Mark Bolden, Trusted Space, Inc.; Tim Craychee, Trusted Space, Inc.; Erin Griggs, Trusted Space, Inc.
Keywords: Cislunar, Space Situational Awareness (SSA)/Space Domain Awareness (SDA), Space Traffic Management (STM), Astrodynamics, Modeling. Instrumentation, Sensors and Systems, Optical Systems, Space-Based Assets
Abstract:
When designing an effective architecture for Space Domain Awareness (SDA) it is necessary to perform extensive modelling across multiple disciplines to effectively design network distributions, sensing payloads, concepts of operations (CONOPs), and Orbit Determination (OD) algorithms. To produce timely domain awareness requires sufficient coverage from well calibrated sensors with real-time data reduction, orbit generation, object characterization, event alerting, and dissemination. The cislunar domain introduces new SDA challenges, such as larger volumes to monitor, lower signal to noise ratios, line of sight obstruction from the Moon, on-board processing/orbit generation to maintain timeliness, lack of GPS for position, navigation, and timing (PNT), limited observatory size weight and power (SWaP), limited communication availability, three body orbit instabilities, and a vast diversity of potential orbit trajectories due to the lunar gravitational forces. Architecting solutions that address these complex challenges requires significant modeling and simulation from diverse areas of expertise to ensure all major contingencies are well represented. This paper will discuss three major challenges for performing cislunar domain awareness: low Signal-to-Noise Ratios (SNRs), Too-Short-Arcs (TSAs), and Observing Constellation Orbit Stability (OCOS).
The 4? steradian cislunar domain that extends out to the L2 position encompasses a volume of 3.79×1017 km3. This volume dwarfs the typically discussed SDA volume that extends out to the geosynchronous belt, only 3.14×1014 km3. With volumes of this size, the ranges from observer to object are substantial with a maximum potential range of 898,200 km. This introduces two key challenges: low SNRs due to range and TSAs due to slow apparent angular motion. For active systems, SNR drops proportionately to 1/range4, and for passive systems, by 1/range2. For example, a diffuse sphere in geosynchronous orbit with an albedo of 0.17 and a surface area of 1 meter2 is >15 visual magnitudes when viewed from the Earths surface, whereas at maximum distance in the cislunar domain for the same object size is >22 visual magnitudes. The increased range also effects the observed arclength when viewing an object from another distant object. This presents a significant challenge when performing Initial Orbit Determination (IOD) due to poor sampling of the new objects in-track velocity. This is a well-known problem when observing geosynchronous objects from the Earths surface and is commonly referred to as the Too Short Arc (TSA) problem. These two challenges push the trade space for Cislunar SDA to consider space-based platforms that are distributed throughout the cislunar domain to reduce ranges from observer to target. Expanding the trade space to include cislunar orbits introduces significant challenges for Observing Constellation Orbit Stability (OCOS) due to the three-body Earth-Moon-observer challenge.
The cislunar orbit trade space for the three-body Earth-Moon-observer is complex. This is due to the gravitational forces applied to the observers orbit within the cislunar volume, specifically, the relationship between the observers location and the Moon. The Moons gravity has the potential to make observers orbit chaotic in nature to include high swings in orbital parameters, specifically inclination, apogee, and perigee values. In some cases, if not properly monitored and controlled, the lunar gravity can create an Earth reentry scenario. The Moons gravity also has the potential to induce orbit stability as well. The best example of these locations are the Lagrange points, specifically, the L4 and L5 points which are extremely stable. Additionally, there are multiple families of periodic orbits including Halo orbits around a Lagrange point. The three-body problem induces complexities when performing initial orbit determination, by requiring multiple factors to be considered (e.g. Earth Centric System, Moon Centric System) and the timelines involved. Without diverse geometric collection, the information gained between observations is limited resulting in extremely large uncertainties.
This paper will discuss a process for modeling and simulating the cislunar domain to evaluate concepts for Cislunar Space Domain Awareness (SDA) network architectures with specific architecture examples. Specific challenges to be discussed include low Signal-to-Noise Ratios (SNRs), Too-Short-Arcs (TSAs), and Observing Constellation Orbit Stability (OCOS). Example cislunar SDA network architecture simulations will be discussed for attempting to detect and obtain custody of an object on a lunar free-return trajectory. A comparison of the effectiveness for each architecture design will be discussed with common metrics of performance.
Date of Conference: September 15-18, 2020
Track: Cislunar SSA