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.

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.

Deployable optics can bring major cost reductions to the field of Earth Observation. One of the key challenges in the development of a deployable optical system, however, is making sure that it can meet its performance targets following its deployment. In this paper, a novel active correction system for a deployable telescope is described. The correction system co-aligns and phases the primary mirror segments and subsequently corrects remaining aberrations using a deformable mirror. A novel phasing sensor called PistonCam can bring telescope segments into phase while the telescope is staring at extended scenes. By only sampling segment boundaries, PistonCam is able to isolate piston and tip/tilt errors which allows the errors to be corrected more effectively. After phasing process has been completed, a moving scene sharpness optimization technique is used to correct the remaining aberrations with a deformable mirror, The technique does not require a constant scene, unlike existing sharpness optimization techniques. As such, the telescope does not need to track a ground scene during the correction process. The technique can also be used for continuous correction of telescope deformations. The active optics system offers robust aberration correction, is computationally inexpensive and requires limited additional optical hardware.

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.