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.
• May 2019: Measuring Earth's energy budget from space is an essential ingredient for understanding and predicting climate. Observational requirements to achieve climate accuracy and continuity with existing datasets drive cost and define measurement approaches. The RAVAN (Radiometer Assessment using Vertically Aligned Nanotubes) 3U CubeSat mission is a pathfinder that demonstrates technologies for the measurement of Earth's radiation budget. RAVAN collected data in orbit for over 20 months, successfully demonstrating the use of vertically aligned carbon nanotubes (VACNTs) as absorbers in broadband radiometers for measuring Earth's outgoing radiation and the total solar irradiance and a pair of gallium phasechange black body cells that are used as a stable reference to monitor the degradation of RAVAN's radiometer sensors. Four radiometers (two VACNT, two cavity) show excellent long-term stability over the course of the mission and a high correlation between the VACNT and cavity radiometer technologies. Short-term variability is a challenge to climate accuracy that remains, consistent with insufficient thermal knowledge and control on a CubeSat. This paper includes details of the in-space validation and efforts to evaluate the applicability of the RAVAN CubeSat as a whole to measurement of Earth's energy budget, such as short- and long-term stability and comparison with independent spaceborne measurements. 17)
- Launched in November 2016, the RAVAN payload collected data for 20 months (the spacecraft is still operating as of this writing, with the payload powered off). The largest obstacle to operations was local UHF ground interference at 450 MHz, which made continuous operations as designed impossible. Fortunately, this did not limit the primary technology demonstration.
- Calibration of the RAVAN radiometers was achieved on orbit with periodic observations of dark space (for offsets) and the Sun (for absolute scale). Prerequisites for well-behaved ERB measurements are stable or smoothly varying offsets and gains. The long-term behavior was excellent, with the VACNT Total and SW (Short Wave) channels changing smoothly by less than 0.4% and 0.5% over the course of the project, respectively. The heater gains were stable for both channels at less than a few tenths of a percent. Measurements of the total solar irradiance (solar views) were stable and also varied by only a few tenths of a percent. A similar assessment of stability can be found from the gallium black body calibrations. Over the course of the entire project, the cavity radiometer signal slowly changed by less than 2.5%, as shown in Figure 7. Based on the presumption that the gallium black body transitions occurred at an invariant temperature of the melting point of pure gallium and that the black body emissivity remained constant, changes in the infrared measurements from one phase transition to the next is taken to be the variability of the radiometer itself. The black body associated with the primary (VACNT) radiometer failed in March 2017, but the other black body continued to operate throughout the entire mission. The observed change in signal, if from the radiometer, could have resulted from changes in the radiometer offsets. Although RAVAN had excellent long-term stability, its shorter-term variability, however, may be a challenge for measurements at the level ultimately required for "climate accuracy" (~0.1%).
Figure 7: Relative changes in (a) VACNT and (b) cavity radiometer Total channel response to gallium black body melts as a function of days since RAVAN launch (11 November 2016). Note that the black body associated with the primary (VACNT) radiometer failed in March 2017 (image credit: JHU/APL)
- Inter-calibration of the VACNT- and cavity-based radiometers viewing the Earth shown in Figure 8 have overall good correlation between the new and "old" technologies, with r2 values exceeding 0.99 for the Total and SW channels. Linear fits of the comparisons indicate that on average, the VACNT Total channel was about 3% higher than the cavity Total; the VACNT SW channel was about 6% higher than the cavity SW. These biases are larger than the 0.1% goal.
Figure 8: Radiometer intercomparison for the (a) Total and (b) SW channels. Linear fits forced through the origin are indicated. Data between RAVAN days 200 and 400 are shown, focusing on the period of greatest calibration stability. The data points are color-coded by time, with earlier times more blue and later times more red (image credit: JHU/APL)
- RAVAN was designed to measure Earth outgoing radiation continuously, apart from calibration maneuvers, and our original plan was to compare global monthly mean values of outgoing radiation with that derived from the Clouds and the Earth's Radiant Energy System (CERES) instruments. However, due to downlink constraints and other limitations, the RAVAN dataset is episodic, and simple quantitative monthly mean comparisons are not possible. As a preliminary attempt at CERES comparison—more qualitative in nature— we binned the entire RAVAN nadir irradiance dataset into a 10° x 10° latitude–longitude geographic grid, irrespective of month. The results are shown in Figure 9. The RAVAN data are not continuous, so there is temporal aliasing, but despite this limitation there is qualitative agreement between RAVAN and CERES, showing similar latitudinal and even longitudinal patterns.
Figure 9: Earth outgoing flux measurements from space. (Left) RAVAN Earth outgoing flux measurements (at spacecraft altitude) for the SW and longwave (Total–SW), binned into a 10º x 10º latitude–longitude geographic grid for the entire mission. (Right) Analogous SW and longwave top-of-the-atmosphere flux (at 20 km) from a 10-year mean of CERES Energy Balanced And Filled (EBAF) data (image credit: JHU/APL)
- In summary, the RAVAN project successfully demonstrated the use of vertically aligned carbon nanotubes as broadband radiometer absorbers and gallium phase-change black body cells on a 3U CubeSat, launched November 2016. RAVAN made periodic observations of the Earth outgoing irradiance, the Sun, and deep space, with its four radiometers: Total and shortwave channels based on VACNT and cavity absorber technologies. In addition, RAVAN also employed observations of its internal gallium black bodies to monitor long-term radiometer drift. Apart from the failure of one of the gallium black bodies after four months, the payload worked well and proved to be radiation-tolerant; technical difficulties associated with the CubeSat bus could be resolved straightforwardly in a future mission.
- Beyond the technology demonstration, RAVAN represents a benchmark for making Earth radiation budget measurements from a CubeSat. RAVAN had excellent long-term stability; shorter-term variability may be a challenge for measurements at the level required for climate accuracy, on the order of tenths of a W/m2 (or about 0.1% of the roughly 340 W/m2 Earth outgoing radiation signal). The short-term variability is not unlike the results from the Earth Radiation Budget Experiment (ERBE) nonscanner, which was ultimately limited by insufficient thermal knowledge and control.
- Broadband observations from a RAVAN-like platform or its underlying technologies may be a part of the solution to the sustainable ERB challenge. For example, RAVAN technologies enable a constellation mission (either small satellite or hosted payload). The results from RAVAN, while not at the level required for climate measurements, are an important step for the development of science-grade ERB instruments on small satellite platforms.
• September 27, 2017: RAVAN began collecting data from Earth's orbit on Jan. 25, 2017, and the technology demonstration was declared a success in early August. — But the solar eclipse on Aug. 21 gave researchers a unique opportunity to further test an important carbon nanotube attribute: its strong sensitivity to rapidly changing energy outputs. While designed to measure the amount of reflected solar and thermal energy emitted from Earth into space, during the eclipse RAVAN's highly sensitive nanotubes would be trained instead on the sun to detect changes in the amount of incoming solar energy. 18)
- Because the researchers knew the CubeSat's location and the percentage of eclipse it would measure, it was easy for the team to compare the satellite's data to the known solar irradiance. Due to RAVAN's position in orbit, it did not catch eclipse totality — where the moon completely blocks the sun's light. Instead, from its position high above the U.S., RAVAN was to collect data of an approximately 80 percent eclipse, similar as to what was observed from principal investigator Bill Swartz's home organization, the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, which leads the mission.
- As the moon passed between Earth and the sun, RAVAN's instruments responded rapidly and accurately to measure the diminishing solar energy that was visible to the satellite's detectors. Swartz explained, "Although RAVAN routinely views the sun for solar calibration, it tracked the sudden change in solar energy afforded by the eclipse as expected."
Figure 10: The data plotted here were collected by one of RAVAN's carbon nanotube radiometer sensors. The plot shows that these sensors can rapidly respond to changes in the sun's or Earth's irradiance. The Sun graphics depict the eclipse's extent during the observation (image credit: JHU/APL, Bill Swartz)
- Now, with eclipse-tested technology, RAVAN is trained back at Earth as Swartz and his team continue to monitor the satellite's instrument performance, perform data analysis, and compare its measurements with existing model simulations of Earth's outgoing radiation.
- RAVAN's current test and validation mission is the first step to enable a future constellation of CubeSats that would orbit Earth and provide continuous global coverage of Earth's radiation imbalance to improve on current measurements, which are taken by instruments housed on a few large satellites.
- Having smaller satellites placed uniformly around the planet could offer an advantage when it comes to studying Earth's energy imbalance. "The radiant energy emerging from the Earth changes rapidly in time and space, particularly as viewed from a satellite constellation speeding along in low-Earth orbit," Swartz said. "The solar eclipse provided a unique opportunity to test the RAVAN measurement responsiveness in a controlled fashion, further proving the technique for Earth observation."
Figure 11: On Aug. 21, 2017, RAVAN observed the sun during the solar eclipse. Here, an artistic rendering depicts RAVAN's view just prior to the event (image credit: NASA)
• August 2017: The RAVAN Spacecraft operations are being conducted by BCT (Blue Canyon Technologies) in Boulder, Colorado. The first month on orbit was used for commissioning and checking-out the RAVAN spacecraft, during which time the radiometers were protected from spacecraft outgassing. During the first month, the thermal environment of the payload was characterized and the gallium black bodies were tested, exercising them through multiple freeze–melt cycles. 19) 20)
- Following the checkout phase, RAVAN began and continues operations comprising continuous nadir Earth observations with interspersed calibration maneuvers, as summarized in Table 2. The CubeSat slews for the solar and deep space views, using the Sun to provide absolute calibration of the radiometers and on-orbit characterization of the radiometer performance. During the operations phase, the RAVAN spacecraft has demonstrated using VACNTs for Earth radiometry.
Table 2: RAVAN modes of operation
- So far, the project has already collected enough data to meet most of the required objectives. The RAVAN project has demonstrated using VACNTs for Earth radiometry and gallium black bodies for on-orbit calibration, with the goal of achieving the accuracy, precision, and stability needed for climate measurements. Continued observations will allow for a determination of the RAVAN sensors' on-orbit stability.
• August 9, 2017: RAVAN has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate. The RAVAN CubeSat was launched into LEO on Nov. 11, 2016, in order to test new technologies that help to measure Earth's radiation imbalance, which is the difference between the amount of energy from the Sun that reaches Earth and the amount that is reflected and emitted back into space. That difference, estimated to be less than 1%, is responsible for global warming and climate change. 21)
- Designed to measure the amount of reflected solar and thermal energy that is emitted into space, RAVAN employs two technologies that have never before been used on an orbiting spacecraft: carbon nanotubes that absorb outbound radiation and a gallium phase change blackbody for calibration.
- Among the blackest known materials, carbon nanotubes absorb virtually all energy across the electromagnetic spectrum. Their absorptive property makes them well suited for accurately measuring the amount of energy reflected and emitted from Earth. Gallium is a metal that melts — or changes phase — at around body temperature, making it a consistent reference point. RAVAN's radiometers measure the amount of energy absorbed by the carbon nanotubes, and the gallium phase change cells monitor the stability of the radiometers.
- RAVAN began collecting and sending radiation data on Jan. 25 and has now been in operation for well past its original six-month mission timeframe.
- "We've been making Earth radiation measurements with the carbon nanotubes and doing calibrations with the gallium phase change cells, so we've successfully met our mission objectives," said Principal Investigator Bill Swartz of Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. He and his team are now monitoring RAVAN in the longer term to see how much the instrument changes over time and are also performing data analysis and comparing its measurements with existing model simulations of outgoing Earth radiation.
- While the technology demonstration comprises a single CubeSat, in practice a future RAVAN mission would operate many CubeSats in a constellation. Instruments for measuring Earth's outgoing energy are currently housed aboard a few large satellites, and while they have a high spatial resolution they cannot observe the entire planet simultaneously the way a constellation of RAVAN CubeSats could, Swartz explained.
- "This successful technology demonstration realizes the potential of a new observation scenario to get at a very difficult measurement using constellation missions," said Charles Norton, program area associate for ESTO (Earth Science Technology Office) at NASA/JPL (Jet Propulsion Laboratory) in Pasadena, California. "In terms of its impact for CubeSats and Smallsats of NASA, I think It has helped to bring forward another example of how this platform can be successfully used for technology maturation, validation and science."
Figure 12: RAVAN is a 3U CubeSat that successfully demonstrated new technologies for measuring the amount of reflected solar and thermal energy that is emitted into space. These observations have the potential to improve spaceborne measurements of Earth's energy imbalance (image credit:JHU/APL)
• May 31, 2017: Mission extended. NASA has authorized continued operations to BCT (Blue Canyon Technologies) 3U CubeSat for the RAVAN mission, which has been successfully operating in LEO since November 2016, through April 2018. After recording its "first light" data when its radiometer doors opened on January 25, 2017, the RAVAN mission, funded by NASA's Earth Science Technology Office, has authorized continued operations. The RAVAN project is being led by JHU/APL of Laurel, MD. 22) 23)
- RAVEN is measuring the Earth's radiation imbalance, the difference in energy reaching the top of Earth's atmosphere from the sun and the energy reflected and radiated to space from the Earth. NASA is making solar measurements that will enable absolute radiometric calibration of RAVAN's observations. RAVAN is positioned as a demonstration mission for a potential follow-on constellation of CubeSats that will measure Earth's radiation imbalance around the globe.
• 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. 24)
- 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 13. 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 13: 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. 24).
• 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. 25)
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 14: 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 14. 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 14) 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 15: 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).