Introduction
The GHOST project was proposed by a team of six Masters students studying Aerospace Engineering at the University of Southampton. GHOST is a spacecraft payload that aims to enhance non-Earth imaging (NEI) of geostationary orbit (GEO) satellites - addressing the need for in-orbit space domain awareness (SDA) technology in this particular region.
Optical sensors found onboard spacecraft are typically designed for extremely high ranges, but this compromises their field of view. For example, the Holmes sensor from HEO Robotics has a resolution of 1.7 m for objects 500 km away from the sensor, but only a 0.4 degree FOV. As a result, these sensitive instruments demand high precision from their host spacecraft's attitude control system to point the sensor in the correct direction. This costs the spacecraft precious power and/or propellant, disrupts the primary mission, and introduces more complexity to the spacecraft.
The HEO Holmes Imager (Source: HEO Robotics)
GHOST intends to relieve the host spacecraft of this burden by shifting the viewing direction of the sensor within the spacecraft - eliminating the need for spacecraft reorientation and allowing the spacecraft's primary mission to continue uninterrupted.
Project Overview
Through collaboration with HEO Robotics, the GHOST payload intends to prove that it can increase the viewing capacity of their Holmes sensor, all within the volume envelope of their larger Adler sensor (480 mm x 220 mm x 224 mm). Only a proof of concept is required for this university group project spanning a single academic year (October 2024 to May 2025). The next step after the university project is revising the prototype into a space-hardened system.
Initial designs involved moving the sensor inside the spacecraft to increase its viewing capacity. However, these proposals were unfeasible due to the limited space available within the Adler volume. Moving the sensor would also apply large disturbance torques to the host spacecraft, disturbing its attitude. These would have to be counteracted using the spacecraft's ACS, defeating the purpose of the payload to reduce the strain on this subsystem.
Instead, the team used an innovation from the viewing direction control mechanism onboard ESA's SPOT-4 satellite. The final design uses two mirrors driven by motors to rotate about horizontal and vertical axes. The mirrors reflect light from the target into the sensor's lens, and as the target moves, the mirrors rotate accordingly to continue capturing the target's light. The mirrors have a much lower mass than the sensor, which results in much lower disturbance torques at a given angular velocity, compared to rotating the sensor itself.
Development Process
In order to evaluate GHOST's ability to increase an optical sensor like Holmes, a commercial off-the-shelf telescope was required with smaller dimensions than the Holmes sensor. A compatible electronic eyepiece was also required to capture images from the telescope. Consequently, the group selected the Celestron C70 Mini Mak spotting scope and the SVBONY SV205 eyepiece. The mount for this telescope (shown below) was designed with the same length, width and height of the Holmes sensor - proving that GHOST could contain the Holmes sensor inside the Adler volume. Fasteners clamp opposing halves of the mount onto the telescope.
Choosing motors to rotate the mirrors was a particularly important decision. Their accuracy and precision solely dictates the entire payload's performance. In addition to this, the motors were subject to many constraints, such as size, cost, power rating, and step size (the angle between motor position increments). After speaking with a number of motor suppliers, the group chose maxon for their drive system. They also supplied appropriate encoders and electronic controller boards, and offered additional support through their Young Engineers Program.
Finally, a testing rig was needed to integrate all the system components. It also had to be designed and manufactured with the same length, width and height of the Adler sensor, to prove that GHOST can be contained within this volume. The team was very fortunate to receive the RS Student Fund, and a testing rig was designed using parts from their comprehensive RS PRO Structural Systems Guide.
An assembly of aluminium profile struts (850-8476) , aluminium plates (018-7328) and angle brackets (180-9138) can be seen in the image above. Other miscellaneous items were also ordered with the funding, such as a Raspberry Pi 5 (021-9255) to remotely control the motors, as well as various fasteners.
Project Timeline
Describe the road to delivering your event/outreach/competition, from
start to finish.
- October 2024: Beginning of the academic year and the group project. Began defining the target customer, mission requirements, and initial design proposals.
- November 2024: Reiterating design, selected telescope and eyepiece, won the RS Student Fund!
- December 2024: Selected motor supplier, finalised design.
- January 2025: Ordered motors from maxon. Attended the Telespazio T-TeC Award Ceremony at the 17th European Space Conference in Brussels, Belgium, won a pre-incubation programme from cesah GmbH. Only university group from the UK to receive an award!
- February 2025: Received testing rig parts and other items from RS Components, assembled and connected electronics and hardware.
- March 2025: Received machined aluminium plate and maxon motors, began testing.
Challenges and Solutions
maxon motors are based in Switzerland, which lead to a standard delivery of 11 working days. However, our motor configuration was not readily available and could not be shipped immediately after order. Therefore, a delay of almost two months followed. During this time, there was also a two-week delay for the university's waterjet cutter, which was required to precisely machine the aluminium plates from RS. Despite this, the team remained productive and progressed with all other aspects of the system. For example, the electronics team finished soldering and connecting the other electronics components - allowing the motors to be added to the system as soon as they arrived.
Our project supervisors also warned us of the system's susceptibility to vibrations during testing. If the mirrors or telescope move too much, the light from the target will scatter across the telescope lens, resulting in a blurry image. A dimly lit target may not be captured at all. Therefore, the design team took measures to minimise the possibility of this scenario. Anti-vibration rubber pads (019-61315) were supplied by RS and added to interfaces between the mounts and the testing rig, as shown below. These pads heavily damped any vibrations applied to the telescope and mirrors, ensuring the telescope would provide a clearer, sharper image. They were also added between opposing halves of each mount to provide a secure clamping fit around the telescope and motors.
Results and Impact
With our system fully assembled, the team has just begun testing. Our first test assesses the payload's ability to visualise targets in space using the mirrors. A simulation was constructed in Systems Toolkit to provide a realistic view of the host spacecraft's view (seen below). Many more tests are soon to commence, such as air bearing table tests to evaluate the motors' impact on the spacecraft's attitude, and shaker table tests to learn how microvibrations affect the system's ability to return a clear image.
Conclusion
The team has thoroughly enjoyed working with each other to create GHOST. They hope to further develop the project, as they strongly believe a space-hardened payload can significantly benefit spacecraft working towards a safer space environment. Regardless, the team would not have accomplished as much without the support from RS Components - to whom they are deeply grateful.
The GHOST team, from left to right: Angelika Kochajkiewicz, Tom Love, Harry Johnson, Gianluca Borgo, Georgia Skelton, and Kian Patel (yours truly).
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