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. The SPHEREx Observatory is comprised of a spacecraft bus provided by Ball Aerospace and a passively-cooled infrared instrument built by JPL and Caltech. 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 gigabits of science data each day. As a survey mission, SPHEREx relies on high system availability and reliable data return to meet its science goals. However, SPHEREx's infrared instrument requires a stable thermal environment that can be disrupted if it is pointed too close to the Earth or Sun, which imposes geometric pointing constraints that limit downlink operations.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, telecom subsystem, attitude control operations, and ground tools. The Spacecraft bus provides 600 megabit-per-second Ka-band downlink that will be received by the NASA Near Space Network Direct-To-Earth (NSN-DTE) service. The instrument's thermal constraints limit the vehicle's ability to point the Ka-band antenna to a station, and typical body-fixed high gain antenna designs did not provide sufficient access. To solve this problem, SPHEREx traded across several parts of the system design and balanced multiple key project budgets, which resulted in architectural decisions in the flight telecom architecture and led to the development of an innovative planning tool that includes an optimizer capable of maximizing downlink time within constraints.In this paper, we will discuss the end-to-end architecture of the SPHEREx science data downlink path and summarize 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, a detailed discussion of the optimizer that enables the mission to achieve sufficient downlink time, and the integration of the optimizer with pointing analysis tools that integrate downlinks with the science plan.