System Level Availability Budget for the Multi-Vehicle Sun Radio Interferometer Space Experiment

The Sun Radio Interferometer Space Experiment (SunRISE) will be the first space-based radio interferometer. Comprised of six identical 6U vehicles carrying decametric-hectometric radio and GNSS receiver payloads, the SunRISE Observatory orbits Earth slightly above GEO to measure solar radio bursts associated with solar coronal mass ejections (CMEs) and solar flares. Because the solar radio bursts that SunRISE will observe are inherently unpredictable, the SunRISE Observatory must have a long enough observing time in order to observe a sufficient number of solar radio bursts to satisfy the Level 1 Science requirements. The observing time is the product of two factors, the total duration of the prime science mission and the fraction of that time that can be used for science or the “Observatory availability”. In practice, solar radio bursts associated with CMEs, so-called Type II bursts, are less frequent, and the requirement to observe a sufficient number of them drives the resulting set of lower-level requirements. The “Observatory availability budget” is a mission-unique system-level budget that aims to guarantee science closure by synthesizing various, apparently disjoint, factors. The factors considered range from typical to more exotic. Typical considerations include time spent downlinking data or conducting orbital maneuvers, considerations of reliability and infant mortality, and expectations for spacecraft and payload hardware and software. Special considerations include the three-dimensional geometric arrangement of the vehicles in space and the extent to which they are in a configuration and with separation sufficient to form an interferometer with the required performance. This paper describes these factors and shows how we have assessed the Observatory availability budget to ensure that the Observatory is online greater than 90% of the time in order to observe a sufficient number of Type II radio bursts. The relevant requirements decomposed from this budget pertain to upset frequencies and durations, planned outage limits, mission duration, and fault protection/management and autonomy needs for the vehicles. This Observatory availability budget also informs mission plans for downlink/uplink and maneuver durations and cadences. This paper provides background on how the 90% availability requirement was developed, and focus on how the Observatory availability budget factors were identified and estimated, the statistical Monte Carlo-based model built to synthesize them, and the philosophy developed for how to interpret the results and evaluate compliance and margin. Of particular interest, we will discuss how SunRISE's architecture of six identical vehicles provides on-orbit redundancy, which is critical to the resiliency of this budget, allowing it to degrade gracefully with anomalies. At current, the SunRISE team is working to close this analysis and verify the related requirements in preparation for launch. SunRISE is a PI-led NASA science mission with PI and science data system at University of Michigan, managed by and with payloads, mission operations, and systems engineering provided by NASA's Jet Propulsion Laboratory, and spacecraft buses, assembly/integration/test from Space Dynamics Laboratory. SunRISE is in Phase D and currently preparing for hardware delivery to launch storage.