## 7 Conclusions  
Accurate, long-term, consistent data are essential to climate science. Such data need to be calibrated to an absolute standard. The nature of satellite observations is the key to obtain global data of the earth and atmospheric system. However, so far, calibration directly traceable to a reference has not been possible for satellite observations. The principle of referring measurements to an absolute standard is not new, as it has been realized for in situ observations which are traceable to standards with well quantified uncertainties. This paper describes a novel space-based system, providing a similar traceability and accuracy for space-based radiance measurements. The realization of the LIBRA system will enable climate monitoring with much higher sensitivity to trend detection and provide benefits in science and economic terms [44].  
LIBRA has important potential contributions to make to future climate observing systems, both directly through well-calibrated measurements and indirectly through facilitating intercalibration of the data from other platforms. LIBRA will measure solar reflected and infrared emitted high spectral resolution benchmark radiances, and provide accurate, credible, and tested climate records. These measurements will be used to detect climate change trends and test, validate, and improve climate prediction models. LIBRA can be used to calibrate other solar and infrared space-borne sensors and thereby improve climate monitoring accuracy for a wide range of measurements across the global space-based observing system.  
As a complementary project to CLARREO and TRUTHS, LIBRA is expected to join an Earth observing satellite constellation and intends to contribute to space-based climate studies via publicly available data. Intercalibration of data from space-based observations falls under the auspices of the international CEOS Working Group on Calibration and Validation (WGCV) and GSICS. Therefore, intensive cooperation with the WGCV and GSICS is highly recommended during instrument development and data utilization of the LIBRA project.  
Figure 12: Sketch of flight test of the balloon-based demonstration system.  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] proposed the framework of the manuscript and supervised the progress. [PERSON], [PERSON] ([PERSON]), [PERSON] ([PERSON]), [PERSON], [PERSON], [PERSON] contributed to the designing of reference instruments, intercalibration modes, and Balloon-borne demonstration system. [PERSON], [PERSON], and [PERSON] aided in the manuscript revision. [PERSON] prepared and finalised the manuscript. All authors contributed to the writing and commented on the manuscript. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work has been supported by the National Key Research and Development Program of China (2018 YFB0504900; 2018 YFB0504800), and the Bureau of International Co-operation Chinese Academy of Sciences (181811 KYSB20160040).  
**Acknowledgments:** The authors do appreciate the discussion and comments from [PERSON], [PERSON] of EUMETSAT, [PERSON] of NOAA, [PERSON] of NPL, [PERSON] of NASA, [PERSON] of University of Colorado, and [PERSON] of WMO. The authors are grateful to [PERSON] and [PERSON] of Shanghai Institute of Satellite Engineering for their work concerning the orbit simulation and demonstration of Chinese LIBRA satellite.  
**Conflicts of Interest:** The authors declare that they have no conflict of interest.