The Vegetation Instrument is composed of three individual Three Mirror Anastigmat telescopes. The manufacturability and testability of the TMA mirrors and the structure as well as the possibility to align the mirrors within the optical tolerances to meet the stringent image quality requirements was demonstrated in a separate ESA GSTP project (development of a compact wide field of Three Mirror Anastigmatic telescope). These TMA’s are completely built from aluminium, structure and mirrors included. This way, thermal gradients and resulting optical deformation can be reduced heavily.
The three TMA’s are equipped with VNIR and SWIR sensors to image the earth in a multispectral way. The SWIR detector is developed in a separate ESA GSTP project, which demonstrated the development of a mechanically butted detector composed of three 1024 InGaAs pixel arrays.
The three TMA are mounted on monolithic aluminium Optical Base Plate. Additionally, the two star trackers are also mounted on the same optical bench. This architecture was needed to limit as much as possible the thermo-elastic deformations between the imaging optics and the star trackers. Apart from the FPA Front End Electronics, which is located near the two detectors in each TMA, all electronics is separate from the Optical Base Plate. The read-out electronics (ROE) for the detectors is mounted through a flex to the PCB’s underneath the satellite platform panel. A Data Handling Unit (DHU), which is the central data processing unit and controls the three different ROE circuits, is mounted on the top side of the satellite panel. The Power Supply Unit (PSU) is also mounted on top of the satellite panel, separated from the optomechanics. Quasi isostatic mounting feet are supporting the optomechanics to isolate as much as possible the relative deformations between the instrument and the platform. Due to the limited amount of power available to drive all sub-units, an active thermal control cannot be accommodated. Instead, all thermal control is performed passively by a radiator connected to the optomechanics in front of the instrument by means of copper thermal straps. As the instrument is facing nadir, i.e. the surface of the earth, the radiator is also facing nadir. This is different from a conventional radiator which is normally facing deep space.
The optomechanics and the radiator is completely thermally isolated from the instrument: the optomechanics is covered completely in MLI to isolate it radiatively from the satellite surrounding. The titanium quasi isostatic mounting feet also conductively isolate the instrument from the platform.
The instrument can operate in three different operational modes, depending on its location in the orbit.
– Survival mode
– Stand-by mode
– Imaging mode