# Passive Radar-Based Parameter Estimation of Low Earth Orbit Debris Targets
###### Abstract
Whereas this system has demonstrated space debris detection capability, the observation volume illuminated is still limited, and the RCS is dependent on backscatter in the Rayleigh region. Due to the increased demand for SDA technologies, the private sector has become involved. LeoLabs (Menlo Park, CA, USA), for example, has built several S-band phased array (electronically beamformed) radars in North and Central America, Europe, Australia, and New Zealand [15]. The Kiwi Space Radar is the New Zealand-based array that became operational in 2019 and was designed to detect targets as small as 2 cm in diameter. Previous research efforts propose passive bistatic systems for imaging and tracking space debris [16]. In this study, the Murchison Widefield Array (MWA) in Western Australia can be additionally tasked to receive commercial FM reflections from the moon. The processing methods were shown to have high feasibility with some drawbacks due to the radar configuration and electromagnetic phenomena. The relatively large wavelength of FM signals incites Rayleigh scattering for smaller debris targets, which may result in significantly smaller RCS values. In addition, the debris motion results in RCS and radar geometry variation, causing decreased image quality. This results in reduced performance for targets less than 1 m in diameter. Another novel approach involves the combination of active and passive radar modalities [17]. Logically, employing the benefits of both radar modalities would be ideal. However, this relies on the concept of sensor fusion, which introduces additional complex ity to the signal processing, time synchronization, and sources of error. A point of concern noted in [17] was that the bistatic plots often have no correspondence with active plots for the same debris target. It is obvious from the examples listed above that there are many techniques employed in pursuit of effective SDA. The common theme is to build or repurpose sensors across the globe to maximize orbital coverage. However, these designs come at significant time and capital costs. The arrays are large and require years to design and build. The problem of spectrum sharing must also be mentioned. Due to advancements in imaging, communications, navigation, and handheld devices, there is significant demand for operation over the same frequency bands. This has forced the need for radio spectrum frequency allocations [18]. The spectrum constraint has inspired research into adaptive frequency-hopping techniques [19; 20; 21; 22]. This work proposes a forward scatter passive bistatic radar processing chain purposed towards enhancing SDA efforts through Doppler extraction. Most operational radars are active, i.e., they both transmit and receive signals. Passive radars are receive-only systems that exploit transmitters that are already available and in use for other applications, thereby allowing for several benefits [23]. The first benefit is time and cost. Only a receiver module or station must be designed and built. The next benefit is the lack of transmitted signals and the cost of generating these. This alleviates the issue of spectrum sharing, earning passive radar the title of 'Green Radar' [24] as the operation depends solely on illuminators of opportunity (IOOs), which are signals already present in the environment. An added advantage of green radar is its covert operation. Receive-only systems are generally impossible to detect and jam. The third benefit is the coverage. Since bistatic system performance is also dependent on the IOO [25], one can be chosen to maximize coverage. In this paper, it is proposed that the IOO be the L1 coarse/acquisition (C/A) GPS signal, which oscillates at 1575.42 MHz. This signal illuminates about 38% of the earth's surface at any given instant [26], allowing receivers to be networked across continents. As a result of the GPS satellite altitude, the radar geometry is virtually constant, leading to consistencies in RCS values and overall system performance.