The Multi Spectral Imager (MSI) onboard EarthCARE consists of a separately accommodated 10 kg Instrument Control Unit (ICU) and a 40 kg Optical Bench Module (OBM).

The OBM comprises two cameras, one dedicated to the solar channels and one to the Thermal Infrared (TIR) channels, both of which are mounted on the same optical bench and use a common Front End Electronics (FEE) unit. The FEE acquires and digitises the analogue signals generated by the optical detectors in response to the illumination from the Earth scene being observed. The ICU implements the routing of command and telemetry between the satellite and the instrument, as well as performing monitoring functions. It buffers data and assembles it with relevant house-keeping data into Instrument Source Packets.

TIR Camera Module Description

The TIR module uses a two-part imaging system, consisting of a telescope and a relay, to focus MSI’s three channels onto a single area array detector. A rotating mirror allows the line of sight to be redirected onto an internal, warm blackbody and to deep space to perform a calibration once per orbit.

During imaging, the Earth view is reflected by the calibration mirror onto a lens that forms the aperture. The fore optics forms an image of the scene at long focal length. The beam then enters the Filter and Dichroic Optical Components (FDOC) where it is split by two dichroic beam-splitters into three beams, which are reflected onto a common image plane and pass through their respective filters that define the response for each of the channels.

In the fore optics, temperature control in the order of 500 mK is adequate. At the detector, signal from this part of the structure and optics will be seen only in the subtended scene and via the filter transmission.

The beams enter the rear optics through three slots in a temperature-controlled plate. In the rear optics they are then reimaged onto the microbolometer array with substantial de-magnification, with the final image at approximately f/1 with a ±11.3° field of view. The rear optics are contained in a thermally isolated enclosure, controlled in temperature to better than 100 mK.

VNS Camera Module Description

The Visible/Near Infrared (NIR)/Short Wave Infrared (SWIR) (VNS) camera images light from the scene onto four different focal planes equipped with linear 512 photodiode arrays, with one focal plane for each channel. The Visible (VIS), NIR and SWIR-1 bands share a common 4.7 mm aperture, and are separated by a system of dichroics and beam splitters.

The SWIR-2 band has a larger, separate 10.4 mm aperture and the corresponding focal plane is stabilised at 235 K via a passive cooling system.

A solar viewing port allows calibration of the four channels by detecting the solar light passed through a pair of diffusers placed in front of the aperture stop. The instrument aperture can be closed to calibrate the dark signal.