## 4 Current Status and Identified Needs
### Level 1 TOA Radiometry and Geometry
#### 4.1.2 In-Orbit Calibration  
The SI-traceability breaks down, in rigorous terms, when the sensor is launched into orbit and exposed to the harsh space environment. As a matter of fact, while full traceability to SI units can be ensured and regularly maintained on-ground, through comparison to primary standards in well-controlled laboratory conditions, the same SI-traceability cannot be achieved in orbit. In practice, dedicated on-board devices are designed to allow the calibration factors to be regularly updated and improved post-launch. Despite not being fully SI-traceable, these on-board devices allow to account for the natural ageing of the satellite sensor during the mission lifetime and to closely assess its radiometric and spectral response.  
In-orbit radiometric calibration of optical sensors in the visible and short-wave infrared domains is carried out routinely using diffuser panels, which have nearly-Lambertian reflectance properties and are exposed on a regular basis to the Sun to provide the reference measurement for calibrating the sensor radiometric and spectral response of the Earth-view. In the real case scenario, the on-board diffusers can exhibit a non-Lambertian behaviour, e.g., due to imperfections of the used coating material, and can be subject of ageing due to long exposure to the space environment. The availability of secondary diffusers, which are exposed less frequently to ensure pristine reference measurements, allow for improving the accuracy of the in-flight radiometric calibration and for accurately characterising the ageing of the operational diffuser [70; 71].  
However, some space-borne systems lack for on-board calibration devices, see for instance Proba-V [72] and the large majority of small-satellite constellations. Furthermore, the on-board calibration system provides a reference source, which is, by definition, applicable only to the considered optical sensor and cannot be transferred to another sensor. In such cases, the use of independent calibration targets (_vicarious targets_), accessible by all sensors, is crucial for absolute radiometric assessment in case of missing or damaged on-board diffuser(s) and for establishing a common radiometric scale to estimate inter-sensor radiometric biases.  
The vicarious targets are temporally stable, spatially and spectrally uniform Earth and planetary targets whose radiometric signal can be accurately modelled with a high degree of confidence. In the case of Earth's targets, the vicarious calibration methods rely on a Radiative Transfer Model (RTM), which is used to simulate the at-sensor signal over the considered site. This simulated value is used to validate the radiometry of the considered sensors, to monitor its variation over time and to inter-compare various sensors observing the same targets [73]. Vicarious approaches are systematically used at ESA for radiometric assessment and intercalibration of S2 and S3 optical sensors, using the DIMITRI (Database for Imaging Multi-spectral Instruments and Tools for Radiometric Intercomparison) tool [74].  
An exhaustive review of the natural targets, which are currently used for vicarious calibration, is out of scope for this paper. We recall in the following paragraphs the most commonly used methods and the current and planned initiatives to address the remaining challenges for assessing radiometric accuracy at TOA level.