Kullen Waggoner, Air Force Institute of Technology; David Curtis, Air Force Institute of Technology
Keywords: SDA, SSA, cislunar, Passive RF, TDOA, FDOA,
Abstract:
This paper delves into the utilization and analysis of a cislunar-based receiver to enhance a network of terrestrial and near-earth space-based passive RF receivers for conducting Space Situational Awareness (SSA) of objects in cislunar space. Over the past five years, the proliferation of US, foreign, and commercial missions to and around the moon has necessitated expanded and improved spacecraft tracking and SSA capabilities. Due to limitations in current electro-optical and radar tracking architectures for objects around the moon, there’s a growing interest in novel methods for maintaining SSA. Consequently, passive RF estimation research has garnered increased attention as a means to address some of the gaps in spacecraft tracking within the cislunar regime. Multi-receiver passive RF systems measure a signal sent from a single emitter and compare the signal as it received at multiple receiver stations to calculate a time difference of arrival (TDOA) and frequency difference of arrival (FDOA). Previous research conducted at the University of Arizona demonstrated that terrestrial-based receivers can successfully track spacecraft between the Earth and the Moon using passive RF. This paper models and explores the integration of a cislunar receiver into a network of terrestrial and GEO-based receivers. The added receivers are in various periodic, closed orbits around the Earth-Moon Lagrange points L1, L4, and L5. The addition of the cislunar receiver is potentially beneficial as the estimation accuracy of passive RF systems is based on the geometric orientation of its receivers. This orientation and its usefulness for estimation is often measured through the metric of the dilution of precision (DOP) which is based on the Cramer-Rao Lower Bound. However, an unexplored and potential significant source of error for such a system is that own-craft position, navigation, and timing (PNT) information is limited for a satellite in L1, L4, and L5 orbits. Therefore, the operational utility of having a passive RF receiver for tracking of other spacecraft in cislunar space is dependent on how accurate the satellite is able to estimate its own position, velocity, and time.
This research effort applies and adapts previous research into UAV-based passive RF estimation where receiver position and velocity uncertainty are considered and adapts/applies it to the problem of cislunar passive RF SSA. This research derives a relationship between the receivers’ position and velocity uncertainties and relates that to the Cramer-Rao Lower Bound of the anticipated final estimation accuracy of the cislunar emitter. Initial results show that the addition of an L1 receiver to a combined terrestrial/GEO receiver architecture improves the predicted median DOP by over an order of magnitude when it is tracking an emitting object in a distant retrograde orbit around the moon. The median DOP improves from 0.3017 km/nsec to 0.0216 km/nsec. Additionally, the uncertainty analysis demonstrated that to get the maximum improvement in the DOP, the cislunar receiver must know its own position to within a standard deviation of less than 10m. Furthermore, the analysis showed that if the cislunar receiver has an own position error with a standard deviation of greater than 2 km, then it provides very little improvement in the final predicted estimation accuracy of the emitting object over the terrestrial and GEO architecture on its own. The paper and presentation will show a derivation of the relationship between receiver uncertainties and final estimation accuracies and further analyze the benefit of the addition of a cislunar passive RF receiver within multiple orbits and architectures.
Date of Conference: September 17-20, 2024
Track: Cislunar SDA