Elvis D. Silva, Ball Aerospace; Jeffrey Van Cleve, Ball Aerospace; Rob Philbrick, Ball Aerospace; Christopher J. Grant, Ball Aerospace; Jacob D. Griesbach, Ball Aerospace; Michael P. Mahoney, Ball Aerospace
Keywords: Cislunar, Astrodynamics, Imaging, Sensors, Optical Systems, Space Domain Awareness, Lunar Exploration, Lunar Mapping
Abstract:
Supporting cislunar space and lunar surface domain awareness has typically required the design and use of intrinsically distinct payloads with different operation modes and techniques: one for detection and tracking of very dim moving point sources against the background of space, another for surface imaging, processing, and monitoring. Here, we outline a conceptual design and utilization of a single sensor architecture achieved by using an electro-optic (EO) signal chain that is capable of hybrid SDA and lunar surface monitoring functions. The sensor includes telescope optics, a focal plane array (FPA), and electronics capable of fast simple pixel arithmetic and storage, either on the sensor chip or off-chip in an FPGA
Telescope design is tightly coupled to mission design, with an obvious example being the relationship between target flux, FOV, FOR, IFOV, revisit time and required SNR. In our assessment, we develop a qualitative apportionment of merit to cislunar SDA and lunar surface imaging to develop a strawman concept that meets first-order requirements of SNR> = 6 for a benchmark SDA target at 90° phase and a range of 55,000km and a sub-2m GSD at altitudes below 150km for lunar surface imaging. In particular, we explore six different mission architectures using 1-Ball or 2-Ball sensor constellations. The system shown here uses a modern imaging CMOS FPA sensor with a hybrid (framing + TDI) capability to define a single telescope design in combination with phased orbit configuration spacecraft to perform Lunar surface and Lunar space domain awareness missions. At apoapsis, the hybrid camera is used as a framing camera and has access to the Moon, low altitude lunar orbits, L1/L2 Lagrange points, and objects entering the lunar domain through the L1/L2 Lagrangian gates. At periapse, the hybrid camera is used as a TDI camera through subframe stacking.
Depending on the type of mission domain (cislunar SDA or lunar surface monitoring), careful selection of adequate orbits plays a role in the lighting, geometric, and access conditions to each domain. In our assessment, we explore a diversity of orbit geometries, sizes, eccentricities, and orientations relative to the lunar topography and cislunar space. Variation of these parameters seek to explore favorable geometric and lighting conditions to enable space/astronomical access near apoapse and lunar surface access near periapse. Coupling low periapse altitudes (i.e., 30km) with apoapse altitudes enables an ability to design for sub-2m surface imaging access periods (range of orbit within 150km altitude). In addition to traditional orbit configurations, our assessment explores the performance of extremely low periapse Near Rectilinear Halo Orbits (NRHO) in their application to cislunar SDA and lunar surface imaging (Whitley & Martinez, 2015). NRHOs have a continuous line of sight and good lighting conditions for objects at both L1 and L2 for most of the month and can be phased to cover Sun avoidance gaps for sensors near the Earth-Moon line (Earth surface, L1, etc.). The disadvantage of NRHOs is more restricted access to lunar topography, specifically at lower latitudes.
In contrast to conventional approaches, this concept provides a single telescope design approach and combines it with a selection of orbital configurations and phasing to give a 1-Ball configuration with relevant access metrics or a 2-ball constellation with a 100%-time access to the lunar space domain, ~100% area access to the lunar surface domain, and favorable lighting and geometric viewing conditions for both. The Hybrid sensor concept would well-complement a high-performance SDA sensor at L1 in a comprehensive cislunar SDA architecture to perform traffic management or monitoring of the cislunar Earth-Moon transit corridor. This sensor concept and its utilization would also be valuable for scientific purposes, such as measuring the contemporary small-impactor rate by observing small new craters on the lunar surface while performing lunar surface imaging in TDI mode or observing impact flashes while observing the dark side of the Moon in framing mode (Cahill & Speyerer, 2020; Liakos+, 2019)
Date of Conference: September 14-17, 2021
Track: Cislunar SSA