Designing the Ka-Band System Architecture for Science Data Return on the SPHEREx Mission

The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) mission, a part of the NASA Astrophysics Medium Explorer (MIDEX) program, will produce the first all-sky near infrared spectral survey of the universe, collecting images every 6” in spectral bands between 0.75 and 5.0 μm. Over the course of its 27-month mission, the SPHEREx Observatory will image the entire sky four times from a 650 km sun-synchronous polar orbit, producing up to 175 Gbits per day of science data which will be returned via the NASA Near Space Network Direct-To-Earth (NSN-DTE) Ka-band service at a rate of 600 Mbits per second. As a survey mission, SPHEREx relies on high system availability and reliable data return to meet its science goals. The constraints of the instrument, however, create key challenges in designing a downlink system design that is capable of returning the mission data set while maintaining the performance of the project’s one instrument. SPHEREx’s passively-cooled infrared instrument is sensitive to thermal disruptions from the environment and the shields that protect it from environmental heating provide only a limited available pointing zone, which in turn constrains the available maneuverability of the vehicle to track an NSN-DTE station. This piece of the SPHEREx design presents an excellent case study in systems engineering. To achieve a cost-effective design that meets both mission constraints and mission needs, SPHEREx needed to balance designs across the instrument, spacecraft telecom subsystem, spacecraft attitude control operations, and ground system, touching multiple key project budgets and resulting in major architectural decisions in both the flight telecom architecture and the planning systems on the ground. In this paper, we will discuss the end-to-end architecture of the SPHEREx science data downlink path and describe the unique challenges and design trades the project executed to develop it. The discussion will include the constraints of the mission and vehicle that drive the downlink design, the resulting spacecraft Ka-band architecture, the development of an optimizer ground tool to plan downlinks, and pointing analysis tools that integrate downlinks with the science plan.