither the vertically aligned carbon nanotubes nor the gallium black body has been used before in an orbiting instrument.
The 0.3 W m-2 absolute calibration requirement for climate accuracy is exceedingly stringent, and to meet this requirement an involved calibration procedure is planned for before launch and during the mission. Ground calibration includes component-level and end-to-end calibrations that are tied directly to NIST standards. Laser-based measurements will be performed for the SW channels, along with calibration of the Total channels with a ground-based fixed-point gallium blackbody, which is known to 0.005 K (1σ), corresponding to 0.03 W m-2). On orbit, the integral gallium blackbody emitters serve as a transfer standard for the Total channels to the pre-launch ground calibration. The on-board blackbodies will also be used to monitor degradation of both the primary and secondary Total channels.
On-orbit nadir-pointing and calibration maneuvers drive the attitude control requirements imposed on the CubeSat bus (0.5º pointing control; 0.1º pointing knowledge). The Sun will be the primary calibration standard, and a series of calibration and inter-calibration modes will be employed, summarized in Table 1. The calibration procedures will be performed at weekly and monthly intervals (monthly for full calibrations).
Embedded in RAVAN's sensor head contamination covers (Figure 8) are two gallium fixed-point black bodies that serve as on-orbit infrared sources that, when coupled with deep space looks, provide an additional means to determine the offset for the total channels. The black bodies consist of a high-purity gallium cell located directly over the detector. We use the gallium solid–liquid phase transition (29.76°C) as a stable reference for the black body emission. The calibration sources are used as stable and repeatable references to track the long-term degradation of the radiometer sensors. Gallium is not toxic, and only stable isotopes are used in RAVAN (different than those used in medical imaging), so its presence poses no human risk during spacecraft integration.
Figure 9: Alternate view of the deployed RAVAN spacecraft (image credit: BCT)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).