A Multi-Constellation Approach to Space-Based RF Emissions Monitoring Through Opportunistic Satellite Cluster Collisions in LEO

Abstract

Abstract. The deployment of small Low Earth Orbit (LEO) satellites equipped with radio-frequency (RF) tracking sensors marks a transformative shift in global, space-based RF emissions monitoring. Leveraging cost-effective software-defined radio (SDR) technology and advanced micro-antenna arrays, these satellites – organized into tandems or clusters – can efficiently scan the Earth's surface. Their payload can generate Time Difference of Arrival (TDOA) or Angle of Arrival (AOA) observables that can be used to accurately detect and locate RF emitters. Applications range from identifying known distress signals for search and rescue operations to detecting unknown Radio Frequency Interference (RFI) sources that may disrupt protected bands, such as GNSS. Currently operational LEO satellite constellations already offer such data acquisition, with commercial providers offering varying performances based on the number of deployed satellites and revisit times. To achieve unparalleled monitoring coverage and responsiveness, this paper introduces the innovative concept of a multi-constellation approach, where a centralized RF monitoring service provider accesses data from multiple constellations. The cost-benefit of this more expensive solution hinges not on the sum of individual constellation capabilities but on the potential to capitalize on additional multi-constellation opportunistic events, termed ‘collisions.’ These occur when satellites from different constellations form new clusters, allowing simultaneous observation of the same area and enabling additional TDOA or AOA measurements not associated to a single constellation. The effectiveness of these collisions depends on relative geometry and other critical performance indicators, and not all events can be considered advantageous for a target RF detection and localization scenario. This paper develops a methodology to establish metrics that quantify the occurrence and utility of such interactions, providing collision frequency data. The methodology involves an analytical description of satellite constellations with varying orbit sizes, considering their relative kinematics to a reference signal emitter on Earth's surface, the Cramér-Rao Lower Bound (CRLB), Dilution of Precision (DOP), and incorporating constraints on the Line of Sight (LOS). Case studies in LEO mission applications, exemplary of search and rescue operations and spectrum monitoring in the E1/L1 GNSS frequency bands, are discussed. The proposed solution offers a crucial tool for monitoring service providers to evaluate the advantages of multi-constellation data access, assess the expected number of beneficial collisions, and optimize sensor acquisition scheduling to maximize multi-constellation RF detection and localization accuracy.