SITSat for GSICS

By Manik Bali and Lawrence E Flynn (NOAA)

Table of Contents

• GSICS Introduction
• SITSat missions as in-orbit GSICS References
• Simultaneous Nadir Overpass
  • Simultaneous Nadir Overpass Prediction
  • GSICS Inter-Calibration Products
• Tying GSICS Products to an Absolute (SITSat) Scale ( Tim Hewison, 2020)
• CLARREO, LIBRA and TRUTHS Measurement Spectrum ( Peng Zhang, 2020)
  • CLARREO Pathfinder Mission
• Opportunities for Inter-Calibration, SITSat Orbits
• Tying GSICS Data Maturity to SITSat mission maturity
• Open Questions
• SITSat Meetings

GSICS Introduction

The Global Space-based Inter-Calibration System (GSICS) is an international collaborative effort initiated in 2005 by the World Meteorological Organization (WMO) and the Coordination Group for Meteorological Satellites (CGMS) to 1-monitor, 2-improve and 3-harmonize the quality of observations from operational weather and environmental satellites of the Global Observing System (GOS).

GSICS aims at ensuring consistent accuracy among space-based observations worldwide for climate monitoring, weather forecasting, and environmental applications.

A SITSat is a satellite-based sensor designed to provide verifiable evidence of all significant contributions to the uncertainty of its measurements. This is done openly and transparently, with traceability to the International System of Units (SI) at the specific location and time of the measurement. The community must consider the uncertainty level of ‘fiducial reference’ quality, meaning it is state-of-the-art within a defined spectral domain or application and can serve as a definitive reference for similar measurements from other sensors. The uncertainty of a SITSat should meet or approach the requirements for key climate science objectives, as outlined in the “SI-Traceable Space-based Climate Observing System: a CEOS and GCICS Workshop” (https://doi.org/10.47120/npl.9319). When used as a reference, a SITSat is expected to have a measurement uncertainty of less than 0.5 compared to its non-SITSat counterparts

GSICS Wiki: http://gsics.atmos.umd.edu/wiki/Home

GSICS Coordination Center: https://www.star.nesdis.noaa.gov/smcd/GCC/

SITSat Missions

Three Sitraceable missions are planned to be launched. These are CLARREO(NASA), TRUTHS (ESA), and Libra (CSRB).

SITSat missions as in-orbit GSICS references

The SITSat missions are envisaged to be a game-changer for the GSICS community. For the first time, in-orbit Si Traceable measurements from Sit SAT missions can be used as a reference to compare measurements of concurrently flying missions with overlapping spectral bands during a Simultaneous Nadir Overpass event.

Figure Above: Simultaneous Nadir Observation (SNO) inter-calibration transfer mode (Geostationary earth orbit (GEO)-Low earth orbit (LEO) or LEO-LEO). (Image credit: Zhang et al., 2020)

Simultaneous Nadir Overpass Prediction ( can be extended to SITSat )

The National Calibration Center led by Changyong Cao (NOAA) has developed a public website that gives the prediction of Simultaneous Nadir Overpasses. We envisage that the algorithms used by Changyong can be directly used to predict and visualize the Simultaneous Overpasses of the SitSAT missions with comparing satellites

https://ncc.nesdis.noaa.gov/VIIRS/SNOPredictions/index.php

GSICS Inter-Calibration Products ( can be extended to SITSat)

Once the SITSat mission is operational and its data is available GSICS and likewise communities would start monitoring Inter-calibration products. It is envisaged that these products would follow CF-x metadata conventions and would be along the lines of existing GSICS Products.

Existing GSICS Products can be found on the GSICS Product Catalog

https://www.star.nesdis.noaa.gov/smcd/GCC/ProductCatalog.php

*CLARREO, LIBRA and TRUTHS Measurement Spectrum*

Figure Above Curtsey Pen Zhang 2021 Calibration and Recalibration: Theory, Practice, and Future Perspective

CLARREO (Climate Absolute Radiance and Refractivity Observatory)

• Objective: CLARREO is designed to provide highly accurate, SI-traceable measurements of the Earth’s radiation budget, which is crucial for understanding and predicting climate change.
• Scope: It aims to deliver benchmark climate records, which will help improve climate models and predictions.
• Instruments: CLARREO will carry instruments capable of observing reflected solar radiation and emitted thermal radiation with unprecedented accuracy.

CLARREO Pathfinder Mission

• Objective**: CLARREO Pathfinder is a smaller, precursor mission to CLARREO, aimed at demonstrating the technologies and methodologies required for the full CLARREO mission.
• Scope**: This mission is intended to validate the instruments and techniques that will be used in the full CLARREO mission, thereby reducing risk and ensuring the success of the larger mission.
• Instruments**: CLARREO Pathfinder will carry a Reflected Solar instrument, which will be mounted on the International Space Station (ISS) to test the key technologies. Key Differences
• Mission Size and Scope**: CLARREO is a full-scale mission with a broader range of objectives and instruments, whereas CLARREO Pathfinder is a smaller, targeted mission focused on technology demonstration.
• Purpose**: CLARREO aims to provide long-term climate data records, while CLARREO Pathfinder is designed to ensure the feasibility and reliability of the technologies for the full mission.
• Deployment**: CLARREO Pathfinder operates from the ISS, whereas the full CLARREO mission would involve dedicated satellites.

CLARREO Pathfinder is a critical step towards the full implementation of the CLARREO mission, ensuring that the necessary technology and methodologies are validated and ready for the comprehensive climate monitoring goals of CLARREO.

Tying GSICS Products to an Absolute Scale

By Heiwson et al 2020

Many satellite missions attempt to calibrate their instruments against SI standards before launch. GSICS, working together with CEOS, aims to define best practices to characterize satellite instruments pre-launch using SI-traceable references to tie them to an absolute radiance scale. However, this traceability chain may be compromised during launch, due to uncontrolled changes to the instrument and its operating environment. For this reason, GSICS endorses the establishment of an observing system with calibration directly traceable to SI-standards on-orbit to act as an inter-calibration reference, which could be used to anchor inter-calibration products to an absolute scale—a long-term aim of GSICS.

A satellite mission, such as CLARREO Pathfinder with HyperSpectral Imager for Climate Science (HySICS) 17], TRUTHS 18], in which an instrument is launched, whose SI-traceability is verifiable in orbit could be used as a reference to achieve this goal. These are generically referred to here as SI-Traceable Satellite instruments (SITSATs).

In the simplest case, shown by the green arrow in Figure 1a, such a SITSAT can be used as a reference in direct inter-calibration of the monitored instrument by SNO or Ray-matching instead of the current reference instrument(s) for the IR case, or the PICTs in the VIS/NIR case. However, the coverage of the available collocated observations can be limited, depending on the design of the SITSAT—and the duration of such inter-calibration products is limited to their operating lifetime.

Currently, GSICS products take the reference instruments as the truth—no uncertainty is associated with their calibration. To compare the radiances after applying the GSICS correction to an absolute scale requires the reference instruments’ uncertainty to be included—and this typically dominates their overall uncertainty budget. This situation would change if it were possible to tie the inter-calibration to an absolute scale through the use of an SI-Traceable Satellite Instrument (SITSAT). SITSAT designs typically have a better calibration accuracy than the current reference instruments used in GSICS. Furthermore, SITSAT sensors can be designed with pointing abilities, significantly reducing the angular matching uncertainty compared to current cross-track scanning reference sensors. Therefore, their introduction could reduce the overall uncertainty with which inter-calibration can be applied to an absolute scale

SITSat Orbits

Each of the three SITSat missions has a distinct orbit

CLARREO Pathfinder would be onboard the International Space Station (ISS) (red) at 400 km altitude and 51.6° orbit inclination, crossing under the Suomi-NPP or NOAA-20 orbit track (green) at ~830 km altitude, in a 13:30 Local Time sun-synchronous 98.7° orbit. Lukashin et al., 2013 have detailed the SNO opportunities

Figure Above: Predicted inter-calibration events between CLARREO Pathfinder on ISS and CERES/VIIRS on NOAA-20 during 1 year of operations. There are about 1,300 viable inter-calibration events. (Courtesy NASA)

Tying GSICS Data Maturity to SITSat data maturity

GSICS Procedure for Product Acceptance (GPPA) is the ...

• GSICS product developers pathway to obtain a "Stamp of Approval" for a potential product
• GSICS data users window to GSICS product quality and "fitness for purpose"
• GSICS governing body reference for judging GSICS product fitness

GPPA is Composed of the Four Following Phases ...

• Product Submission Phase:A GSICS Product Acceptance Form for a potential product has been satisfactorily completed and submitted to the GSICS Coordination Center
• Demonstration Phase:The Product is determined to be within GSICS scope, its fundamental founding concepts are understood, and it meets GSICS data format/content guidelines. In this phase, the Product is released solely for the purpose of evaluation within GSICS and by potential product users
• Pre-Operational Phase:The Product has been determined to be a valuable part of the GSICS product portfolio, and has developed and understood methodology, software, supporting models and measurements, uncertainty, quality indicator, and traceability to a community or SI standard
• Operational Phase:In addition to its Pre-Operational Phase attributes, the Product has developed and understood generation, distribution, version control and archive strategies. It also has an available User's Guide. At this point, Product is fully accepted and maintained within GSICS

How is GPPA connected to SITSat missions

The GPPA is a special case of the QA4EO architecture. The maturity of reference data that the SitSAT mission would provide for GSICS style inter-calibration is a key parameter that is used to assign maturity to GSICS inter-calibration products.

Open Questions

1) Would the parent QA4EO be modified to assign maturity to the SITSat mission observations?

2) What would be the metadata conventions to be used in the file formats of the SITSat missions

3) What is the expected timeliness of SITSat be available to users for inter-calibration ( real time and what platforms)

Clarreo Path Finder Data Dissemination and Availability

CLARREO Pathfinder data will be publicly available from the Atmospheric Science Data Center (ASDC), which has been assigned as the mission’s Distributed Active Archive Center. More details on https://clarreo-pathfinder.larc.nasa.gov/science-overview/#cpf-data-products

GSICS SITSat meetings

SI-Traceable Space-based Climate Observing System Workshop (SITSCOS) -

Event Dates: 9-11 September 2019 - National Physical Laboratory in London, UK

https://calvalportal.ceos.org/sitscos-ws

This meeting was followed by a recent TRUTHS workshop.

Earth's Spectrally Resolved Reflected Radiation - TRUTHS for Climate Workshop

Event Dates: 27-28 June, 2024 Harwell Science and Innovation Campus, UK.

https://nikal.eventsair.com/truths-workshop/

References

Cao, C., M. Weinreb, and H. Xu, 2004: Predicting Simultaneous Nadir Overpasses among Polar-Orbiting Meteorological Satellites for the Intersatellite Calibration of Radiometers. J. Atmos. Oceanic Technol., 21, 537–542, https://doi.org/10.1175/1520-0426(2004)021<0537:PSNOAP>2.0.CO;2.

Hewison, T.J.; Doelling, D.R.; Lukashin, C.; Tobin, D.; O. John, V.; Joro, S.; Bojkov, B. Extending the Global Space-Based Inter-Calibration System (GSICS) to Tie Satellite Radiances to an Absolute Scale. Remote Sens. 2020, 12, 1782. https://doi.org/10.3390/rs12111782

Lukashin, C., B. Wielicki, D. Young, K. J. Thome, Z. Jin, and W. Sun (2013), Uncertainty Estimates for Imager Reference Inter-calibration with CLARREO Reflected Solar Spectrometer, IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, 51, doi:10.1109/TGRS.2012.2233480.

Zhang, P.; Lu, N.; Li, C.; Ding, L.; Zheng, X.; Zhang, X.; Hu, X.; Ye, X.; Ma, L.; Xu, N.; et al. Development of the Chinese Space-Based Radiometric Benchmark Mission LIBRA. Remote Sens. 2020, 12 , 2179. https://doi.org/10.3390/rs12142179

Libra Specifications: https://www.eoportal.org/satellite-missions/libra#overview

https://clarreo-pathfinder.larc.nasa.gov/