CIRAS (CubeSat Infrared Atmospheric Sounder) - a pathfinder mission for EON-IR (Earth Observation Nanosatellite -IR)
The NASA Science Mission Directorate has selected proposals for the InVEST (In-Space Validation of Earth Science Technologies) Program in support of the ESD (Earth Science Division). CIRAS is a NASA/JPL instrument under development to measure upwelling infrared radiation of the Earth in the MWIR region of the spectrum from space, a pathfinder for a nanosatellite mission. The observed radiances can be assimilated into weather forecast models and be used to retrieve lower tropospheric temperature and water vapor for climate studies. 1) 2) 3)
The objective of the proposed effort is to develop a sounder instrument, to be flown on a 6U CubeSat, capable of meeting the temperature and water vapor measurement requirements of the AIRS and CrIS instruments data products in the lower troposphere. The method employed will use a infrared grating spectrometer with infrared detectors and micro-cryocooler. The measurement will be made in the 4-5 µm spectral region. This work is significant in that if selected, the CIRAS will demonstrate a critical science and meteorological measurement in a significantly smaller package enabling use in constellations for improved latency. CIRAS can also be used as a gap filler in the event of a failure of the CrIS, and thereby providing insurance for the long-term data continuity of AIRS.
Multiple units can be flown to improve temporal coverage or apply different spectral ranges to look at other atmospheric trace gases in the infrared including CO, CO2, CH4, O3, and SO2. Higher spatial resolution units can be developed with the same optics and detector array, with a lower detector operating temperature and consequently more power from the spacecraft. Higher spatial resolution and formation flying can provide new data products including 3D motion vector winds. CIRAS incorporates the following new instrument technologies.
• The first is a 2D array of High Operating Temperature Barrier Infrared Detector (HOT-BIRD) material, selected for its high uniformity, low cost, low noise and higher operating temperatures than traditional materials. The detectors are hybridized to a commercial ROIC and commercial camera electronics and dewar package.
• The second technology is an MGS (MWIR Grating Spectrometer)) to be designed and developed by Ball Aerospace to provide imaging spectroscopy for atmospheric sounding in a CubeSat volume. The MGS has no moving parts and is based on heritage spectrometers including the Ball Aerospace SIRAS-G (Spaceborne Infrared Atmospheric Sounder for GEO) IIP of 2007.
JPL will also develop the mechanical, electronic and thermal subsystems for CIRAS, with satellite components procured from Blue Canyon Technologies. The integrated system will be a complete 6U CubeSat capable of measuring temperature and water vapor profiles with good lower tropospheric sensitivity. The CIRAS is the first step towards the development of an EON-IR (Earth Observation Nanosatellite -Infrared) capable of meeting the replacement needs of the CrIS, including both the MWIR and LWIR regions of the spectrum.
Background: NWP (Numerical Weather Prediction) centers worldwide have demonstrated the value of hyperspectral infrared sounders to improving weather forecasts. The AIRS (Atmospheric Infrared Sounder) on the NASA Earth Observing System Aqua spacecraft was the first hyperspectral infrared sounder to be used for operational forecast improvement. The IR sounder radiances are assimilated into Global Circulation Models and NWP centers worldwide including the NCEP (National Center for Environmental Prediction), the ECMWF (European Center for Medium-Range Weather Forecast) and the UK Met Office.
Six hours of forecast model improvement on the 5 day forecast has been achieved by assimilating AIRS data at NCEP and ECMWF by assimilating only 1 in 18 footprints. 4) An additional 5 hours of improvement on the 5 day forecast or more has been shown to be possible using cloud cleared radiances. 5) The AIRS and the IASI (Infrared Atmospheric Sounding Interferometer) impacts to operational 24-hour forecasts at ECMWF are roughly comparable and are second only to the collective impact of four AMSU units. Finally, the CrIS (Cross-track Infrared Sounder) on the JPSS (Joint Polar Satellite System) has demonstrated comparable performance to the AIRS and IASI. The sounders have a wide field of view enabling coverage of the full context of severe weather events (Figure 1).
Figure 1: Wide swath scanning of the IR sounders enable daily revisit of severe weather events (image credit: NASA/JPL, E. Olsen)
While the data from AIRS is assimilated into the operational forecasts at NCEP today, AIRS is expected to complete its mission when the Aqua spacecraft runs out of fuel in the 2022 timeframe. We can expect CrIS and IASI instruments and their nearly identical replacements to be operational into the late 2030's. Maintaining continuity of these important weather forecasting and climate data sets is critical to NASA and NOAA. NOAA has identified the need for an EON-IR (Earth Observing Nanosatellite - IR) as a low cost-to-orbit way to mitigate a potential gap in data of the CrIS on JPSS. A key objective of CIRAS is to demonstrate the technologies needed for an operational IR sounder, like EON-IR. In that respect, CIRAS is a pathfinder for EON-IR.
Not all requirements from the current operational IR sounders need to be satisfied in the EON-IR. The primary requirement to meet is the ability to provide radiances with sensitivity to temperature and water vapor profiles in the lower troposphere (below 500 mb). It is also recognized that the spatial coverage will depend on the orbit of the CubeSat, and that the CubeSat need not be placed in the same orbit as the operational sounders. This would enable sounding information at times that are currently not available (e.g. early morning). EON-IR would also provide a substantial cost saving (over 10x) compared to the current operational sounders, and can provide for new architectures, including a constellation for near-realtime global coverage of atmospheric humidity and temperature. Finally, with an improvement in the spatial resolution (zoom mode) and 3 tandem formation satellites in formation separated by 5-10 minutes, the EON-IR would be able to measure atmospheric motion vector winds in 3D using the water vapor profiles retrieved. EON-IR offers robustness to loss of failure and options for alternate orbits that complement the existing suite of sounders and pave the way for a future lower-cost, more robust observing system. CIRAS is the pathfinder mission to demonstrate several of these capabilities.
A comparison of measurement and resource capabilities the legacy sounders, AIRS, IASI, CrIS and requirements for CIRAS is shown in Table 1. CIRAS is designed for a LEO (Low Earth Orbit) that is most likely lower than the legacy sounders. A wide field scan range is possible from the instrument but will result in a shorter swath than the legacy sounders. CIRAS has one band, discussed below, and is significantly smaller and lighter.
Table 1: Comparison of measurement and resource capabilities for legacy sounders and CIRAS
Spectral resolution and range:
The CIRAS is designed to operate in the MWIR from 1950-2450 cm-1 to reduce size, cost and resource requirements of the IR sounders. This region was selected since it contains both temperature sounding and water vapor sounding spectroscopic lines, and the detectors can operate at higher temperatures than the LWIR (Longwave Infrared). The spectral range and resolution for CIRAS and the legacy sounders in this region are shown in Figure 2 along with a typical spectrum of the atmosphere (after convolution with the CIRAS SRF (Spectral Response Function)), highlighting the water vapor lines and CO2 branch used for temperature sounding. Also shown are the AIRS channels used by the UK Met Office for data assimilation.
Figure 2: Spectral resolution and coverage for CIRAS compared to legacy IR sounders (image credit: NASA/JPL)
The CIRAS spectral resolution is comparable to AIRS in the temperature sounding region. Note: AIRS and CrIS do not sound in the 5 µm water band (2000 cm-1) since they use longer wavelengths for water vapor. The CIRAS spectral resolution improves naturally in the water vapor band (since gratings operate with constant spectral bandwidth in wavelength domain). This improvement preserves information content and allows the project to resolve the lines.
The legacy sounders also have the LWIR (Longwave Infrared) for temperature sounding. A concern with using only the MWIR region of the spectrum for temperature sounding is contamination by solar reflected energy between 4 and 4.5µm. However, the AIRS temperature profile retrieval algorithm has successfully dealt with the solar reflected component by separately solving for shortwave emissivity and reflectivity. 6) Despite not having LWIR sensing capability, the CIRAS spectral band is expected to provide comparable temperature sounding to legacy sounders in the lower troposphere.
An earlier study showed that the DOFS (Degrees of Freedom of Signal) for AIRS are comparable to a midwave sounder using a similar band to CIRAS6. In this early study, the DOFS for temperature were nearly the same, but the water vapor showed better lower tropospheric sensitivity and poorer upper tropospheric sensitivity. A more recent study of the information content of CIRAS for this paper leads us to the same conclusion. Figure 3 shows the row sum of the AK (Averaging Kernels) for CIRAS compared to CrIS for temperature and water vapor. The row sum of the averaging kernel matrix is an estimation of the sensitivity of the retrieval to changes in the atmospheric state, although it lacks the vertical resolution information or cross-species sensitivities found in the full averaging kernel. The DOFS are the trace of the averaging kernel for each species, and is a measure of the information content of the retrieval. Calculations shown here are for comparison purposes, using idealized systems over mid-latitude ocean in clear sky, and assumed summertime atmospheric states and "a priori" covariances. All channels were used. Effects from the spectral resolution and expected noise are included, but systematic errors are not.
Figure 3: Row sum of AK shows CIRAS has good lower tropospheric sensitivity (image credit: NASA/JPL)
Spatial resolution and coverage:
CIRAS is designed to match and exceed legacy sounder spatial resolution requirements. Table 1 shows the comparison of the nominal spatial resolution. CIRAS can achieve a nominal 13.5 km spatial resolution from any orbit. This is achieved by adjusting the pixel binning along-track and scan rate and number of frames averaged for the cross-track direction. Figure 4 shows the binning configuration and projection of the slit on to the focal plane for the nominal resolution. CIRAS maintains a constant frame rate and integration time keeping internal functions of the instrument common between the two modes. The scan pattern for CIRAS rotates the slit with scan angle. The pattern is deterministic and isn't expected to pose a problem.
CIRAS can also adjust the scan rate and number of pixels along-track and frames cross-track averaged to achieve a 3 km footprint. The project calls this "zoom" mode. The instrument cannot download this entire data volume, and is therefore limited to a swath of about 160 km. In addition to higher spatial resolution, the Zoom mode can target anywhere in the full swath of the Global Mode. The higher spatial resolution will enable more soundings per unit area than the current operational sounders, and allow more clear observations. CIRAS will be the highest spatial resolution hyperspectral IR sounder to fly in space. This mode will demonstrate the value of higher spatial resolution IR sounding for future science and operational weather missions. This mode will be possible with the EON-IR given successful demonstration of this technology.
Figure 4: CIRAS uses pixel and frame binning and an adjustable scan rate to achieve 3km (Zoom) or 13.5 km (Global) FOVs from any orbit (13.5 km FOV from 600km orbit altitude shown), image credit: NASA/JPL
The radiometric sensitivity of the IR sounders in the temperature sounding band must be better than 0.2 K at 280 K. The AIRS and CIRAS are grating spectrometers and have good sensitivity in this region for temperature sounding. CrIS also has good sensitivity in this region, but like the other legacy sounders use the long wavelength region for temperature sounding as mentioned above. Figure 5 shows the predicted NEdT (Noise Equivalent differential Temperature) for CIRAS in Global and Zoom modes compared to the legacy sounders in this spectral region. The Zoom mode NEdT is higher due to fewer pixels to average in the along-track direction.
Figure 5: CIRAS NEdT at 280K compared to legacy IR sounders (image credit: NASA/JPL)
CIRAS (CubeSat Infrared Atmospheric Sounder) Instrument
The CIRAS payload (Figure 6) includes a scan mirror capable of rotating 360º to view Earth, cold space and an internal blackbody for calibration. The Scan Mirror Assembly consists of a single planar gold coated aluminum mirror mounted at 45º to the axis of a stepper motor. Gold provides low polarization and high reflectance in the CIRAS band. The footprint rotation is not a concern since all spectral channels share the same slit (same for AIRS), and the slit rotation angle is deterministic, making geolocation straightforward. The blackbody is a simple flat plate composed of black silicon, heat sunk and instrumented with a temperature sensor, and provides high emissivity and durability in a compact design. Energy from the scan mirror is collected using a 3-element all-refractive telescope. Energy from the telescope is focused onto the entrance slit of an all refractive MGS (MWIR Grating Spectrometer). The telescope and spectrometer are to be developed by Ball Aerospace. The spectrometer disperses the energy across the spectral range and produces a 2-dimensional image at the focal plane with one direction spatial (504 pixels) and the other spectral (625 channels).
Figure 6: Illustration of the CIRAS payload and spacecraft (image credit: NASA/JPL)
The detector array uses the JPL HOT-BIRD photosensitive material mounted on a Lockheed Martin SBF (Santa Barbara Focalplane) 193 ROIC (Readout Integrated Circuit). The ROIC is mounted in a custom ICDA (Integrated Cooler Dewar Assembly) to be developed by IR Cameras. The dewar contains a cold filter mounted close to the focal plane, and a window at the interface between the dewar and the optics.
The CIRAS subsystem electronics are primarily commercial. A payload electronics board will be developed to interface the various subsystems. Clocks, biases and A/D conversion are performed using military-grade electronics also provided by IR Cameras. JPL will develop the payload electronics to interface with the scanner, camera, cryocoolers, blackbody and spacecraft electronics.
Cooling of the spectrometer to 190 K and the narrow spectral range of the filters minimize background loading on the detector. Use of a Ricor cryocooler for the optics is included in the current design but alternate coolers will be investigated. The detector is cooled to 120 K also using a Ricor cryocooler, heat sunk to the warm radiator. Electronics, cryocooler and spacecraft waste heat is dissipated in warm temperature radiators on all remaining surfaces except nadir and anti-nadir.
CIRAS will be developed as a stand-alone payload with mechanical, thermal, and electrical interfaces to the spacecraft. The radiators will be detachable to allow easy integration of the payload and reattachment after integration with the spacecraft.
The CIRAS spacecraft will be a commercial 6U system (industry procurement) with deployable solar panels and additional batteries. The spacecraft will provide communications, navigation, power and on-board processing and formatting of the raw data stream from the payload. The spacecraft structure will have custom features to allow a clear scan field of view and for mounting the payload components.
The payload development and test will take 20 months followed by spacecraft integration and test of 2 months. In-flight operations will take a minimum of 3 months after launch consisting of system activation, calibration and data acquisitions. CIRAS is TRL-5 upon entry and TRL -7 after flight demonstration.
MGS (MWIR Grating Spectrometer): CIRAS uses a 15 mm telescope that forms a telecentric image at the entrance slit of the spectrometer. Light passing through the slit is collimated by the collimator optics in the spectrometer, then dispersed by the diffraction grating in the MGS, and focused at the FPA (Focal Plane Assembly). The diffraction grating is built into the backside of a silicon substrate (immersion grating) to maximize dispersion and minimize distortion. The optical system includes fold mirrors to fit the layout within the CubeSat allocated volume. The entrance slit will use precision micro-machined fabrication and black silicon deposition techniques used on several JPL flight projects. Optical materials will be selected to minimize distortion, and establish an athermal system capable of being aligned at room temperature and operated at 190 K without refocus.
Two important requirements of spectrometer optical performance are the spectral ‘smile' (<2 pixels) and the keystone geometrical distortions (<2 pixels), resulting from anamorphic magnification (beam compression) of the diffracted beam. Anamorphic magnification also may cause PSF (Point Spread Function) elongation in the spectral direction, which reduces spectral resolving power. The symmetry of the design and the use of the immersion grating minimizes distortions. The resulting spectral ‘smile' across the spectrum is less than 1 pixel, and the keystone distortion is also less than 1 pixel. The diffraction PSF remains concentric and uniform in the spatial and spectral directions, resulting in a uniform resolution across the FPA. The PSF cross-section follows a diffraction limited power distribution within the focal spot at the FPA, with a spot diameter of ~31µm, or 1.3 pixels. Additional considerations in the design and build will include operating temperature, stray light, ghosting, and fringing.
HOT-BIRD detectors: The current MWIR detector/FPA market is dominated in volume by InSb that has cut-off wavelength of λ=5.6 μm.7 HgCdTe (MCT) detectors, with adjustable cut-off wavelength depending on the Cd fraction in the absorber material, are used for shorter and longer cutoff wavelengths in the MWIR and for very high end applications, such as the low illumination conditions needed here. Single element MWIR MCT detectors show high-performance operation, however, MCT FPAs (Focal Plane Arrays) typically suffer from pixel-to-pixel non-uniformity and pixel operability issues. This requires lower operating temperatures for the FPA to achieve requirements for all pixels. While in principle, it is possible to select astronomy grade MCT FPAs with very good performance8, this process involves fabrication and testing of a large number of FPAs. This approach is risky, time consuming and labor intensive resulting in a very high cost as well as in high risk of delivery delays.
CIRAS will use the recently invented JPL HOT-BIRD (High Operating Temperature Barrier Infrared Detector) technology based on III-V compounds. HOT-BIRD offers a breakthrough solution for the realization of lower development cost, while reducing dark current (Figure 7) and improving uniformity and operability compared to II-VI material (MCT), Table 2. Low 1/f noise and high temporal stability allows CIRAS to use a slow scan for better sensitivity and less frequent calibrations.
Table 2: Key figures of merit for InSb and MCT compared to HOT-BIRD in the MWIR
Figure 7: Dark current vs temperature for a JPL HOT-BIRD device compared to InSb and MCT (image credit: NASA/JPL)
Black silicon blackbody: AIRS used a wedge calibration blackbody, which is excellent and robust, but large. In order to save space the CIRAS calibration target (blackbody) is a flat plate coated with a JPL developed black silicon process. The result is a broadband black surface, exhibiting less than 0.15% reflectance at a wavelength of 5 µm. The calibration target will be mounted to an aluminum block with a temperature sensor and operate at ambient temperatures. Temperature sensing accuracy is expected to be better than the 0.25 K requirement. Black silicon for stray light absorption is currently being used on a number of flight spectrometers, and within the coronagraph for the future WFIRST-AFTA (Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets) of NASA.
A preliminary estimate of the resource requirements for CIRAS have been established. This will allow sizing of the spacecraft for CIRAS as well as the data management system.
Instrument mass: Table3 lists the mass of the key subsystems for the CIRAS. The total mass is less than 9 kg at this time. The maximum mass for the nanosatellite is 14 kg, leaving sufficient margin for spacecraft subsystems and margin.
Table 3: CIRAS payload mass
Power: An estimate of the payload power dissipation is given in Table 4. The power for CIRAS is driven by the cryocooler for the focal plane assembly. Total power is estimated to be 37.5 W.
Table 4: CIRAS payload power
Data rate: The raw data rate for the CIRAS is 13.7 Mbit/s in global mode. Assuming pixel binning and frame averaging to achieve the nominal resolution and 6x compression through data selection (when data are acquired and which subset of the 625 channels are downloaded), the orbital average data rate is 0.33 Mbit/s. This gives approximately 0.32 Gbit of data per orbit. The project estimates that 1Gbit per orbit can be downloaded assuming a ground station in view for that orbit.
The CIRAS spacecraft will be a commercial 6U CubeSat (industry procurement), a preliminary view of the nanosatellite and its components is presented in Figure 8. Further information will be provided when available.
CIRAS started development June 1, 2016 and is expected to be completed by the end of 2017 or early 2018 with a launch in mid to late 2018. The mission duration is only required to be 3 months to demonstrate the technologies, but a much longer life is expected.
Finally, CIRAS will demonstrate the viability of CubeSat compatible technologies that can be used for science missions. A successful CIRAS mission will add to the body of evidence that is growing today that CubeSats can make measurements suitable for operational weather and science missions.
1) Thomas S. Pagano, "CubeSat Infrared Atmospheric Sounder (CIRAS)," 96th AMS (American Meteorological Society) Town Hall Meeting, New Orleans, LA, USA, Jan. 10-14, 2016, URL: https://ams.confex.com/ams/96Annual
2) "Four Projects Awarded Funding Under the In-Space Validation of Earth Science Technologies (InVEST) Program," NASA 2015 InVEST Awards, URL: https://esto.nasa.gov/files/solicitations/
3) Thomas S. Pagano, David Rider, Joao Teixeira, Hartmut Aumann, Mayer Rud, Marc Lane, Allen Kummer, Dean Johnson, Jose Rodriguez, Sarath Gunapala, Dave Ting, Don Rafol, Fredrick Irion, Charles Norton, James Wolfenbarger, John Pereira, David Furlong, Dan Mamula, "The CubeSat Infrared Atmospheric Sounder (CIRAS), Pathfinder for the Earth Observing Nanosatellite-Infrared (EON-IR)," Proceedings of the 30th Annual AIAA/USU SmallSat Conference, Logan UT, USA, August 6-11, 2016, paper: SSC16-SSC16-WK-32, URL: http://digitalcommons.usu.edu/cgi/viewcontent
4) A. P. McNally, P. D. Watts, J. A. Smith, R. Engelen, G. A. Kelly, J. N. Thepaut, M. Matricardi, "The assimilation of AIRS radiance data at ECMWF," Quarterly Journal of the Royal Meteorological Society, Vol. 132, 2006, pp: 935-957, doi: 10.1256/qj.04.171
5) J. Le Marshall, J. Jung, M. Goldberg, C. Barnet, W. Wolf, J. Derber, R. Treadon, S. Lord, "Using cloudy AIRS fields of view in numerical weather prediction". Australian Meteorological Magazine, 2008, vol. 57, pp. 249-254
6) Joel Susskind, John M. Blaisdell , Lena Iredell, "Improved methodology for surface and atmospheric soundings, error estimates, and quality control procedures: the atmospheric infrared sounder science team version-6 retrieval algorithm," Journal of Applied Remote Sensing, Volume 8(1), 084994. doi:10.1117/1.JRS.8.084994
7) Thomas S. Pagano, Hartmut H. Aumann, Dave Rider, Mike Rud, Colin Smith, Dean Johnson, Allen Kummer, Don Rafol, Marc Lane, Larrabee Strow, Chris Barnet,"CubeSat Infrared Atmospheric Sounder (CIRAS)," NASA/JPL, March 22, 2016, URL: http://airs.jpl.nasa.gov/system/presentations
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).