Erik Payton, University of Maryland; Tam Nguyen, University of Maryland
https://doi.org/10.64861/MMQX9178
Keywords: Space-based SSA, cislunar, system design, optical SSA
Abstract:
Current space situational awareness (SSA) architectures are primarily focused on the detection and tracking of Earth-orbiting spacecraft and debris. With recent interests in lunar missions and cislunar operations, SSA capabilities for resident space objects (RSOs) in cislunar space must be developed. Current SSA efforts rely heavily on the space surveillance network (SSN), which primarily consists of ground based sensors. However, cislunar SSA from ground-based sensors is challenging due to sensor gaps and poor viewing angles, even while utilizing a global network. Space-based SSA sensors have the potential to address these shortcomings. A sensor placed in a cislunar orbit would avoid some of the challenges with Earth-based observations; however, complex orbital dynamics in the cislunar regime result in very few orbits with consistent favorable solar illumination angles, long term stability, and extensive optical coverage of the regime.
The first Sun-Earth Lagrange point (SEL1), located 1.5 million kilometers from the Earth-Moon system (EMS), is not often considered for use in near-Earth and cislunar SSA. Detecting spacecraft from these extreme distances is challenging due to significant flux losses. On the other hand, SSA from SEL1 provides unique optical advantages, offering a near-zero phase angle, and a comprehensive view of the EMS with limited exclusion zones. Additionally, orbits of SEL1 have long term stability with minimal delta-v cost for orbit maintenance. This paper shows that cislunar SSA from a spacecraft at the SEL1 point is a feasible mission concept, and considers several optical designs with various hardware and operational constraints.
This study examines the tradespace of possible designs for the optical payload, and provides an overview of the potential capabilities associated with this concept. The approach to this study starts with defining requirements and constraints for the observatory focused on target RSO orbits and photometry, as well as the capabilities of the optical payload. Once outlined, potential configurations of the optical payload are analyzed for their ability to detect the characteristic RSO. The final step is to examine different operational procedures regarding observatory tracking and stacking methods to improve detection statistics. In this work, the representative RSO is a one meter radius lambertian sphere with an albedo of 0.2, appearing from SEL1 as a 22nd visual magnitude object.
The optical parameters under consideration for this design study include the focal ratio (f/number), the size of the aperture, and the quantity of optical tube assemblies (OTAs) within the payload. The specific choice of detector and pixel size are explicitly intertwined with the performance of the observatory. For this analysis, a well documented sensor is chosen as a baseline and its parameters kept constant. Results show that larger aperture sizes and lower f/numbers are preferable to achieve positive detections. Analysis is focused around f/numbers lower than f/4 and optical apertures from 0.1 meters to 0.5 meters. The high costs associated with extremely large apertures motivates the use of multiple co-boresighted OTAs on the payload to achieve better detection statistics if one OTA were insufficient.
The concept of operations are designed to prevent trailing loss for the fastest expected apparent motion of an RSO. The observatory is sidereally tracking to ensure the easy removal of background stars. The integration time (dwell time) is set such that the spacecraft does not move more in angular space than the instantaneous field of view (iFOV) of the OTA in that integration time. If a spacecraft were observed while the observatory was tracking the motion of the Earth, a longer integration time could be utilized due to a decrease in apparent relative motion of the spacecraft to the optical payload, i.e., the effective dwell time. Note that with this tracking concept, removal of background stars would not be possible. To satisfy star removal constraints while maximizing dwell time, we developed a hybrid approach. We synthetically track the Earth which creates co-added frames with this effective dwell time, while the observatory is tracking sidereally. To further increase the SNR of a detected RSO, a blind search is needed to generate un-cued detections. Strategies such as track-before-detect (TBD) algorithms search over many velocity hypotheses to find these RSOs. TBD methods have significant computational requirements due to large search volumes, but more efficient options such as the fast discrete X-ray transform (FaXT) make onboard stacking of the images possible.
Traditional space situational awareness utilizes ground based infrastructure that will be insufficient for use in cislunar space as the satellite and debris population grows in that orbital regime. A spacecraft located at the Sun-Earth L1 point would have the ability to observe cislunar space with near-zero solar illumination angles, and a near-constant view of the region. Selected results show that an example design for an observatory with two co-boresighted 30 cm f/1.4 OTAs has the potential to detect objects of 22nd magnitude with a 50% probability of detection and 1% false alarm rate. Future work on this concept includes understanding coverage gaps behind the Earth and Moon, analyzing the exclusion zones due to reflected light from the Earth and Moon, and optimizing observation schedules to reduce revisit time.
Date of Conference: September 15-18, 2026
Track: Cislunar SDA