Characterisation and performance verification results of the earthcare multi spectral imager VNS camera

The European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) are co-operating to develop the EarthCARE satellite mission with the fundamental objective of improving the understanding of the processes involving clouds, aerosols and radiation in the Earth's atmosphere. A Cloud Profiling Radar (CPR), an Atmospheric LIDAR (ATLID), a Broadband Radiometer (BBR) and a Multi-Spectral Imager (MSI) constitute the payload complement of the EarthCARE satellite. The four instruments will provide synergistic data on cloud and aerosol vertical structure, horizontal cloud structure and radiant flux from sub-satellite cells. By acquiring images of the clouds and aerosol distribution, the MSI instrument will provide important contextual information in support of the radar and LIDAR data processing. The MSI instrument itself consists of two camera units, the Thermal Infrared (TIR) camera and the Visible, Near-Infrared and Shortwave Infrared (VNS) camera, that are readout through a shared Front-End-Electronics (FEE) unit, all controlled by the Instrument Control unit (ICU). The subject of this paper is the characterisation and performance verification results of the TNO designed and built Proto Flight Model (PFM) VNS camera in conjunction with the SSTL designed and built PFM FEE unit. This paper presents an overview of the characterisation and performance verification philosophy, followed by a more detailed presentation of several important measurements sets highlighted below. Optical quality measurements (Modulation Transfer Function) In order to measure the MTF of the VNS camera for several spatial frequencies simultaneously, a dedicated laboratory setup was built that provided the camera with block illumination patterns. Using Fourier analysis these optical block functions could be separated into their higher order components, resulting in acquisition of the MTF performance for several spatial frequencies concurrently. Spectral Response measurements For the VNS camera the spectral response was measured from 300nm up to 2400nm over the entire instrument swath of 360pixels. In order to perform this in an efficient manner a lock-in amplification setup was devised that included a "high" power pulsed tunable laser source, integrating spheres and monitoring detector. In order to control pulse to pulse variations of the laser source and have a correct background correction, the 1kHz pulse frequency of the laser was further modulated by a several Hz chopper, resulting in spectral measurements with ∼1% accuracy. Straylight measurements The straylight requirements for the VNS camera were specified as the maximum allowable amount of signal in an infinite dark area when illuminating the VNS camera with semi-infinite light source in an adjacent area. A dedicated tool was developed to simulate these (semi) infinite areas. Radiometric characterization For the VNS camera the required absolute radiometric accuracy was quite relaxed, 10% (5% goal). However, the interchannel radiometric accuracy between the VNS channels is required to be better than 1%. This last requirement could not be achieved by "standard" radiometric calibration methods and a calibration method was developed using the VNS camera itself in collaboration with an integrating sphere that was used in radiance and irradiance modes.After finalisation of the performance testing and calibration measurements the VNS camera was delivered to SSTL mid 2017 for further integration on the MSI Optical Bench Module and alignment with the TIR camera and other MSI subsystems by SSTL.