## 5 Structure Design Based on Universal Joint Coupling Mechanism  
Based on the configuration presented in Section 4.4, a concrete CAD model of a Cassegrain antenna is established. According to the resulting fixed point \(P\) and axial vector \(\mathbf{v}\), the precise position of the rotation axis can be determined, as shown in Table 2, based on which the process of structure design is demonstrated as follows. **(1) Composite panel**  
Although the reflective surface of the panel can be machined into an ideal paraboloid, its high accuracy cannot be guaranteed under harsh space environments without an elaborate panel design. In space, the environment temperature approximately ranges from \(-100\,\mathrm{\SIUnitSymbolCelsius}\) to \(200\,\mathrm{\SIUnitSymbolCelsius}\); hence, the heat-resistant material is indispensable for fabricating the panel. Also, high-strength and high-stiffness materials are needed to avoid possible resonance during launch, deployment and altitude maneuver. To meet these two requirements, the carbon-fiber-reinforced plastic (CFRP) sandwich structure is adopted. As shown in Figure 10b, the two skins are made of M55J CFRP, and the honeycomb core is made of T300 CFRP. The related research has reported that this kind of sandwich structure is capable of sustaining a high surface accuracy under severe on-orbit thermal environments [64; 65; 66]. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c} \hline \multirow{2}{*}{**Antenna**} & \multicolumn{8}{c}{**Deviation Parameter \(\mathbf{k_{\mathrm{d}}}\)**} & \multicolumn{8}{c}{**Improvement Rate (\%)**} \\ \cline{2-13}  & **0** & **0.1** & **0.2** & **0.3** & **0.4** & **0.5** & **0.6** & **0.7** & **0.8** & **0.9** & **1.0** & \(\mathbf{\eta_{\mathrm{min}}}\) & \(\mathbf{\eta_{\mathrm{max}}}\) \\ \hline Dornier FIRST & 3.96 & 3.97 & 3.99 & 4.01 & 4.03 & 4.06 & 4.09 & 4.12 & 4.16 & 4.20 & 4.25 & 42.80 & 52.87 \\ Dornier MEA & 3.96 & 3.98 & 3.99 & 4.01 & 4.03 & 4.06 & 4.09 & 4.12 & 4.15 & 4.19 & 4.25 & 43.08 & 52.81 \\ RadioAstron & 4.41 & 4.42 & 4.44 & 4.46 & 4.48 & 4.51 & 4.52 & 4.56 & 4.58 & 4.62 & 4.66 & 32.82 & 39.87 \\ MilliMetron & 3.96 & 3.98 & 3.99 & 4.01 & 4.04 & 4.06 & 4.09 & 4.12 & 4.16 & 4.20 & 4.25 & 19.50 & 27.62 \\ NPSSDA & 3.94 & 3.95 & 3.97 & 4.00 & 4.02 & 4.05 & 4.07 & 4.11 & 4.14 & 4.18 & 4.23 & 25.13 & 33.92 \\ Uniaxial Model & 3.34 & 3.35 & 3.37 & 3.40 & 3.43 & 3.46 & 3.49 & 3.52 & 3.56 & 3.61 & 3.67 & 18.92 & 30.23 \\ CFRP Model & 4.87 & 4.86 & 4.88 & 4.90 & 4.92 & 4.95 & 4.97 & 5.00 & 5.03 & 5.06 & 5.10 & 29.29 & 35.62 \\ Sunflower & 3.04 & 3.06 & 3.08 & 3.10 & 3.13 & 3.17 & 3.20 & 3.23 & 3.28 & 3.33 & 3.40 & 33.46 & 48.33 \\ Improved Sunflower & 3.00 & 3.02 & 3.04 & 3.07 & 3.09 & 3.13 & 3.16 & 3.20 & 3.25 & 3.30 & 3.35 & 11.76 & 24.20 \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of RSE II. (2) Supporting framework  
To support the panels and further improve their strength and stiffness, a truss-type framework is also adopted, in which its constituent rods are made of T300 CFRP to reduce the weight, as shown in Figure 10c. In addition, diverse optimizations are performed with respect to the framework in terms of physical dimensions, position and topology to further lower its weight. Note that the fully deployed antenna is closely similar to the ground-based reflector antennas; thus, their topology optimization techniques [67; 68; 69] can be directly applied to this antenna by supplementing only one additional constraint. The constraint requires that the adjacent petals shall not interfere with each other during deployment, which was reported in Ref. [38]. (This optimization research will be presented in future studies.) (3) Deployment mechanismTypically, there are two ways to deploy the antenna: deploying all petals with one motor or deploying with the same number of motors as the petals. The first approach requires a synchronous device; however, its cost may not necessarily be lower than that of the second approach considering the expenditure.