Andrea Lopez, University of Colorado Boulder; Julian Hammerl, University of Colorado Boulder; Hanspeter Schaub, Colorado Center for Astrodynamics Research
Keywords: x-ray, sensor, relative orbit estimation, pasive object detection, angles-only
Abstract:
Space Situational Awareness (SSA) is a topic widely researched in the near-Earth region, involving the detection and tracking of objects orbiting the Earth without direct interaction with these objects. At present only a handful of spacecraft are in the cislunar region further than GEO, but the presence of vehicles in the lunar vicinity is anticipated to increase in the upcoming years, with robotic exploration missions, satellite communication and navigation systems, or the Gateway. With the increased presence on spacecraft in this region, on-orbit cislunar SSA capabilities will become an asset in the near future, not only for debris tracking purposes but also for proximity operation applications with non-cooperative spacecraft. A new method of using ambient plasma-induced x-rays to detect objects in the neighborhood of a spacecraft is proposed for the cislunar region. Energetic electrons (> few 100 eV) present in the ambient plasma interact with atomic nuclei exciting the release of characteristic x-rays and bremsstrahlung x-rays. This project investigates the use of wide FOV x-ray detectors in the form of a cluster of commercial off-the-shelf x-ray sensors to detect objects in the vicinity of a spacecraft, exploiting the natural interactions taking place in the ambient plasma. The sensors considered in this research are Si-PIN photodiode x-ray spectrometers, and the signal is modeled as an on/off signal. A measurement is therefore obtained when the signal changes from on to off (or viceversa) on a particular sensor, knowing that this event takes place when the target object is at the edge of the field of view of that sensor. Thus, at the time of the change-of-signal event, the angle between the pointing of the sensor (known) and the target heading is the field of view cone half angle To increment the number of such events, the cluster of sensors is mounted on a platform that is rotating at a known constant angular rate. This methodology provides an angles-only approach to the relative motion estimation problem, as range measurements are not available. Past work explored a variety of sensor configurations for one or multiple sensor clusters, analyzing the performance for obtaining a heading for a static target (i.e., not moving with respect to the observer spacecraft). For a dynamic target, linear relative motion examples (e.g., unforced double integrator dynamics) introduce dynamical observability challenges. A method for using batch measurements to obtain estimated headings, which are then the input for a sequential filter, was developed in previous work. This work explores more realistic models for the relative orbit dynamics that could be encountered in an on-orbit scenario. Typical approaches for angles-only relative orbit determination (ROD) focus on near-circular orbits and have poor observability as a result of linearizing the dynamics in the filter and measurement models. A common approach for handling the range ambiguity and improving observability in angles-only navigation is conducting prescribed maneuvers that result in range-dependent angle variations. However, the accuracy of the relative orbit estimation achieved with this method is strongly dependent on maneuver-planning and capabilities of the observer spacecraft. More recent approaches to ROD exploit the nonlinearities in the filter dynamics and measurement equations to improve the observability of the problem using vision-based sensors. This work explores the application of this approach to the proposed measurement model.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation