Raymond Wright, Ball Aerospace; Michael Gordon, Ball Aerospace; Jeffrey Van Cleve, Ball Aerospace; Hans Schroots-Barrios, Ball Aerospace
Keywords: Space Domain Awareness, Space Situational Awareness, SDA, SSA, Infrared, Visible, Instruments, Payloads, Imaging
Abstract:
The typical phenomenology Space Domain Awareness (SDA) missions rely on is designing a system that observes in the visible spectrum. Mission design in the visible bandwidth provides commonality with requirements and mission metrics from other missions. However, due to the visible bandwidths reliance on reflected light, issues such as eclipses, poor illumination scenarios, and smaller objects hinder an SDA mission. Using a different frequency band for SDA missions is a solution to some of the limitations present when only observing in the visible. Expanding the SDA mission into the infrared also improves threat detection sensitivity which allows it to see further, enabling threat detection monitoring of the cislunar regime.
The infrared bandwidth spans from approximately 0.7µm to 1000µm, which is more than 1000 times larger than the visible spectrum (approximately 0.4µm to 0.7µm). The visible spectrum is commonly divided into categories called colors and the same can be done in the infrared. While there is no accepted common standard of the division in the infrared, the reflected infrared spectrum occurs approximately between 0.7µm to 3µm; thermal emissions approximately span 3µm to 15µm, and it is in these two categories (reflected and emitted) that an SDA mission can be optimized to detect threats. This paper focuses on the how the reflected and thermally emitted IR can be used in missions to observe in poor illumination scenarios and improves sensitivity. The missions discussed in this paper focus on the Earth-Moon system mission design, but these processes can be applied to all mission types.
Modeling a mission typically starts with what objects are going to be observed. To model an Object correctly, its sources of energy need to be defined. The two sources of light an object can provide in the Visible and IR spectrums are reflected and emitted light. The Sun is the primary source of reflected energy in the Earth-Moon system. The Sun is also the primary source of passive energy that can be converted by the Object into an internal temperature. An Objects makeup will vary the amount of reflection and absorption of the Suns energy, but if an Object is trying to reduce its reflective surface area, then it is increasing its absorption rate, which increases temperature and can increase the reradiation of heat. The Objects complete spectrum will be a combination of reflected Sunlight and its thermal emissions from its temperature. From the conservation of energy, high energy objects produce fewer photons than lower energy objects. With the increased photon generation in the longer wavelengths, the object will appear brighter. This increase in photons from internal thermal emissions over a reliance on reflected light also increases an SDA mission sensitivity enabling it to see the same target that is further away.
The mission (also called an Observer) will have two geometric relationships that define how much light it can observe. Reflection relies on a triangular relationship between the Sun, Object, and Observer. This triangular relationship can create poor viewing angles where the Observer will see little to no reflected sunlight. Observing in the infrared can remove this dependence and instead create a direct line-of-sight relationship between the Observer and the Object. Thermal emissions are radiated out uniformly, removing the triangular dependency of reflected light. If an Object also contains an internal power source, an observer measuring in the thermal IR will be able to detect when this power source becomes powered or unpowered. Direct line of sight, internal emissions, and the Objects uniform radiation allow more photons to be detected at the Observer, improving the sensitivity. The improved sensitivity allows the Observer to detect objects that are further away. Ground based SDA sensitivity suffers in the infrared due to the absorption of the infrared wavelengths from water. Space based SDA missions will not suffer the same degradation of performance as ground-based SDA missions and can take full advantage of detecting threats in this bandwidth.
Finally, while observing in the IR does have advantages over the Visible spectrum, there is an engineering cost. The Observer needs to be sufficiently cooled such that it is not blinded by its own thermal emissions. Modifications from the standard visible design techniques need to be incorporated into a mission design to handle unwanted thermal emissions and stray light. However, the cost does not preclude space-based detection in the infrared for SDA missions since it is a necessary update to achieve mission goals.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation