The TeraByte InfraRed Delivery (TBIRD) system is a 3U payload on a 6U CubeSat launched in May 2022 which has now demonstrated space to ground links of >1 Terabyte (TB) per pass at a max data rate of 200Gbps. As a CubeSat mission, the development of the TBIRD payload was focused on low SWaP and a “rapid prototyping” approach which accepted higher risks to accelerate the schedule and reduce costs. The optomechanical design process followed standard in-house processes to develop a system that would be robust to LEO environmental loads, with a focus on the stability of the transmit (Tx) and receive (Rx) channel performance metrics. The driving requirement of maintaining 20μrad pointing error between the TX and Rx channels forced specific attention to thermal and mechanical load changes over operational conditions, which drove major design decisions. This paper describes some of engineering challenges overcome and approaches used to make TBIRD a successful program, as well as some of the tradeoffs of rapid prototyping precision optical payloads. TBIRD successfully met and exceeded the total downlink requirements listed above, with a bandwidth of 200Gbps and a total downlink of 4.8TB of information in a single pass.
Silicon carbide’s material properties make it a desirable choice as a mirror substrate, particularly for space-based optical systems where thermal stability and low mass are critical. Mirror substrates made from silicon carbide (SiC) often have a small amount of porosity or surface features that make optical surfacing of the bare substrate challenging. To achieve the surface figure and roughness required for diffraction-limited performance at visible wavelengths, it is common to apply a thin layer, or cladding, of a similar material that has properties more conducive to optical surfacing. The introduction of another material, however, has the potential to change the surface figure over temperature, even with small differences in thermal expansion between the cladding and the substrate. This paper presents testing to characterize that effect on a SuperSiC®-SP mirror clad with chemical vapor deposited (CVD) silicon carbide produced by Entegris Specialty Materials. This mirror was fabricated for a two-axis gimbal-mounted scan mirror assembly, which has been developed for the QZSS-HP program. The QZSS-HP is a hosted payload (HP) on the Japanese Quasi-Zenith Satellite System (QZSS) and will be used for space domain awareness (SDA). Thermal expansion measurements and optical surface measurements performed at Massachusetts Institute of Technology Lincoln Laboratory (MIT LL) show that the difference in coefficient of thermal expansion (CTE) is on the order of 0.2 parts per million per degree C.
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