EnMAP thermal cooling system kicked into the right orbit

By Dominik LANG (OHB) & Olivier BERDER (EHP)

Last news update from OHB, dated May 5, 2022:

On April 1, 2022, at 6:24 p.m. Central European Summer Time (CEST), EnMAP started its journey into space as the largest payload on a Falcon 9 rocket from the US space company SpaceX from Cape Canaveral in Florida.

EnMAP on top of the encapsulated Falcon 9’s fairing. Copyright Space-X

After acquisition of the mission reference orbit on April 9, 2022, EnMAP successfully completed its Launch and Early Orbit Phase (LEOP) on April 14, 2022.

The satellite operations are proven to be safe, the X-band downlink is verified using housekeeping data, and all launch locks are released. The activation of the instrument’s thermal control system has shown to be in particular challenging. The thermal control system for the Visible and Near InfraRed (VNIR) camera is already successfully activated.

On April 27, 2022, EnMAP captured the first light image. It recorded a strip about 30 kilometers wide and 180 kilometers long over Istanbul on the Bosporus in Turkey with Europe and Asia, and then downlinked and processed the data. The applied calibration was based on data measured by the instrument in the laboratory. These first images already give a good foretaste of what can be expected by EnMAP. The high quality in all channels is well visible on the one hand in low noise and disturbing image fringes in the extensive dynamic range, which is especially evident in dark areas like water, and on the other hand in typical spectral profiles.

The German satellite EnMAP
Credit: DLR

EnMAP mission:

The EnMAP environmental mission is managed by the German Space Agency at DLR on behalf of the German Federal Ministry for Economic Affairs and Climate Action (BMWK). OHB-System AG has been contracted to develop and build the satellite and its hyperspectral instrument. The Ground Segment is realized by the German Space Operations Center (GSOC). The mission is under the scientific management of the GeoForschungszentrum Potsdam (GFZ).

The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission to monitoring and characterise Earth’s environment on a global scale. EnMAP measures and models key dynamic processes of Earth’s ecosystems by extracting geochemical, biochemical and biophysical parameters that provide information on the status and evolution of various terrestrial and aquatic ecosystems.

The German satellite EnMAP
Credit: OHB/H. von der Fecht

The Instrument Thermal Cooling System (ITCS), EHP and Airbus D&S contribution:

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. EHP and Airbus D&S was involved on the Project in order to perform the detailed design of the Instrument Optical Unit Thermal Control SubSystem (IOU-TCSS), being part of the ITCS, based on mini loop heat pipe technology after completion of a first breadboard demonstrator, and then the manufacturing and thermal performance testing of the cooling system before delivery to OHB. 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 EnMAP HSI STDM thermal vacuum campaign has successfully demonstrated before flight the ability of the system to meet the requirements and the ITCS operational concepts necessary for implementing such a complex system.

Deployment of the radiator panels at EHP premises
Full equipped radiator panel

Few words from Dominik Lang (OHB):

MiniLHP evaporator and gold plated lines network

The Instrument Optical Unit – Thermal Control SubSystem based on miniLHP technology, developed and manufactured by Airbus D&S and EHP, implemented in a temperature controlled configuration provides enormous flexibility in terms of thermal design and allows to cope with challenging mission boundary conditions. It allows to stabilize different equipment to 21°C +/- 1°C while subsystem mode switching and orbit maneuvers are performed. From customer perspective It is highly recommended to take the system into account in un early project development phase in order to correctly implement all required system interfaces and to implement the required operational concept. OHB System AG would like to thank Airbus D&S and EHP for the great work which has been done under the contract and the cooperative collaboration the last years.      

The work presented in was performed on behalf of the German Space Agency DLR with funds of the German Federal Ministry of Economic Affairs and Technology under the grant No. 50 EP 0801

Credits:

https://www.enmap.org/

https://arc.aiaa.org/doi/10.2514/6.2013-3327

https://www.dlr.de/content/de/artikel/missionen-projekte/enmap/enmap-ueberblick-daten.html

https://www.phoenix.de/mission-enmap-a-2730959.html

https://www.dlr.de/content/de/artikel/news/2022/02/20220504_deutscher-umweltsatellit-enmap-sendet-erste-bilder.html