Edwin G. W. Peters, University of New South Wales Canberra; Timothy Bateman, UNSW Canberra Space; Rabbia Saleem, UNSW Canberra Space; Andrew Lambert, UNSW Canberra Space; Melrose Brown, UNSW Canberra Space
Keywords: space based space surveillance, passive RF
Abstract:
With a growing number of human made assets in Earth’s orbit in recent years, and the number these assets set to double in the next years, tracking and monitoring these assets is increasingly paramount for continuous safe access to space. To accurately determine the orbit of a space asset and detect manoeuvres, orbit changes and patterns of life, frequent observations are required. Nowadays, optical sensors and active radio frequency (RF) sensors provide the majority of the sensors used to collect observations on space-based objects. This includes mono and multi-static radar and optical sensors. Active radar systems trade off the field of view with range, effectively limiting the range of wide field of view sensors to the low Earth orbit (LEO) regime. Optical sensors are used for sensing mid and high Earth orbits (MEO and HEO), but sensor operation is depending on weather and illumination conditions. Passive RF sensors provide a critical capability to fill some of these gaps by collecting RF signals emitted by space-based objects. While typically, but not necessarily, limiting the observation to targets actively emitting RF signals, aspects, such as close proximity operations and pattern of life can additionally be determined from passive RF observations. Since passive RF sites forgo the need of a emitter to illuminate targets, only receivers are needed, reducing the cost and spectrum licensing requirements significantly. Additionally, passive RF sites can be deployed close to, or in populated areas and are suitable to track active objects in all orbits. Passive RF observations can broadly be divided in two groups: Emitter observation and passive radar. While emitter observation requires the target to actively emit a signal, the target can reside in any orbit visible from the passive RF site. This includes HEO, MEO and LEO spacecraft and even cis-lunar orbiting spacecraft. The drawback is, that the spacecraft must be emitting RF signals, and the beamwidth is large enough to cover the passive RF sites. On the other hand, passive radar utilises so-called “emitters of opportunity” as illuminators. These comprise of terrestrial and space-based emitters, such as FM and digital TV broadcast emitters. Current passive radar systems can track large LEO objects, but challenges arise when tracking small objects and in higher orbits. Depending on the number of passive RF sites available and their configuration, multiple measurement types can be obtained. This includes frequency of arrival, time of arrival, frequency and time difference of arrival, angle of arrival and signal power and characteristics. These can be used for orbit determination, manoeuvre detection and pattern of life monitoring. General with all the above mentioned sensor types is, that a geographical site is required to install the sensor. The distribution of landmass, operating conditions and access as well as geopolitics limit the installation of a sufficient number of sites to provide global coverage of Earth’s orbits.
Space based space surveillance (SBSS) is a growing area of interest, and an obvious way to extend the coverage of parts of the sky and Earth’s orbits that are not visible from terrestrial sensors. While the majority of current space-based sensors focus on Earth or planetary observations, there are a limited number of optical SBSS sensors, such as NEOSSat, Saphire and GASSP, as well as sensors used for object inspection. Optical sensors in space are not affected with weather effects in Earth’s atmosphere, greatly increasing their operational capabilities. However, the field of view of optical sensors is narrow, which limits the feasibility of using these sensors for surveillance. A number of optical SBSS sensors are utilised for object inspection, where specific objects are inspected. Space based passive RF sensors would provide the large coverage that terrestrial sensors provide, with the prospect of using these to detect unknown objects as well as monitor existing objects. Constellations of passive RF equipped spacecraft in orbit, and around the moon can sense a large part of the sky. Thus, SBSS has the prospect to provide crucial capability essential for object detection, high precision localisation, monitoring and manoeuvre detection in space.
These spacecraft can be equipped with a multitude of sensors, such as wide beamwidth detection sensors, as well as narrow beamwidth RF, optical and event based precision sensors. While space based sensors exist today, the majority are utilised for Earth observations. Challenges however arise; What orbits provide the best coverage?, How should the apertures be sized and configures? Power and size requirements of the spacecraft. In this work, we present a feasibility study on the prospect of space based space surveillance (SBSS). The feasibility study is backed by RF signal collections performed using the software defined radios onboard the M2 spacecraft.
The University of New South Wales, Canberra (UNSW Canberra) embarked on an ambitious Cube Satellite research, development, and education program in 2017 through funding provided by the Royal Australian Air Force (RAAF). M2 is the final mission of the series, comprising of two identical spacecraft that were launched on the 22nd of March 2021 in a 45 degree inclination 550 km altitude orbit. This current work will use data from the passive RF collection campaign on our M2 spacecraft to inform a feasibility study for a dedicated LEO space-based passive RF sensor capability. The goals of this study is to investigate how a passive RF based SBSS capability can augment a ground network of sensors to provide global coverage of Earth’s and cis-lunar orbits. We will investigate different sensor configurations and orbits and provide a feasibility study on the requirements for the RF performance and sensor types required on passive RF based SBSS spacecraft. We will investigate joint operations with combinations of wide field of view passive RF sensors and narrow field of view sensors, such as high gain RF antennas and optical sensors to provide new capabilities for monitoring, object detection and manoeuvre detection in both LEO and other orbits.
Date of Conference: September 19-22, 2023
Track: Space-Based Assets