Lead Organisation: University College London
Project Lead: Michael Emes
Partners: University of Oxford, The Open University, University of Leeds, RAL Space, STAR Dundee
The LOCUS mission concept proposes to perform atmospheric limb sounding of the Earth’s Mesosphere and Lower Thermosphere (hereafter MLT) at both THz frequencies and infra-red (IR) wavelengths. The scientific goals of the LOCUS mission payload can be briefly summarized as the direct correlation between THz and IR measurements of atomic and molecular species such as O, O3, OH and O2 in the MLT region, and Investigating the chemistry and formation mechanisms of Noctilucent clouds and the effect of the Sun’s CME (Coronal Mass Ejection) events and their forced auroral events on the chemical processes and energy particle precipitation.
A previous IOD study identified a standard micro-satellite SSTL-150 (~150kg category) with a proven success track-record of space deployment as a suitable platform. Selection of this satellite structure defined the volume, mass and power constraints for the breadboard development. These constraints form the core of the compact optical design and breadboard architecture as well as the power provision for operation of the mechanical cryocooler required to cool the THz receivers, digital spectrometers and other housing keeping electrical systems.
The LOCUS mission identifies 4 THz bands (0.8, 1.15, 3.5 & 4.7 THz) for the measurement of specific molecular and atomic transitions. Correspondingly, narrow IR photometric bands (2.1, 5.3, 9.3 & 15 μm) related to transitions of the same (or ancillary) chemical species. This technology demonstration activity has focused on one THz and one IR band pertaining to the same molecular species, namely NO (Nitric Oxide). This identifies the 1.15 THz band and the 5.3 μm IR band as those at which testing of the breadboard would occur.
The “Elegant Breadboard” CEOI activity has combined a number of previously funded and ongoing technology developments such as the THz receiver (under the CEOI 7th call) as well as a previous incarnation of the Wide-Band Spectrometer into a single platform (the breadboard) aimed at raising the TRL of such elements while constructing a laboratory breadboard demonstrator. An ESA IOD (In-Orbit Demonstrator) mission study was completed in 2014 ahead of this activity and laid out the requirements for a small Low Earth Orbit satellite and its payload to deliver the science objectives related to the understanding of the Mesosphere-Low Thermosphere and the measurement of key atmospheric species via THz spectroscopy measurements of atomic and molecular transitions.
The innovations demonstrated through this activity are:
- The use of QCLs (Quantum Cascade Lasers) and Waveguide integrated Schottky Diodes as the front end and mixer of THz receivers to reduce the footprint of the receivers as well as increasing their reach into previously unreachable THz regions;
- Very wide-band digital spectrometers allowing recovery of multiple spectral transitions within a single instrument band in a low mass and power-limited environment;
- The design and build of an all-aluminium radiometer optical train and support framework (breadboard) compliant with use at both submm-wave (THz) and IR wavelengths. A key objective was to minimise the breadboard size to allow the overall payload to be accommodated on a small LEO platform keeping in mind thermal constraints of stability that are imposed on the payload due to the LEO environment.
The above innovations have been achieved during this project, with the design and manufacture of a compact telescope that has been integrated with a baseplate and forming the optical breadboard. The telescope meets the required mission parameters and has been manufactured entirely in aluminium with surface quality sufficient for operation across the required THz and mid-IR range. The breadboard includes thermal emission panels that have been designed and built to maximize thermal stability, and novel miniature mechanical cryocoolers.
The team performed thermal testing of the optical breadboard and a pair of operational cryocoolers (the development of which lies outside of this activity) and demonstrated an ambient temperature <100K. The breadboard optical beam quality was first assessed in the THz range by illuminating the telescope with a 3.5THz signal derived from a QCL source mounted at the optical focus and inside a large mechanical cooler and scanning the emerging beam with a sensitive THz detector. The scanning system was especially developed for the project and allowed the measurement of antenna patters from individual QCL devices and the integrated optical assembly. Optical performance in the mid-IR was assessed by mounting an IR detector at the telescope focus and scanning an IR source across the primary entrance plane.
A 1.15THz receiver developed via a previous CEOI 7th Call has demonstrated mixing and a spectral signature has been detected. The presence of water vapour presented significant attenuation along the THz signal path and installation of a chamber that replaced air with nitrogen gas was necessary. The receiver was also integrated with a large-scale cryocooler and a more extensive chamber encompassing the whole breadboard assembly, including the antenna scanner.
The end of this activity sees the development of an instrument assembly (in a dedicated lab) which in principle could be installed on a remote location or flown on a plane or a balloon to perform remote measurements as a demonstrator for a future satellite Flight Model. LOCUS is being proposed as an EE-10 (ESA Earth Explorer 10 call for mission ideas) candidate mission, and this work supports de-risking some of the instrument design.