## 6 Balloon-Borne Demonstration System  
When radiometric benchmark transfer methods are developed, an issue is the validation of the full chain of the space-based radiometric benchmark transfer. This problem is a primary motivation for the development of a balloon-borne demonstration system. There are three notable benefits in using this aerostat: First, the flight altitude of the aerostat can reach at least 18 km, above most of the atmosphere that contributes to the radiances observed from satellite. Second, there is a quasi-zero wind speed layer at the height range of 18-25 km in middle-latitude areas, such as over China, at the end of spring and in summer, so it is easier to have longer observations with the aerostat. Third, the onboard sensors can be recovered after a flight, so that high accuracy calibrations of sensors are feasible before and after an aerostat flight.  
Considering the state of the art of aerostats, a high altitude balloon was chosen as the demonstration platform because it provides capacity for heavy payloads. However, the motion of the aerostat by the wind will affect spatial and temporal data matching between the aerostat and satellite. Therefore, before launching an aerostat flight mission, historical wind field data and the previous two days' radiosonde data will be collected to anticipate the balloon flight path, in order to choose the proper flight time and plan for an optimized flight path. During the flight, the flight control unit of the balloon can update the flight path, based on real-time wind fields, and precisely forecast the short-term trajectory and determine fall point of the payload cabin. Due to atmospheric friction, the payload cabin will rotate around the linkage cable. Therefore, azimuth of the payload is controlled with a flywheel alleviating balloon rotation. Precise calculation of sensor position and attitude will be realized through the onboard high accuracy position and orientation system (POS).  
Currently, there are two reference sensors considered for the balloon test flights. The first is a simulator of the space-based benchmark spectroradiometer, covering a spectral range from 400 to 2500 nm with a spectral resolution of 3.5 nm@(400-1000 nm), 10 nm@(1000-1700 nm), 12 nm@(1700-2500 nm) and a maximum FOV of 10\({}^{\circ}\). The second is a thermal sensor covering the same spectral range and a similar spectral resolution as the future space-based MIR-TIR benchmark sensor. Two blackbodies (BB), i.e., a high temperature (30 \({}^{\circ}\)C) BB and low temperature (10 \({}^{\circ}\)C) BB, are used to perform onboard calibration. The sensors are installed in a special chamber with high shock resistance and they are operated with active temperature control to maintain 20 \({}^{\circ}\)C.  
At least two stratospheric balloon-based test flights will be performed in the demonstration phase. During each flight, there will be several flight phases of the aerostat: (i) launch, (ii) ascend,  
Figure 11: Intercalibration transfer mode using vicarious reference targets.  
(iii) observation, and (iv) descend (Figure 12). The total duration time is about 8 h, ensuring that there are at least 6 h for observations. Considering the narrow swath of balloon-based reference sensors, the balloon needs to observe uniform targets in order to alleviate influences of different resolutions of the to be calibrated satellite instruments. The Golmud area (94.9\({}^{\circ}\)E, 36.4\({}^{\circ}\)N) and Qinghai Lake (99.4\({}^{\circ}\)E, 36.3\({}^{\circ}\)N) are considered as candidate calibration sites. In the experiment, the Chinese high resolution GF series and the ZY series satellites, the FY series satellites, as well as Landsat-8, Sentinel-2, and NPP will be considered as satellites for which intercalibration will be demonstrated.