Jesse Greaves, University of Colorado Boulder; Daniel Scheeres, University of Colorado Boulder
Keywords: cislunar, space, domain, awareness, relative, optical, autonomous, navigation, observation, combined, estimation, proximity, operations
Abstract:
Traditional concerns regarding space domain awareness are exacerbated in cislunar space due to sensitive dynamics, limited viewing geometries, and increasingly overloaded Earth-based resources. These concerns motivate the investigation of space-based navigation and observation capabilities which lend themselves to autonomy. This paper expands upon previous results to demonstrate that relative optical measurements provide simultaneous navigation of a primary spacecraft and observation of a secondary spacecraft. First, linear observability analysis proves that relative optical measurements fully estimate the dual spacecraft system over various orbits. Subsequently, the Optimal Control Based Estimator is modified with the Unscented Transformation to account for the strongly non-linear dynamics and then leveraged to generate a state solution along with an estimated control policy. Finally, the estimated control policy is employed to identify and classify maneuvers. Collectively, this methodology directly enables a chaser craft to navigate to a final docking phase autonomously and engenders further research into autonomous operations in cislunar space.
Cislunar space is a primary target for development due to the regions suitability as a proving ground, and staging point, for deep space exploration. NASAs Artemis program is an exemplary model for cislunar development, but NASA is not alone in its vision for inhabiting this expanse. Nearly every major national space agency, and many commercial companies, have expressed interest in sending spacecraft on cooperative and independent ventures to this region. The combined efforts of the international space community will lead to a myriad of spacecraft traversing and occupying cislunar space. Such an environment necessitates the design of cislunar space domain awareness (SDA) capabilities to ensure all vehicles collective safety and long-term reliability in the region.
Currently, cislunar estimation relies on Earth-based resources leading to various observational challenges. Earth-based radar such as the Deep Space Network (DSN) is already oversubscribed, and scheduling DSN time for all future vehicles will be prohibitive. Ground-based optics require sensors that must avoid objects in exclusion zones around the moon to preserve sensor health. Additionally, all Earth-based observers have a limited viewing geometry of cislunar objects, which results in sparse information and potentially long periods of occultation. To overcome these factors and many other issues, the exploration of new observation platforms is essential. In particular, cislunar observation platforms demonstrate promise to reduce Earth-based resource load, enhance observability, and enable novel missions.
Previous works have studied the benefits of utilizing various cislunar observation platforms for orbit determination in the region. Generally, these studies can be divided into passive vs. active sensing and electro-optical vs. radio-frequency sensing. Observer location also plays a critical role given the vast number of periodic/quasi-periodic orbits and the potential to utilize the lunar surface itself. Space-based optical platforms are an appealing solution as they leverage existing commercial technology and provide equivalent accuracy for cooperative and non-cooperative systems. Additionally, studies have shown that optical measurements are a viable option for autonomous cislunar navigation. The combined advantages of space-based optical sensors lead to the possibility that optical measurements can provide simultaneous autonomous navigation and observation. Thus, this paper seeks to examine relative optical measurements to estimate a primary and secondary spacecraft simultaneously over various orbits.
This paper begins by utilizing linear observability to prove that relative optical measurements lead to a fully observable system. The applied method is a modification of LiAISON results that previously demonstrated that relative radiometric measurements are sufficient to perform dual navigation. Then, a modified version of the Optimal Control Based Estimator (OCBE) with Unscented uncertainty propagation is adapted to provide filtering solutions and an estimated control policy. The Unscented transformation is made to account for the strongly non-linear dynamics and leads to more consistent results. Finally, the control policy from the estimator is used to identify and classify maneuvers. The classification method separates the control policy into 4 categories of maneuvers: none, stable manifold, unstable manifold, and generic. The maneuver categories correspond to station keeping, exponential orbit departures, and generalized maneuvers, respectively. This classification of the OCBE control policy extends previous work that now accounts for uncertainty in the observer state, improves filter performance, and adds maneuver classification. By successfully demonstrating dual spacecraft estimation free from Earth-based resources, this work seeks to further autonomous operations in cislunar space.
Date of Conference: September 14-17, 2021
Track: Cislunar SSA