Jake Singh, Rhea Space Activity; Joshua Baumann, Rhea Space Activity; Matthew Gorban, Rhea Space Activity; Christopher Dinelli, Rhea Space Activity; Eileen Ryan, New Mexico Tech/MRO; William Ryan, New Mexico Tech/MRO
Keywords: Satellite spectral radiance, optical navigation, optical filtering, Starlink
Abstract:
With the rising threat of GPS denial and the proliferation of low Earth orbit (LEO) satellites, optical navigation using artificial satellites has emerged as a promising Alternative Positioning, Navigation, and Timing (Alt-PNT) method for terrestrial applications. This technique determines the observer’s position by analyzing timestamped images of target satellites with known ephemerides against background star fields. The primary challenge of optical navigation in a terrestrial setting is actually detecting the light reflected by satellites, which becomes increasingly difficult with daytime background radiation, cloud coverage, high Sun phase angle, or even plasma formation in the case of high-speed hypersonic flight. Spectral information on the signal would be beneficial for designing optical filters that selectively remove background noise and boost the signal-to-noise ratio (SNR), enabling detection in a wider range of conditions.
The deployment of the Starlink constellation has spawned numerous publications on the satellites’ apparent magnitude and bi-directional reflectance functions (BRDFs), but there remains a clear gap in the literature on the spectral radiance of any Earth-orbiting satellites. Seeking to address this, the authors conducted a four-night observation campaign with the Magdalena Ridge Observatory’s 2.4-meter telescope. Spectrometer collections (over a 380 to 860 nm range) were performed on 50 satellites and orbital objects across LEO, MEO, and GEO. Of these, 22 collections were Starlinks spanning all three satellite generations. For the Starlink family in particular, spectral analysis reveals a local peak in radiance around 475 nm, a shape resembling the solar irradiance curve, and a distinct spike in the near infrared for the Starlink v2 satellites. Spectral characteristics of the other orbital objects will be discussed at length in this paper.
The spectrometer used for these collections reported only relative flux density across wavelength bins, meaning absolute radiance (e.g., in W/cm²-sr-µm) was not directly measured. We outline a procedure used to estimate absolute radiance from relative flux density, based on corresponding apparent magnitude measurements of the satellites. We also provide preliminary methods for estimating the SNR in a CMOS detector across a variety of optical navigation scenarios. The computed SNR serves as a predictor for the performance of target detection algorithms within the image processing stage of optical navigation. Finally, we illustrate some core optical navigation concepts and present an observability study on Starlink satellites for terrestrial applications, providing insight into their Alt-PNT potential.
The dataset from this observation campaign provides valuable insight into the feasibility of optically tracking satellites in challenging observational environments. For instance, short-wave infrared (SWIR) sensors may enhance the ability to track Starlink v2 satellites in daylight or thin cloud cover due to their observed near-infrared radiance peak. Additionally, a reentry vehicle navigating through RF-blocking plasma may detect and track satellites using carefully designed optical filters. Such filters can be optimized to maximize satellite signal while excluding a portion of irradiant plasma, whose spectral emissions can be predicted via dynamic aerothermal simulations. The authors expect this dataset and its corresponding analyses to provide value not only to the Alt-PNT community but also to researchers in Space Domain Awareness (SDA), astronomy, and any other fields concerned with optical detection and tracking of satellites.
Date of Conference: September 16-19, 2025
Track: Satellite Characterization