EnMAP Hyper Spectral Instrument Thermal Design and Test Verification

Abstract

The Environmental Mapping and Analysis Program (EnMAP) is based on a space borne hyper spectral imaging mission capable of measuring the solar radiance reflected from the Earth’s surface as a continuous spectrum over the spectral range from 420 nm up to 2450 nm from a sun-synchronous orbit. The instrument consists of a telescope coupled to two dispersive spectrometers for the visible near infrared (VNIR) and the short wave infrared (SWIR). The dispersive elements are curved glass prisms while the structure and mirror elements are made of aluminum. Two custom high performance 2-D detector arrays record the spectrally and spatially resolved signals allowing to form the hyperspectral image data sets. In addition to the typical thermal control requirements system level radiometric and spectral performance requirements in combination with operational boundary conditions are identified as major design drivers for the thermal control architecture. In nominal operational conditions the Instrument Thermal Control System (ITCS) is required to control the spatial gradients over the telescope assembly and the spectrometers to less than 2 °C for 5 years as well as stabilizing temperatures to better than ± 0.3 °C per week. The ITCS must incorporate a second cold redundant SWIR FPA including spectrometer mounted front end electronics as well as a redundant cryo-cooling system. The operations concept with frequent mode switching for data takes and very limited spacecraft power and volume resources have resulted in a sophisticated ITCS design involving extensive use of actively controlled two-phase heat transport devices. The ITCS uses a configuration of 12 loop heat pipes in controlled variable conductance mode to transport heat from the dissipating units mounted on the optical assembly to a radiator. Reservoir control heaters used as actuators in a cascade control loop architecture allow regulating the effective LHP conductance such that the equipment temperature is stabilized. Operating the reservoir control in a specific inhibition mode allows to use the LHPs as switchable thermal links in order to efficiently incorporate the redundant SWIR FPA and redundant LHPs. The optical assemblies are stabilized using a classical distributed heater concept in conjunction with an active thermal control and large area passive radiative heat disposal. The ITCS design will be presented together with the results of the EnMAP HSI STDM thermal vacuum campaign demonstrating the ability of the system to meet the requirements and the ITCS operational concepts necessary for implementing such a complex system.