Minimize ISS Utilization: CLARREO Pathfinder

ISS Utilization: CLARREO Pathfinder (CPF) Mission

HySICS     Launch    References

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) is a NASA Tier 1 mission recommended by the NRC Decadal Survey 2007. In 2016, NASA allocated funding for a CLARREO-PF ( Pathfinder) mission to demonstrate essential measurement technologies required for the full mission. The allocated funds support the flight of a Reflected Solar spectrometer, hosted on the ISS (International Space Station) in the 2023 timeframe. 1)

One of the major objectives of CLARREO Pathfinder mission is to demonstrate on-orbit sensor inter-calibration. The CLARREO Pathfinder approach for reference inter-calibration is based on measuring spectral reflectance with high accuracy and establishing an on-orbit reference for operating Earth viewing sensors: CERES and VIIRS. The mission goal is to be able to provide CLARREO reference observations that are matched in temporal, spectral, and angular domains with measurements from the aforementioned instruments, with sampling sufficient to overcome the random error sources from imperfect data matching.

The inter-calibration method is to monitor changes in targeted sensor response function parameters: effective offset, gain, non-linearity, spectral response function, and sensitivity to polarization. We will present the CLARREO Pathfinder project status, development of its inter-calibration approach and algorithms, and expected sampling estimates.


Figure 1: CLARREO Pathfinder on ISS: Baseline Mission Objectives (image credit: NASA)

Inter-Calibration Requirements & Objectives

• CLARREO Pathfinder will demonstrate essential technologies for high-accuracy measurement and inter-calibration within reflected solar spectral range:

- Aligned with ESAS 2017 funding of radiance inter-calibration being a targeted observable, and ESAS 2007 Tier-1 CLARREO mission

- Demonstrate on orbit, high accuracy, SI-Traceable calibration for measurement of Earth solar reflectance –4 to 8 times more accurate than current best available sensors on orbit.

- Demonstrate ability to transfer this calibration to other on-orbit assets (VIIRS, CERES, and other assets as opportunities)

• Reflected Solar (RS) Spectrometer, launched to the International Space Station (ISS)

• Category 3 / Class D Mission, nominal 1-year mission life + 1 year science data analysis

• Targeted for launch to ISS: Recommend “Targeted for launch to ISS in late CY2022”(Our latest schedule shows instrument delivery 6/2022 and launch 11/2022, but actual launch will depend on SpaceX)

The CPF (CLARREO Pathfinder) mission began in 2016 and consists of a reflected solar (RS) spectrometer that will be hosted on the International Space Station for one year of operations starting in 2023. CPF will demonstrate essential measurement technologies required to obtain high-accuracy RS climate observations. 2)

CPF Mission Overview

CLARREO Pathfinder has two primary mission objectives. The first objective is to demonstrate on-orbit calibration approaches to achieve and maintain an unprecedented high accuracy with SI-traceability. 3) The second objective is to demonstrate a novel on-orbit inter-calibration approach by inter-calibrating two other reflected solar sensors: Clouds and the Earth’s Radiant Energy System (CERES) shortwave (SW) channel and Visible Infrared Imager Radiometer Suite (VIIRS) RS bands. CPF Level 0, Level 1, and Level 4 data will be publicly available through the Atmospheric Science Data Center (ASDC) at NASA Langley Research Center. 4)

HySICS (HyperSpectral Imager for Climate Science)

The CPF spectrometer is based on the HySICS instrument developed by the University of Colorado/Laboratory for Atmospheric and
Space Physics (LASP) in Boulder, CO. HySICS is a pushbroom imaging spectrometer that is designed to have a radiometric uncertainty of 0.3% (1σ), which is a three to ten times improvement over existing spaceflight RS instruments and is achieved via direct, on-orbit measurements of the accurately known spectral solar irradiance.

HySICS has a 70 km swath at nadir that is comprised of 480 discrete measurement pixels. At each pixel location, HySICS simultaneously measures spectrally-resolved reflected radiance from 350 to 2300 nm with 3 nm sampling. After applying calibration factors, the data comprise “image cubes” which are the spatially- and spectrally-resolved measurements of Earth’s solar reflectance and radiance.

Direct measurements of the Sun are taken using a 0.5 mm aperture and sufficiently short integration time. When HySICS observes the Moon or Earth, it uses a larger diameter aperture of 20 mm and longer integration time. Additional instrument characteristics are shown in Table 1.

Radiometric Uncertainty

0.3% (1-sigma)

Spectral Range

350 – 2300 nm

Spectral Sampling/Resolution

3 nm/6 nm

Polarization Sensitivity

<1%, 350-1800 nm; <2%, 1800-2300 nm

Swath Width

10 degrees

Spatial Resolution (nadir)

0.5 km

Sampling Rate

15 Hz

Table 1: Key characteristics of the CPF (CLARREO Pathfinder) instrument

Both the calibration and inter-calibration approaches will be enabled by the two-axis pointing capability of the instrument, as further described below. The high accuracy measurements from CPF will be a critical demonstration of the measurements needed to develop long-term climate-quality data sets.

Novel Calibration Approach

Earth-observing instruments are calibrated on the ground prior to launch; after launch that pre-flight calibration is typically tracked to evaluate how it changes throughout the instrument’s lifetime. After the instrument is launched into space, on-orbit resources that are either part of the instrument design (e.g. solar diffusers), the Earth system (e.g. pseudo-invariant targets like deep convective clouds or specific desert scenes), or celestial bodies (e.g. Sun, Moon, deep space) are used to monitor the instrument’s sensitivity changes throughout its lifetime in a relative, rather than an absolute, sense.

Alternatively, CPF will achieve its unprecedented accuracy levels primarily with regular on-orbit calibration measurements of the Sun as an on-orbit absolute calibration reference. Figure 1 illustrates the three main operating modes of CPF: nadir (nominal mode, red), solar (orange), and lunar (green) observations. A key benefit of the two-axis pointing capability of the CPF payload is that HySICS is able to view the Sun, Moon, and Earth by way of the same optical path, enabling the novel on-orbit calibration approach and the achievement of the 0.3% radiometric uncertainty.

The instrument slit will be scanned perpendicularly across the solar disk to collect all solar power incident at the input aperture. Comparisons to known spectral solar irradiance provide the instrument’s SI-traceable accuracy. Using this on-orbit solar reference, Earth reflectance measurements are radiometrically calibrated and will be distributed through the CPF Level 1 data products. The sun will also be used for flat field scans for both the 0.5 mm aperture and 20 mm aperture. These flat field scan measurements will be used to correct for pixel-to-pixel variations in detector sensitivity.

In-flight temperature changes are expected to cause small shifts in instrument optical alignment, resulting in shifts in wavelength scale and thermal background. Wavelength scale shifts can cause errors around high gradient spectral features, such as at the edge of absorption lines. A HgAr pen-ray lamp with known spectral atomic lines between 400 nm and 2100 nm will accompany the instrument. Measurements of lamp will be used to determine the wavelength scale of the instrument regularly throughout its lifetime. The instrument will also take dark space measurements to determine the residual blackbody radiation emitted by the instrument. See (Ref. 3) for additional details of the HySICS instrument and calibration design.

The ability of HySICS to take the necessary on-orbit calibration measurements makes the instrument robust to the on-orbit degradation that can plague absolute calibrations of instruments.


Figure 2: An illustration of key CLARREO Pathfinder observation modes: nominal operation at nadir (red) and direct solar (orange) and lunar (green) observations. The solar and lunar scans are two of the observations modes that will be used for regular on-orbit calibration of the CPF instrument (image credit: NASA, CU/LASP)


By measuring spectral radiance and reflectance with high accuracy, the CPF instrument will be able to serve as an on-orbit radiometric calibration reference for other operational Earth-viewing reflected solar sensors. CPF will demonstrate this capability by inter-calibrating the SW channel of CERES and RS bands of VIIRS. Two-axis pointing, a spectral range of 350-2300 nm, and spectral sampling of 3 nm enable the CPF instrument to provide nearly coincident temporal (within 10 minutes), spatial, angular, and spectral matching observations with inter-calibration targets, providing sufficient sampling to reduce random errors. 5) Figure 2 shows the CPF instrument matching its boresight to that of either the CERES or VIIRS instrument on JPSS-1, indicated by the overlapping red and green lines and shaded areas.

The CPF inter-calibration team is developing a series of algorithms that will be used to construct CPF-VIIRS and CPF-CERES Level 4 data products that will include all the information needed to conduct the inter-calibration data analysis. These algorithms include the spatial convolution, spectral convolution, and angular corrections needed to account for the spatial, spectral, and angular differences between CLARREO Pathfinder and each of its target (CERES & VIIRS) inter-calibration measurements. The mission requirement for the uncertainty contribution from these algorithms is 0.3%, in order to not exceed the radiometric uncertainty of the CPF instrument. 6)

The inter-calibration data analysis will refine knowledge of the CERES and VIIRS effective offsets, gains, non-linearities, spectral responses, and polarization sensitivity (VIIRS only).


Figure 3: Illustration of the CPF on ISS inter-calibration approach showing how nearly-concurrent measurements from CPF as the reference (red swath shows CPF line-of-sight) and measurements taken by the CERES and VIIRS on JPSS-1 (green swath), image credit: NASA, CU/LASP

Development status of CPF

• ”April 6, 2022: NASA and the Australian Space Agency are continuing their 60-year successful partnership to study Earth's changing climate. 7)

- NASA Deputy Administrator Pam Melroy and Australian Space Agency Head Enrico Palermo signed a joint statement of intent for cooperation in Earth science on April 4, 2022 at the 37th Space Symposium in Colorado Springs, Colorado. The partnership highlights complementary Earth observation missions currently in development at both agencies.

- NASA's Climate Absolute Radiance and Refractivity Observatory Pathfinder (CLARREO-PF) will make measurements of reflected sunlight from the International Space Station that are five to 10 times more accurate, and with higher spectral resolution, than those of existing reflected solar sensors. The Australian Space Agency is developing a set of Satellite Cross-Calibration Radiometer (SCR) satellites that will serve to improve the calibration of smaller Earth-observing optical satellites.

- “We are pleased to complement and strengthen the value of our CLARREO-PF mission and global Earth observations through this statement of intent to continue our strong partnership in space with Australia,” said NASA Deputy Administrator Pam Melroy.

- Measurements of reflected sunlight are critical to understanding the health of Earth's climate, and the accuracy of CLARREO-PF will allow it to serve as an inter-calibration reference to other orbiting sensors. The flight instrument is in development at the University of Colorado Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado and is scheduled to launch to the space station in December 2023.

- This signing follows a previous joint statement of intent on expanding space cooperation between the Australian Space Agency and NASA signed at NASA Headquarters in Washington in September 2019.


Figure 4: Australian Space Agency Head Enrico Palermo, left, and NASA Deputy Administrator Pam Melroy, sign a joint statement of intent for cooperation in Earth science during the 37th Space Symposium, Monday, April 4, 2022, in Colorado Springs, Colorado (photo credit: NASA, Bill Ingalls)

• March 2020: The CPF project passed its Critical Design Review (CDR) in March 2020. The meeting was hosted virtually from NASA/LaRC (Langley Research Center) and included the review board, participants, and presenters who joined the review online from Goddard Space Flight Center, Glenn Research Center, Johnson Space Center, Headquarters, the University of Colorado Laboratory for Atmospheric and Space Physics (LASP) and the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. 8)

- Passing CDR means that the matured payload and mission design met the performance requirements and that the project is ready to continue with payload fabrication, assembly, integration, and test. The project will now proceed toward the Pre-Environmental Review, which will be held in November 2021.

- The current plan is for the CPF mission to be operational starting in 2023.

• The KDP-C (Key Decision Point-C) for CPF was passed on July 8, 2019.

• September 27, 2018: NASA has awarded a contract to the University of Colorado Boulder’s LASP (Laboratory for Atmospheric and Space Physics) for development of a reflected solar spectrometer for the agency’s Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder mission. 9)

- The cost-no-fee contract provides for the formulation, implementation, launch, operation and analysis of the CLARREO Pathfinder mission’s reflected solar spectrometer. An indefinite-delivery, indefinite-quantity component provides for special studies and related services as required. The period of performance spans eight years and the total contract value is $57.4 million.

- CLARREO Pathfinder will demonstrate the technology needed to assess coastal flooding risks more effectively and better inform policy. The Science Directorate at NASA’s Langley Research Center in Hampton, Virginia, manages the mission for the agency’s Science Mission Directorate in Washington.

• September 15, 2016: The Climate Absolute Radiance and Refractivity Observatory (CLARREO) Pathfinder Project held a Mission Concept Review (MCR) for the Pathfinder mission Aug. 24 through 26 at NASA’s Langley Research Center in Hampton, Virginia. 10)

- “The MCR affirmed the need for the CLARREO Pathfinder (CPF) mission, demonstrated the feasibility of the planned approach, and demonstrated that the team has developed the necessary project infrastructure and work plans to proceed to the next phase of project execution,“ said Gary Fleming, CPF project manager.

- The CLARREO Pathfinder Project will launch a reflected solar spectrometer to the International Space Station (ISS) in the 2020 timeframe to demonstrate essential measurement technologies required for a future full CLARREO mission.

- The instrument will measure energy from the sun being reflected from Earth with higher accuracy than current space-based sensors. These measurements will be used to calibrate the sensors of other satellites that will cross the instrument’s path while in orbit, such as Clouds and the Earth’s Radiant Energy System (CERES) and Visible Infrared Imaging Radiometer Suite (VIIRS).

- Pathfinder will reduce risk, demonstrate essential techniques and technologies, and advance science in preparation for a future full CLARREO mission. The full CLARREO mission would produce highly accurate climate projections to improve climate models and ultimately enable sound policy decisions.

HySICS instrument

The Hyperspectral Imager for Climate Science (HySICS) is the imaging spectrometer that is the heart of the CPF payload. HySICS will take spectral measurements of sunlight reflected by Earth and the Moon. The instrument is being developed and built by the University of Colorado at Boulder's Laboratory for Atmospheric and Space Physics (LASP). Its design builds on over 10 years of LASP-led science research and technology development.


Figure 5: The CLARREO Pathfinder (CPF) instrument HySICS will establish unprecedented boundaries in accuracy (0.3%,1σ) of reflected sunlight measurements of Earth (image credit: LASP) 11)

Launch: A launch is the CPF mission to the ISS is expected in 2023.


Figure 6: Artist’s rendition of the location where CPF will be hosted and mounted. The rendered orientation of HySICS shows it in its nadir-pointing mode. (image credit: NASA & LASP)

1) C. Lukashin, Y. Shea, K. Thome, B. A. Wielicki, ”CLARREO Pathfinder Inter-calibration:Requirements and Objectives,” NASA/LaRC, CALCON (Conference on Characterization and Radiometric Calibration for Remote Sensing) USU (Utah State University), June 2019, URL:

2) Bruce A. Wielicki, D. F. Young, M. G. Mlynczak, K. J. Thome, S. Leroy, J. Corliss, J. G. Anderson, C. O. Ao, R. Bantges, F. Best, K. Bowman, H. Brindley, J. J. Butler, W. Collins, J. A. Dykema, D. R. Doelling, D. R. Feldman, N. Fox, X. Huang, R. Holz, Y. Huang, Z. Jin, D. Jennings, D. G. Johnson, K. Jucks, S. Kato, D. B. Kirk-Davidoff, R. Knuteson, G. Kopp, D. P. Kratz, X. Liu, C. Lukashin, A. J. Mannucci, N. Phojanamongkolkij, P. Pilewskie, V. Ramaswamy, H. Revercomb, J. Rice, Y. Roberts, C. M. Roithmayr, F. Rose, S. Sandford, E. L. Shirley, W. L. Smith, B. Soden, P. W. Speth, W. Sun, P. C. Taylor, D. Tobin, X. Xiong, ”Achieving climate change absolute accuracy in orbit,” BAMS (Bulletin of the American Meteorological Society), Volume 94, Issue No 10, pp: 1519-1539, Published: 1 October 2013,

3) Greg Kopp, Paul Smith, Chris Belting, Zach Castleman, Ginger Drake, Joey Espejo, Karl Heuerman, James Lanzi, and David Stuchlik, ”Radiometric flight results from the HyperSpectral Imager for Climate Science (HySICS),” EGU (Geoscience Instrumentation Methods and Data Systems, Vol. 6, Issue 1, 169–191, 2017) 04 April 2017, 2017, URL:

4) Yolanda Shea, Gary Fleming, Greg Kopp, Constantine Lukashin, Peter Pilewskie, Paul Smith, Kurt Thome, Bruce Wielicki, Xu Liu, Wan Wu, ”CLARREO Pathfinder: Mission Overview and current status,” IGARRS 2020,

5) Bruce A. Wielicki, David R. Doelling, David F. Young, Norman G. Loeb, Donald P. Garber, and David G. MacDonnell"Climate quality broadband and narrowband solar reflected radiance calibration between sensors in orbit," IGARSS 2008, 2008IEEE International Geoscience and Remote Sensing Symposium, 1, pp.257-260, 2008,

6) Carlos M. Roithmayr, Constantine Lukashin, Paul W. Speth, Greg Kopp, Kurt Thome, Bruce A. Wielicki, and David F. Young. "CLARREO approach for reference intercalibration of reflected solar sensors: On-orbit data matching and sampling," IEEE Transactions on Geoscience and Remote Sensing, Vol. 52, No 10, pp.6762-6774, 2014, URL:

7) Joe Atkinson, ”NASA, Australian Space Agency Collaborate on Earth Science,” NASA Feature, 6 April 2022, URL:

8) ”CLARREO Current Status,”

9) Steve Cole, Jeremy Eggers, ”NASA Awards Contract for Climate Pathfinder Mission Instrument,” Contract Release C18-030, 27 September 2018, URL:

10) MaryAnn Jackson, ”CLARREO Pathfinder Undergoes Successful Mission Concept Review,” NASA/LaRC, 15 September 2016, URL:

11) Yolanda L. Shea, Gary Fleming, Greg Kopp, Constantine Lukashin, Peter Pilewskie , Paul Smith, Kurt Thome, Greg Ucker, Bruce A. Wielicki, 2020,

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 (

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