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LIBRA

Nov 27, 2023

EO

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Planned

Chinese Space-based Radiometric Benchmark (CSRB) is a climate monitoring project proposed and funded by the Ministry of Science and Technology (MOST). The goal of CSRB is to launch an International System of Units (SI) traceable satellite named LIBRA in 2025 to contribute to space-based climate studies via publicly available data.

Quick facts

Overview

Mission typeEO
Mission statusPlanned

Design chart of Earth-Moon Imaging Spectrometer (EMIS) and Total Solar Irradiance (TSI) instrument on LIBRA mission (Image credit: MDPI)

Summary

Mission Capabilities

The LIBRA satellite will house four payloads: the Infrared Spectrometer (IRS), the Earth-Moon Imaging Spectrometer (EMIS), the Total Solar Irradiance (TSI) instrument, and the Solar spectral Irradiance monitoring instrument Traceable to Quantum benchmark (SITQ). IRS will provide high spectral resolution measurements across the infrared spectrum, traceable to the SI standard. EMIS consists of a telescope and a hyperspectral imaging spectrometer to measure reflected solar spectrum radiance. Both TSI and SITQ will measure solar spectral irradiance through temperature changes based on precisely measured equivalent electrical power, and Spontaneous Parametric Down-Conversion (SPDC) respectively.

Performance Specifications

The Infrared Spectrometer (IRS) is a highly sensitive infrared sounder with hyperspectral resolution (0.5 cm-1) and a spectral range of 600-2700 cm-1.

The Earth-Moon Imaging Spectrometer (EMIS) is made up of a telescope and hyperspectral imaging spectrometer with a spectral range of 380-2350 nm. It makes a trade off between the spectral and spatial resolution, with spectral and spatial sampling better than 10 nm and 100 m, respectively.

The Total Solar Irradiance (TSI) instrument operates by alternately exposing the detector to a radiant energy source (solar irradiance) and then to known internal electrical heating. TSI has a spectral range of 0.2-35 μm. 

Solar spectral Irradiance Traceable to Quantum benchmark (SITQ) operates by tracing the solar spectral irradiance to Planck’s constant and photon count number. SITQ is a highly sensitive instrument with a spectral range of 380-2500 nm.

Launch is planned for 2025. LIBRA will be launched into a Low Earth Orbit (LEO). 

Space and Hardware Components

Additional to the four instruments on board the LIBRA satellite (IRS, EMIS, TSI and SITQ), key technologies of the system include miniature phase-change cells providing fixed-temperature points, a cryogenic absolute radiometer, and a Spontaneous Parametric Down-Conversion (SPDC) detector. 
 

Overview

While the global observing system for climate continues to better support the needs of an increasingly wider user community, such data needs to be calibrated to an absolute standard. It is well established that the current capability for climate monitoring from space is insufficient. To respond to this need for reference-type missions to harmonise global satellite climate observations, the Ministry of Science and Technology (MOST) proposed the Chinese Space-based Radiometric Benchmark (CSRB) project in 2006. 1) 2) 3)

In the early stages of China’s Earth observation satellite programs, there were hardly any onboard calibration systems. The primary goal of CSRB is to launch a SI traceable satellite named LIBRA to contribute to space-based climate studies via publicly available data. LIBRA will offer measurements of the outgoing radiation from the Earth and the incoming radiation from the Sun with high spectral resolution. 4)

LIBRA is a complementary mission to Climate Absolute Radiance and Refractivity Observatory (CLARREO) developed by the USA, and the Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) developed by Europe and the UK. Together, these satellites will form a constellation of Earth observation satellites that will contribute to space-based climate studies via publicly available data.

LIBRA will be built in three phases:

  • Phase A (extended to 2018), which focused on calibrating the thermal infrared band (IR) and the reflective solar band (RSB) to SI standards
  • Phase B (from 2018), which focuses on developing engineering models for the reference units
  • Phase C, which will focus on developing a flight model for launch.
Figure 1: The roadmap of the Chinese Space-based Radiometric Benchmark (CSRB) project. Image credit: MDPI)

Spacecraft

The experimental prototype of the Space Cryogenic Absolute Radiometer (SCAR) has been in development since 2015. The light power benchmark of SCAR is converted to a radiance benchmark by the Transfer Radiometer (TR) and multiple laser diodes.

The Stryn-type Pulse Tube Cryocooler (SPTC) is used to obtain the 20 K working temperature, which is optimised to provide the refrigerating capacity of 350 mW at 20 K for space applications. The heating effects of the incident light and electrical heater are nearly equivalent at 20 K, therefore reducing non-equivalence corrections.

Using an internal mirror, the IRS will enable nadir, off-nadir, internal calibration, and deep-space (zenith) observations.

The Earth-Moon Imaging Spectrometer (EMIS) and Total Solar Irradiance (TSI) instruments will be co-boresighted and mounted to a two-axis gimbal to enable nadir (nominal operations) and off-nadir lunar and solar observations. The spacecraft bus should provide an independent means for acquiring necessary position and attitude information. Upon the completion of phases B and C, further specifications relating to size and structure of the LIBRA satellite will be finalised.

The LIBRA experimental observatory has been designed with a spacecraft operation system that will minimise the cost and risk of operations and maximise use of spacecraft and ground system fault detection, reporting and protection tools. The observatory has been designed for an operational lifetime of five years with consumables for eight years. The Chinese Space Station (CSS) is also being considered as an instrument platform for LIBRA.

Launch

Launch is planned for 2025. LIBRA will be launched into a Low Earth Orbit (LEO). Lagrangian orbit locations are also being investigated for complementary observations.

The Simultaneous Nadir Overpass (SNO) cross intercalibration transfer mode is a crucial aspect of the orbit that allows for intercalibration between the reference satellite LIBRA and the targeted satellite flying in different orbital planes. The intercalibration is carried out in near real-time in the nadir zone. 5)

Figure 2: Simultaneous Nadir Observation (SNO) intercalibration transfer mode (Geostationary earth orbit (GEO)-Low earth orbit (LEO) or LEO-LEO). (Image credit: Zhang et al., 2020)

Mission Status

  • 2018: The prototype payload of the Earth-Moon Imaging Spectrometer (EMIS) and Total Solar Irradiance (TSI) begin development.
  • 2018: Phase A of the CSRB project complete and phase B started. Fundamental problems of building the SI-traceable calibration for the thermal infrared band and reflected solar band have been resolved.
  • 2014: CSRB project approved and initial funding provided by the Ministry of Science and Technology (MOST).
  • 2006: The CSRB project was proposed by an expert team from MOST on Earth observation and navigation. 6)

Sensor Complement

The LIBRA satellite will house four payloads: the Infrared Spectrometer (IRS), the Earth-Moon Imaging Spectrometer (EMIS), the Total Solar Irradiance (TSI) instrument, and the Solar spectral Irradiance monitoring instrument Traceable to Quantum benchmark (SITQ).

Table 1: Overview of instruments on board LIBRA.

Instrument

Payload Requirements

Key Technology

IRS

Spectral range: 600-2700 cm-1

Spectral resolution: 0.5 cm-1

IFOV: 24 mrad

Sensitivity: 0.1 K @270 K

Emissivity of BB: ≥ 0.999

Measurement uncertainty: 0.15 K (k = 2)

Miniature fixed-temperature phase-change cells

EMIS

Spectral range: 380-2350 nm

Spectral resolution: 10 nm

Spectral precision: 0.5 nm

Spatial resolution: 100 m

Coverage: 50 km

Measurement uncertainty: 1% (k = 2)

Space Cryogenic Absolute Radiometer

TSI

Spectral range: 0.2-35 μm

Measurement uncertainty: 0.05% (k = 2)

Long-term stability: 0.005%

Space Cryogenic Absolute Radiometer

SITQ

Spectral range: 380-2500 nm

Spectral resolution: 3 nm (380-1000 nm) 8 nm (1000-2500 nm)

Spectral precision: 0.1-0.3 nm

Self-calibration uncertainty: 0.2%

Measurement uncertainty: 0.35% (k = 2)

Spontaneous Parametric Down-Conversion

 

Infrared Spectrometer (IRS)

The Infrared Spectrometer (IRS) is an infrared sounder with hyperspectral resolution traceable to international units that will perform high precision measurements of the infrared spectrum.

IRS is designed with fourier infrared spectrum detection technology and miniature phase-change cells. Accurate blackbody temperatures (fixed points) are based on ‘phase-change cells’ technology. The high sensitivity response over the wide spectral band is achieved by using a small-array detector. The high stability performance is realised by system temperature control technology using multiple temperature zones. High accuracy spectral calibration is achieved by referring to a standard spectral line source (high frequency infrared laser and gas absorption line).

Three on-orbit absolute radiance IR calibrators realise on-orbit self-calibration of the cavity blackbody. An on-orbit temperature scale from 270 to 350 K is established using ITS-90 miniature phase-change cells traceable to ITS-90 with an uncertainty of better than 10 mK (k = 2). 9)

Earth-Moon Imaging Spectrometer (EMIS)

The optical design of EMIS consists of a telescope and a hyperspectral imaging spectrometer. The telescope utilises a four-mirror anastigmat to eliminate aberrations. EMIS makes a trade-off between the spectral and spatial resolution, with spectral and spatial sampling better than 10 nm and 100 m, respectively. The swath width is approximately 50 km at nadir from a 600 km orbit.

The Space Cryogenic Absolute Radiometer (SCAR) is used to realise accurate radiometry on-satellite by referencing the ground-based optical standard. The reflected solar spectral radiance is measured by EMIS, which is regularly calibrated by SCAR and the Benchmark Transfer Chain (BTC) in order to improve the measurement accuracy and long-term stability.10)

Figure 3: Schematic diagram of hyperspectral calibration on satellite.

Total Solar Irradiance (TSI) instrument

TSI operates by alternately exposing the detector to a radiant energy source (solar irradiance) and then to known internal electrical heating. The temperature increase due to the absorbed solar irradiance is then compared to the same temperature increase due to a precisely measured equivalent electrical power.

SCAR is also the benchmark of TSI calibration onboard the satellite, which will improve the long-term stability of TSI measurements. The irradiance scale of the TSI is traced to SCAR via simultaneous solar observations, and the system deviation is corrected by the real-time correction based on a super stable voltage reference.

Solar spectral Irradiance Traceable to Quantum benchmark (SITQ)

Spontaneous Parametric Down-Conversion (SPDC) is a quantum optical effect occurring in a non-linear crystal pumped by a laser beam. It will be used on board LIBRA to trace the solar spectral irradiance to Planck’s constant and photon count number.

In the calibration mode, correlated photons are directed by a rotating mirror into the middle optics, and then, are spectrally separated by two monochromators, before finally reaching avalanche photodiodes. This approach is especially useful as a reference or benchmark in space. In the observation mode, solar radiation passes an entrance aperture and propagates along the same optical path before it is received by analogue photodiodes.

Ground Segment

Presently, no ground segment has been identified in the CRSB project. The CSRB project plans for ground system fault detection through the LIBRA experimental observatory.

References  

1) Cooke, Roger, et al. “Value of information for Climate Observing Systems.” Environment Systems and Decisions, vol. 34, no. 1, 23 July 2013, pp. 98–109, URL: https://link.springer.com/article/10.1007/s10669-013-9451-8

2) World Meteorological Organization, (WMO). 2015, pp. 165–209, Status of the Global Observing System for Climate, URL: https://library.wmo.int/viewer/54812

3) Zhang, Peng, et al. “Development of the Chinese space-based Radiometric Benchmark Mission libra.” Remote Sensing, vol. 12, no. 14, 8 May 2020, p. 2179, URL: https://www.mdpi.com/2072-4292/12/14/2179 

4) Xingfa, Gu, and Tong Xudong. “Overview of china earth observation satellite programs [space agencies].” IEEE Geoscience and Remote Sensing Magazine, vol. 3, no. 3, 30 Sept. 2015, pp. 113–129, URL: https://ieeexplore.ieee.org/abstract/document/7284779 

5) Zhang, Peng. WGCV-51 Progress on Chinese Space-Based Radiometric Benchmark Project, 3 Oct. 2022, URL: https://ceos.org/document_management/Working_Groups/WGCV/Meetings/WGCV-51/Presentation/3.5_Zhang_WGCV-51_Chinses%20SITSAT_v1.pdf 

6) LU, Naimeng, et al. “Introduction of the radiometric benchmark satellite being developed in China for Remote Sensing.” National Remote Sensing Bulletin, vol. 24, no. 6, 7 June 2020, pp. 672–680, URL: https://www.ygxb.ac.cn/en/article/doi/10.11834/jrs.20200011/ 

7) Ohring, George, et al. “Satellite instrument calibration for measuring global climate change: Report of a workshop.” Bulletin of the American Meteorological Society, vol. 86, no. 9, 1 Sept. 2005, pp. 1303–1314, URL: https://journals.ametsoc.org/view/journals/bams/86/9/bams-86-9-1303.xml?tab_body=abstract-display 

8) Intergovernmental Panel on Climate Change, IPCC. Climate Change 2014 Synthesis Report, Intergovernmental Panel on Climate Change (IPCC), 2014, URL: https://archive.ipcc.ch/pdf/assessment-report/ar5/syr/AR5_SYR_FINAL_All_Topics.pdf 

9) Hao, X. P., et al. “Miniature fixed points as temperature standards for in situ calibration of temperature sensors.” International Journal of Thermophysics, vol. 38, no. 6, 11 Apr. 2017, URL: https://link.springer.com/article/10.1007/s10765-017-2223-9 

10) Boesch, H, et al. “Si-traceable space-based climate observation system: A CEOS and GSICS workshop. National Physical Laboratory, UK, 9-11 Sept 2019.” National Physical Laboratory (NPL), 25 Jan. 2022, URL: https://eprintspublications.npl.co.uk/9319/ 

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 (eoportal@symbios.space).

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