In 2019, the European Space Agency ESA awarded the project “LEOB” (Large Deployable Reflector for Earth Observation) to a European industrial team led by HPS (D). Major contributors are LSS (D), FHP (PT), RUAG Space Germany (D), vH&S (D), INEGI (PT), TICRA (DK), and a number of suppliers. The subject of LEOB is the development and testing of an Engineering Model (EM) of an 8 m aperture-class unfurlable mesh reflector as a technology demonstration activity for the first European-developed antenna reflector of this class. Specifically, the future COPERNICUS Earth Observation mission CIMR for ocean monitoring (COPERNICUS Imaging Microwave Radiometer Mission) is planning to use this type of reflector. The industrial team working on LEOB has been confirmed by ESA in mid-2021 as the provider of the Large Deployable Reflector System (LDRS) for CIMR, being a key element of the radiometer instrument on the mission. In the consortium, LSS GmbH is the prime for the Large Deployable Reflector (LDR) itself and is infusing key heritage and expertise on unfurlable reflectors gathered over the past more than 30 years by LSS key personnel.
The LEOB EM is seen as a pathfinder for the CIMR LDR and by now also has become a critical element to demonstrate a TRL of 6 for the CIMR LDRS which is needed before CIMR can move into flight H/W development (phase C/D). Specifically, the LEOB EM has been designed to be compatible with environmental testing, therefore representing a meaningful testbed at LDR system level for justifying TRL 6 in many aspects of the design.
In orbit, the CIMR LDR will be held at the required position relative to the antenna focal plane and the satellite platform by an articulated boom which after deployment is latched in the preferred, final configuration while offering the desired stiffness. For launch, the boom is folded via its hinges held down to a satellite panel. Likewise, the LDR itself for launch is folded, forming a cylindrical package which in turn is held down by hold-down and release mechanisms. The LEOB EM design follows exactly this concept whereas of course the design details differ in several respects, even though the LEOB antenna optics are already representative of the CIMR optics. From a technological perspective – the reflector concept, innovated and developed by LSS, using a peripheral ring with a double pantograph from CFRP tubes and front and rear networks to form the RF surface with the RF reflecting material being a metallic mesh – , the LEOB LDR EM is fully representative of the CIMR LDR, including the use of materials.
The CIMR mission is associated with several unique design drivers imposed on the LDRS: 1) required frequency range going from L-band to Ka-band, i.e. necessitating exquisite surface accuracy and shape stability of the RF surface, 2) required beam spot size and beam efficiency which necessitates an RF aperture between 7 and 8 m, 3) conical scanning mode of the deployed LDRS and of the radiometer instrument front end relative to the satellite with a rotation period of around 10 seconds, requiring momentum compensation at S/C level and countermeasures inherent in the LDR design against deformation of the reflector in response to inertial loads, 4) on-orbit longevity of the LDRS, necessitating adequate resiliency of reflector parts, materials and subassemblies to maintain the desired shape of the RF surface over the CIMR mission’s planned ~7 year lifetime in orbit.
The testing program that the LEOB EM is undergoing – starting in late 2021 and spanning the period until about June 2022 – is run in a way very similar to a qualification campaign, with the associated PA and QA effort. Overall LEOB reflector development included an intense structural design and analysis effort for proper sizing against launch loads (reflector and boom being stowed and locked) and for ensuring the required stiffnesses and shape stability in the deployed configuration. Testing of the LEOB reflector EM includes (in chronological order): deployment functional tests at ambient with weight offloading along with optimization of the process of reflector folding, measurements of the shape of the RF surface after deployment through Laser Tracker and Laser Radar systems, modal tests of the deployed reflector to determine the Eigenmodes for validation of FE model predictions, vibration test of folded reflector to simulate launch loads, thermal vacuum cycle testing with primary reflector hold-down and release mechanism functional tests within the vacuum chamber, subsequent deployment at ambient, surface shape measurement and modal test, subsequent reflector folding followed by secondary reflector hold-down and release mechanism functional test inside the thermal vacuum chamber with initial motor-driven reflector deployment. Finally, a test at ambient with the reflector in “reverse” orientation (so-called “cup-down”) will be performed, to assess aspects of the influence of gravity on the RF surface: the shapes of the RF surface as measured from both reflector orientations would then be used to predict a “0-g RF surface”.
To support above described testing, various test MGSE had to be developed that allow reflector deployment with gravity compensation as well as for thermal vacuum tests, among others, and which constitute a critical task within the LEOB, and later, CIMR, projects.
In this paper, key test results from LEOB will be presented along with an outlook towards TRL 6 achievement for the LDR for CIMR and outlining the CIMR PDR design status.