Growing interest in free-space optical communication, due to the high bandwidth and security provided by these links, has generated the necessity of designing high-performance satellite terminals. In order to develop these terminals, the opto-thermo-mechanical phenomena that appear in the space environment and their effect on optical communication links have to be understood in detail. A review of the opto-thermo-mechanical phenomena occurring in spaceborne terminals is presented, describing the relevance of each of them. The methods found to compute the impact on the communication performance due to opto-thermo-mechanical phenomena are collected by building the bridge between the optical and communication performance parameters. Finally, techniques available to mitigate the detrimental effects of these phenomena are classified, and the relevant research challenges are identified.@en

This paper shows the roadmap of the development of a deployable space telescope at Delft University of Technology and explores its applications, key innovations and the design of a small demonstrator. Deployable space telescopes allow for smaller stowed dimensions during launch and it is expected that increasing demand for Earth observation at a high temporal resolution enables large constellations for which the reduction in launch cost per telescope will eventually break even with the required investment. The long term development of deployable space telescopes at TU Delft foresees a primary mirror of up to 1.5 m and the measurements in the visual spectrum, the thermal infrared spectrum and specific spectral lines for trace gasses. The near term objective is to develop a small demonstrator performing wide-band monitoring in the thermal infrared spectrum. This demonstrator comprises of a fixed 30 cm primary mirror and a deployed secondary mirror and baffle. The stowed instrument has roughly half the volume of its deployed configuration. Currently, critical technologies are being designed and proto-typed. The M2 is deployed and suspended by three carbon-fiber booms with custom-designed hinges and actuators at its root. Different configurations have been tested on their correct and accurate deployment.@en

Piezoelectric actuators are a very attractive technology for active optics mechanisms in space applications due to their very high precision and reliability. However, self-heating of these actuators may be limit their use in space or under high loads, due to the limited ability to evacuate heat. Test procedures that reproduce the operational conditions of these actuators are important to check these operational limits. Here an effort to characterise the thermal emission of pre-stressed piezoelectric actuators in operation is presented. The technique allows direct measurement of the power dissipated by the test item via the control of the different heat transfer mechanisms, using the fall in power provided as measure of power dissipated by the actuators, instead of relying on direct temperature sensors. This allows the construction of a thermal model with a dissipation term readily integrated in system-level modelling to account for the dissipated heat of the piezo. The technique may also be applied to other piezo low power applications in the order of 1 W of thermal emission, and is adaptable to emulate the boundary conditions encountered in operations.

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Pose estimation in robotics is often achieved using images from known and purposefully applied markers or fiducials taken by a monocular camera. This low-cost system architecture can provide accurate and precise pose estimation measurements. However, to prevent the restriction of robotic movement and occlusions of features, the fiducial markers are often planar. While numerous planar fiducials exist, the performance of these markers suffers from pose ambiguities and loss of precision under frontal observations. These issues are most prevalent in systems with less-than-ideal specifications such as low-resolution detectors, low field of view optics, far-range measurements etc. To mitigate these issues, encoding markers have been proposed in literature. These markers encode an extra dimension of information in the signal between marker and sensor, thus increasing the robustness of the pose solution. In this work, we provide a survey of these encoding markers and show that existing solutions are complex, require optical elements and are not scalable. Therefore, we present a novel encoding element based on the compound eye of insects such as the Mantis. The encoding element encodes a virtual point in space in its signal without the use of optical elements. The features provided by the encoding element are mathematically equivalent to those of a protrusion. Where existing encoding fiducials require custom software, the projected virtual point can be used with standard pose solving algorithms. The encoding element is simple, can be produced using a consumer 3D printer and is fully scalable. The end-to-end implementation of the encoding element proposed in this work significantly increases the pose estimation performance of existing planar fiducials, enabling robust pose estimation for robotic systems.@en

Laser satellite communications provides an attractive application for optical communications. It provides more bandwidth with improved security at potentially a lower cost due to reduced size weight and power (SWAP) and reduced license cost compared to radio frequencies (RF) communications. These advantages have been recognized for years and lead to the successful European data relay service (EDRS) [1]. This service provides the proof that optical communications can be used as a reliable means of satellite communications. In addition, the inherently save quantum key distribution (QKD) technology is demonstrated in a satellite to ground link in [2]...@en

A 6U CubeSat for Earth observation in 230–350 km orbits with sub-meter resolution is presented. The proposed Stable and Highly Accurate Pointing Earth-Imager (SHAPE) system’s attitude determination and control system (ADCS) is composed of a single momentum bias wheel with magnetic bearings at rotational speeds of 6000–7000 rpm and refined magnetorquers. Reaction wheels as instability source are absent. The ADCS stabilizes the spacecraft attitude by counteracting the torques from external disturbances in the thermosphere down to < 1° pointing accuracy and < 0.1° instability. The momentum wheel was sized to an angular momentum of 1 Nms based on the worst-case atmospheric density of the next solar cycle. The 0.5 Am² magnetorquer dipole moment provides with low power consumption, mass and cost, high reliability and sufficient torque. The ADCS initialisation study revealed three stable start-up modes, while the all-spun state is achieved using a set of thrusters. De-tumbling analysis show that the magnetorquers reduce the tumbling rates with magnitudes of up to 35°/s to mean motion values in less than an orbit using a static gain B-dot controller. A 3U camera design capable of sub-meter spatial resolution at 230 km altitude is presented which complies with the SHAPE spacecraft system design. The instrument has a single deployable primary mirror enabled by a deployment hinge design with hysteresis < 0.5 μ. This payload combined with air-breathing electric propulsion technology at 230 km nominal altitude boosts the SHAPE system Earth observation potential down to sub-meter spatial resolution and enables tuning of the mission lifetime by orbit keeping.

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Deployable optics promise a revolution in the capability of observing the universe by delivering drastically reduced mass and volume needs for a desired level of performance compared to their conventional counterparts. However, this places new demands on the mechanical and thermal designs of new telescopes, essentially trading mass and volume for structural and control complexity. We compile the thermomechanical challenges that should be taken into consideration when designing optical space systems, as well as summarize 14 projects proposed to address them. Stringent deployment repeatability requirements demand low hysteresis, whereas stability requirements require high stiffness, proper thermal management, and active optics.

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Quantum key distribution from satellites becomes particularly valuable when it can be used on a large network and on-demand to provide a symmetric encryption key to any two nodes. A constellation model is described which enables QKD-derived encryption keys to be established between any two ground stations with low latency. This is achieved through the use of low earth orbit trusted-node QKD satellites which create a buffer of keys with the ground stations they pass over, and geostationary relay satellites to transfer secure combinations of the keys to the ground stations. Regional and global network models are considered and the use of inter-satellite QKD links for balancing keys is assessed.

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The following research topics, running at the Delft University of Technology, aim to increase the spatial resolution of Earth Observation systems: 1. Small instruments and sensor development for PocketQubes and CubeSats 2. A Stable Highly Accurate Pointing EO (SHAPE) 6U spacecraft 3. The Stratospheric balloon project Stratocruiser 4. The Delft Deployable Space Telescope (DST) project The focus of this paper is topic 4 which is driven by the need for cheaper, lighter and smaller telescopes imposed by the on-going trend to deliver more refined Earth observation data at a lower price. Evident reasons to incorporate deployable telescope structures are firstly to fit in a launcher and secondly to decrease launch mass and volume. The Deployable Space Telescope (DST), being developed at the Delft University of Technology, aims to reduce volume (> 4 times) and mass (< 100 kg) by using innovative deployable optics. The WorldView-4 satellite was chosen as benchmark for its development. The DST overall systems design is driven by a strict bottom-up versus top-down systems engineering approach. The coarse alignment budget is treated as a one-off deployment precision performance, with the drift and stability budgets as low and high frequency stability margins. Most critical subsystem is the first DST mirror M1: its position has to be accurate up to 2 µm in all directions whilst the tilts shall be within 2 µrad. In orbit the dynamic thermo-mechanical conditions require these parameters to be within 5 nm position and 10 nrad as stability budget. The M1 calibration and actuation is controlled by a wave front error algorithm. The novel actuation system, implementing the active optics strategy, is mounted on the mirror support structure. The allowable deployment errors in tip, tilt and piston are 16, 10 and 13 µm whilst the actuation precision is 51, 32 and 10 nm. To support these critical budgets the development and testing of the first key DST hardware comprises a 3D printed COmpliant Rolling contact Element (CORE) hinge design. Hinges of this design were not applied in space so far. A good mechanical hinge design for high-precision deployment is identically one that exhibits low-hysteresis response to load cycling. The hysteresis was tested by a technique called Digital Image Correlation, which is normally used to detect micro-cracks in composite layers. The test setup proved to be very suitable for the hysteresis characterisation with a precision down to 100 nm. The maximum hysteresis found was 0.3 µm for over 50 load cycles. The CORE hinge design is currently tested for hysteresis response and thermal gradient behaviour. This paper describes the status of the optical, thermo-mechanical and active optics systems design. The concurrent design approach combined with a strict bottom-up and top-down compliant systems engineering approach shows that the DST is a healthy system concept.

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The increase of spatial and temporal resolution for Earth observation (EO) is the ultimate driver for science and societal applications. However, the state-of-the-art EO missions like DigitalGlobe's Worldview-3, are very costly. Moreover, this system has a high mass of 2800 kg and limited swath width of about 15 km which limits the temporal resolution. In this article, we present the status of the deployable space telescope (DST) project, which has been running for 6 years now at the Delft University of Technology, as a cutting-edge solution to solve this issue. Deployable optics have the potential of revolutionising the field of high resolution EO. By splitting up the primary mirror (M1) of a telescope into deployable segments and placing the secondary mirror (M2) on a deployable boom, the launch volume of a telescope can be reduced by a factor of 4 or more, allowing for much lower launch costs. This allows for larger EO constellations, providing image data with a much better revisit time than existing solutions. The DST specification baseline, based on Wordview-3, aims to provide images with a ground resolution of 25 cm (panchromatic 450-650 nm) from an orbital altitude of 500 km. In this paper, the current status of the optical, thermo-mechanical, and active optics systems design are described. The concurrent design approach combined with a strict bottom-up and top-down compliant systems engineering approach show that the DST is a healthy system concept.

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Quantum communication is a prime space technology application and offers near-term possibilities for long-distance quantum key distribution (QKD) and experimental tests of quantum entanglement. However, there exists considerable developmental risks and subsequent costs and time required to raise the technological readiness level of terrestrial quantum technologies and to adapt them for space operations. The small-space revolution is a promising route by which synergistic advances in miniaturization of both satellite systems and quantum technologies can be combined to leap-frog conventional space systems development. Here, we outline a recent proposal to perform orbit-to-ground transmission of entanglement and QKD using a CubeSat platform deployed from the International Space Station (ISS). This ambitious mission exploits advances in nanosatellite attitude determination and control systems (ADCS), miniaturised target acquisition and tracking sensors, compact and robust sources of single and entangled photons, and high-speed classical communications systems, all to be incorporated within a 10 kg 6 litre mass-volume envelope. The CubeSat Quantum Communications Mission (CQuCoM) would be a pathfinder for advanced nanosatellite payloads and operations, and would establish the basis for a constellation of low-Earth orbit trusted-nodes for QKD service provision.

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We provide a proof of the technical feasibility of LOUPE, the first integral-field snapshot spectropolarimeter, designed to monitor the reflected flux and polarization spectrum of Earth. These are to be used as benchmark data for the retrieval of biomarkers and atmospheric and surface characteristics from future direct observations of exoplanets. We perform a design tradeoff for an implementation in which LOUPE performs snapshot integral-field spectropolarimetry at visible wavelengths. We used off-the-shelf optics to construct a polarization modulator, in which polarization information is encoded into the spectrum as a wavelength-dependent modulation, while spatial resolution is maintained using a micro-lens array. The performance of this design concept is validated in a laboratory setup. Our proof-of-concept is capable of measuring a grid of 50 x 50 polarization spectra between 610 and 780 nm of a mock target planet - proving the merit of this design. The measurements are affected by systematic noise on the percent level, and we discuss how to mitigate this in future iterations. We conclude that LOUPE can be small and robust while meeting the science goals of this particular space application, and note the many potential applications that may benefit from our concept for doing snapshot integral-field spectropolarimetry.

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