RELATIVE NAVIGATION AND POINTING ERROR BUDGET FOR AN X-RAY ASTRONOMY FORMATION-FLYING MISSION

This paper presents an error budget for the relative navigation and intersatellite pointing of a novel formation-flying mission, Cal X-1. Despite extensive ground calibration campaigns, cross-comparison of orbiting X-ray observatories reveals systematic discrepancies in measured celestial source fluxes of more than 10%. The Cal X-1 mission will address this problem by establishing an in-orbit X-ray flux standard using a pair of SmallSats flying in formation. The first spacecraft will host an X-ray telescope while the second spacecraft will host an absolutely calibrated X-ray source. The mission design requires precise inter-satellite pointing, but constraints on size, weight, power, and cost preclude the use of specialized hardware. This paper seeks to demonstrate that meeting the challenging intersatellite pointing requirements is feasible through advanced relative navigation techniques. The performance of a suitable relative navigation system is demonstrated in high fidelity simulation. Next, a mathematical model which accounts for errors stemming from relative navigation, attitude determination, and the spacecraft structural assembly is developed to allow the computation of a pointing knowledge error. By comparing this pointing knowledge error with the requirements of the Cal X-1 mission, the feasibility of the proposed inter-satellite pointing methodology is demonstrated.