## 1 Introduction  
The role of satellites in Earth observation has increased substantially over the past sixty years since the first meteorological satellite TIROS 1 was launched in 1960. With satellite observations, we are able to probe the Earth system (atmosphere, ocean, and land surface) at increasingly high temporal, spatial, and spectral resolution [1]. However, the capability for climate monitoring from space is insufficient [2; 3; 4]. Even though satellite observed radiances today generally have a radiometric precision of 0.2 K or better, radiances assimilated in numerical weather prediction (NWP) models still need to be corrected for significant biases. For climate monitoring, current measurements are not yet adequate because small trends of the order of a tenth of a degree need to be measured with high confidence [5; 6]. It is essential for climate monitoring and desirable for NWP that measurements are basically bias-free [7]. Without accurate, high quality observations on relevant time and space scales, climate science applications and services will be limited [8]. To anchor satellite-based climate observation, several reference-type missions against an absolute standard are being considered by several space agencies [9]. The Traceable Radiometry Underpinning Terrestrial- and Helio-Studies (TRUTHS [10] has been accepted by the ESA (European Space Agency) as a climate mission with an unprecedented SI-traceable accuracy. The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission has been underway by the National Aeronautics and Space Administration (NASA) since the mid-2000s. CLARREO also aims to provide accurate and SI-traceable observations, allowing the assessment of decadal changes of climate [11]. More recently, a CLARREO Pathfinder mission with only the reflected solar spectrometer is being built for the International Space Station (ISS), with launch tentatively planned for 2023. The proposed CLARREO and TRUTHS missions would create a climate observing system measuring both the incoming and reflected solar energy with a 10-fold increase in data accuracy compared to existing systems. They would provide spectrally resolved thermal infrared and reflected solar radiation with high absolute accuracy. In addition, such systems will serve as on-orbit references for a wide range of visible and infrared Earth observation sensors in LEO and GEO orbits. This approach has already been realized by the Global Space-based Intercalibration System (GSICS) using the most stable current instruments as references [12]. In the recent Vision 2040 document of the WMO Integrated Observing System (WIGOS), it is presumed that operational meteorological satellite systems will remain the key elements of a space-based climate observing system capable of unambiguously monitoring indicators of changes in the Earth's climate [13]. Satellite agencies are therefore encouraged to develop new satellite instruments with climate applications in mind. The proposed reference systems would go a long way in that direction. This will also enable the production of much improved Essential Climate Variables (ECVs) in accordance with established key requirements for climate monitoring [14]. Realizing the importance of reference-type missions for improving climate science and for harmonizing global satellite observations, an expert team on Earth observation and navigation of Ministry of Science and Technology (MOST) proposed the concept of the Chinese Space-based Radiometric Benchmark (CSRB) in 2006. The CSRB project was approved and initially funded by MOST in 2014. To date, Phase A of the CSRB project, completed in 2018, resolved the fundament problems of building the SI-traceable calibrator for the thermal infrared band and reflected solar band. The ultimate goal of the CSRB project is to build a flight model of the Chinese radiometric benchmark satellite, named 'LIBRA', for launch during 2022-2025. The roadmap of the CSRB project is presented in Section 2, and the requirements and specifications for LIBRA follow in Section 3.