Advancements in space reflector technology promise exciting possibilities. How do they impact future space missions?
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Overcoming challenges in space reflector technology

Space reflector technology advancements offer exciting prospects, potentially revolutionising future space missions. How do they impact future space missions?

Large transformable space-based reflectors hold promise for space exploration and expanding our understanding of the universe. They serve various purposes:

  1. Capturing and concentrating solar energy to power spacecraft and stations.
  2. Reflecting or focusing light to generate high-power laser beams for long-distance communication or scientific research.
  3. Creating giant mirrors for telescopes, enhancing resolution and observation capabilities.
  4. Generating artificial gravity by altering spacecraft trajectories.
  5. Shielding spacecraft from micrometeorites and space debris.
  6. Converting solar energy into electricity.

However, implementing these reflectors poses significant technical challenges. Ensuring stability in zero gravity and cosmic radiation is paramount. Developing efficient, reliable systems for transformation is crucial, considering factors such as temperature and their impact on the reflective surface’s shape.

In outer space, it is necessary to take into account many different factors, such as large temperature differences, radiation, lack of gravity, etc. All this leads to solving complex technical problems.

Fedor Mitin

The appearance of a large-sized transformable reflector

Various space reflectors exist, broadly categorized into cable-stayed, umbrella, truss, and inflatable designs. Each type has unique characteristics in production, deployment, transportation to orbit, operation, and control.

To address the need for a large aperture while keeping structural weight limited and volume minimal during launch, transformable antennas have been developed. Examples include the AstroMesh ring antenna, the JAXA Engineering Test Satellite antenna, and the TerreStar and Skyterra antennas.

A large-size space-based transformable reflector (LSTR) (refer to Figure 1) utilizing a cable-stayed system comprises several components: a spacecraft (1) serving as the deployment reference, solar arrays (2) supplying energy to the setup, and a system (3) for illuminating the reflective surface. Additionally, the reflector includes a rod (4) extending the reflector (5) to the necessary distance and a net (6) shaping the required radiation pattern. Engines (7) positioned at points A, B, and C facilitate the transition of the LSTR from its folded transport configuration to the deployed standard position.

Figure 1. LSTR with the use of a cable system for maintaining the shape of the reflector
Credit. Author

In general, the task of LSTR deployment at each stage is solved by influencing the design of actuators.

Challenges in optimizing the net of the reflector setup process

The novelty of the development lies in the selection of the optimal control algorithm and the optimization of the configuration process. Structural elements of large-sized spacecraft are attenuated for a long time. For instance, reflectors with significant diameters experience long-term damping oscillations even six months after deployment in orbit, rendering the satellite inoperable. Rods, spokes, and nets oscillate due to internal dissipative forces, necessitating a hiatus in usage for up to six months.

Satellites are subject to external disturbances, in particular, radiation and temperature changes. The regulated operation of the reflector takes up to 15 years. Therefore, six months of fluctuations is a significant time interval that needs to be reduced.

This prompts consideration of the real-time application of optimal control algorithms. The primary criterion is minimizing fluctuations. Special mathematical models and algorithms have been developed to facilitate real-time deployment with minimal oscillation, reducing the transition to the operating position from six months to one month. Consequently, the reflector can begin functioning six times faster.

Large antennas can’t be made rigid. The reflective netting, crucial for focusing signals, is mesh-based.  Impact on one point initiates oscillations throughout the net-like waves, necessitating shape retention.

Minimizing energy costs is crucial. Operating in outer space where solar panels solely supply energy, resources are limited. Besides vibration reduction, minimizing energy expenditure is imperative.

Figure 2. An example of a net-adjusting control system
Credit. Author

Formulation of the problem of adjusting the shape of the radio-reflective surface

Ensuring the specified characteristics of the antenna, such as the radiation pattern, and the width of the main lobe, is directly related to maintaining the specified shape of the radio-reflecting net. Even a slight deviation of the active surface from the intended shape significantly affects antenna performance.

The task of controlling the shape of the active part of the reflector in real-time is urgent to ensure the required characteristics of the spacecraft antenna under conditions of uncertainty during operation. At the same time, the design of the control system and control algorithms must ensure the minimization of oscillations of the net and energy efficiency due to the limited energy reserves on the spacecraft.

Figure 3 illustrates the spoke (1), the two main radial cords of the front (2) and back (3) nets, and the cables (4) connecting them. At the spoke’s right end, it splits into two parts to define the desired shape of the radio-reflective surface. Each cable (4) contains an actuator (5), typically a DC motor, for this purpose.

It is necessary to adjust the length of the cables (4) with the help of actuators (5) and set the shape of the radio-reflecting net cloth determined by the front network (2) while minimizing energy consumption.

Figure 3. Diagram of the spoke of a space-based reflector
Credit. Author

Optimizing the real-time control process of DC motors poses challenges, primarily stemming from the complexity of ensuring the convergence of solutions arising from the maximum principle of two-point boundary value problems. Consequently, developing diverse optimal control algorithms is imperative to attain a dependable solution.

Testing of the proposed algorithms on a prototype

To validate all algorithms and design solutions, prototypes (refer to Fig. 4, 5) are essential for refining the proposed solutions.

Figure 4. Layout of the net adjusting system
Credit. Author
Figure 5. Layout of the net adjusting system
Credit. Author

The use of the developed optimal control algorithm made it possible to ensure the successful completion of the task without difficulties in convergence, unlike classical methods.

In this instance, employing an optimal regulator reduces electrical costs by approximately 13% compared to current approaches. It ensures the necessary accuracy and quality of system regulation across a broad spectrum of terminal conditions.

Accounting for disturbances and sensor noises affecting the system under study presents challenges in processing measurement results and planning. The explored algorithm can be integrated into more intricate solutions involving measurement processing and optimization of observation intervals.

These research findings hold significance in calculating energy costs and selecting actuators and power systems for space-based reflectors.

Ongoing research in this domain continues to advance.

Developing large-sized transformable reflectors is a very technologically advanced and expensive production. Creating these structures is possible for large corporations such as NASA, Roscosmos, CNSA, ESA. Competition creates good trends for the development of the industry. However, for a significant breakthrough, it is necessary to organize the exchange of experiences and international conferences with the participation of all leading companies. This will allow us to solve more global problems in space exploration. An example of such cooperation is the International Space Station. Space exploration is the task of all of humanity.

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Journal reference

Mitin, F. V., & Krivushov, A. I. (2023). Improving the efficiency of DC motor control to adjust the reflective surface of the space-based reflector. Russian Aeronautics66(1), 162-169. https://doi.org/10.3103/S1068799823010221

Fedor Mitin is an Associate Professor of Control Systems and Information Processing at the Baltic State Technical University "Voenmeh". He holds a Ph.D. and an engineering degree. Dr. Mitin's research focuses on the control of technical systems, with particular emphasis on studying and implementing optimal control algorithms for unmanned aerial vehicles, land and underwater mobile objects, and spacecraft. His research has been published in journals such as Acta Astronautica and Russian Aeronautics, as well as other leading journals in the field of space engineering. The results of his research have been utilized in the development and operation of existing satellites.