Michael Thompson, Advanced Space, LLC; Nathan Parrish Ré, Advanced Space, LLC; Cameron Meek, Advanced Space, LLC; Bradley Cheetham, Advanced Space, LLC
Keywords: cislunar, SSA, SDA
Abstract:
Building on previous work in cislunar orbit determination at Advanced Space, this paper demonstrates cislunar orbit determination via simulated optical measurements from another spacecraft in cislunar space. The goal of this work is to evaluate the potential performance (in terms of the uncertainties in the state of a target object) of space-based optical tracking filters in the vicinity of the Moon.
Estimating a spacecraft state via ground-based or space-based optical measurements up to GEO is a well-studied problem and is operationally performed by several companies and agencies. Space-based tracking in the cislunar domain is a more novel problem given challenges with lunar exclusion angles, short data arcs, and nonlinear dynamics that stress basic assumptions and simplifications of most tracking filters. In this study, angular measurements are generated between two cislunar spacecraft based on simulated optical measurements against reference background stars. Random angular noise is added based on realistic values for optical sensors, and these noisy measurements are processed in a sequential filter to refine the 6-dimensional state and covariance of the target over time.
This study demonstrates bounds on the potential performance of cislunar space-based surveillance systems from the standpoint of tracking filter design, trajectory design, and basic optical modeling. A specific focus is placed upon the 9:2 synodic resonant Near Rectilinear Halo Orbit as the observation orbit, although other cislunar trajectories are studied as well. This observation orbit is of interest to the authors given that it is the planned orbit for the upcoming CAPSTONE mission, which will be pathfinding mission design and navigation for NASAs future Lunar Gateway and performing inter-spacecraft navigation experiments. These experiments include the demonstration of the Cislunar Autonomous Position System, which is an architecture for autonomous navigation of cooperating spacecraft in the cislunar domain. Advanced Space is leading the CAPSTONE mission and will own the system and control operations during the mission. This includes navigation for the mission and operations of the RF and optical payloads.
Given that any SSA modeling is dependent upon the optical characteristics of both the observing sensor and target spacecraft, this study will seek to be as optically agnostic as possible, and each examined case will be performed with a several optical cutoffs corresponding to different levels of target brightness and observer sensitivity. While this work is not intended as a sensitivity study of target brightness or sensor sensitivity requirements, some insights are naturally provided in these areas as different optical cutoffs are explored.
While this work focuses on the standard space-based SSA approach of an observer with a well-known state estimating the unknown state of a target, future work could demonstrate co-estimation of both target and observer. Hill, Born, Parker, and several other authors have shown that the cislunar environment, particularly near Earth-Moon L1 and L2, has some key features in dynamical systems theory that can allow for the co-estimation of both relative and absolute states of two cooperating spacecraft with radiometric measurements between them. In the context of SSA, it obviously cannot be assumed that any two spacecraft are cooperative, but optical measurements in the cislunar environment could provide the necessary information for co-estimating the states of multiple spacecraft and could lead to a future where autonomous navigation coupled with cislunar SSA is possible.
The cislunar SSA/SDA field is still developing, and a number of studies thus far have focused either on high-fidelity simulations of the apparent magnitude of cislunar objects, or on performing line-of-sight coverage analyses (including occultations and solar constraints) with potential observers and targets in order to inform constellation design. This study seeks to inform the field by simulating tracking in an operational-like filter and quantifying to what degree custody of targets can be maintained at multiple observer and target locations, and multiple optical cutoffs. Previous literature has shown that a multiple-observer constellation is typically required to maintain persistent SSA coverage of the cislunar domain through varied solar geometries of the lunar month. This work focuses on the tracking filter performance of a single observer tracking a single target, but the filter development and testing could be applied to a constellation-level tracking scheme in the future.
Date of Conference: September 14-17, 2021
Track: Cislunar SSA