As modern space operations become more complex, the need for precise attitude determination and advanced situational awareness continues to grow. Star trackers have been a key part of satellite navigation, providing unmatched accuracy in determining spacecraft orientation. Recent advancements in sensor technology have introduced a new capability, allowing star trackers to support Space Domain Awareness. By combining both functions, these systems offer an efficient solution for tracking celestial reference points and monitoring objects in orbit.
This article explores the fundamentals of star trackers, innovations in testing with starfield simulators, and their expanding role in space surveillance, providing insight into the evolving landscape of space-based sensing technologies.
Star Trackers are indispensable optical devices that enable satellites to determine their orientation in space by referencing star fields. As a core part of the Attitude Determination and Control System (ADCS), they provide unmatched precision and reliability for various space missions, from Earth observation to deep-space exploration.
Key Functional Processes:
Importance in ADCS:
Challenges and Innovations:
Star Trackers face challenges like susceptibility to space radiation and interference from bright celestial objects. Yet, developments in sensor durability and AI-driven algorithms are paving the way for more adaptive, efficient, and resilient star-tracking systems, ensuring they remain vital to the space industry.
Ensuring the accuracy and reliability of star trackers before deployment is a crucial step in satellite mission preparation. Ground testing is necessary to validate performance under controlled conditions, but traditional night-sky testing presents logistical challenges such as weather dependency, restricted visibility, and limited access to observatories. To address these issues, engineers have developed starfield simulators which are sophisticated optical systems designed to generate realistic star patterns in laboratory settings. These simulators enable both open-loop and closed-loop testing, allowing for a comprehensive assessment of star tracker functionality before launch.
Open-Loop Testing: Functional Verification
In open-loop testing, a star tracker passively observes a simulated star field without interacting with a control system. This method allows engineers to verify basic functions such as star recognition, centroiding accuracy, and attitude determination algorithms. The Jet Propulsion Laboratory (JPL) has developed a compact Star Field Simulator (SFS) specifically designed for CubeSat-scale star trackers. The JPL SFS projects dynamic star fields onto a high-resolution display, which, when paired with a collimating lens, mimics the appearance of stars at infinite distances. This setup eliminates the need for physical modifications to the star tracker, preserving a "test-as-you-fly" verification approach.
Similarly, the German Aerospace Center (DLR) has developed an Optical Sky Field Simulator (OSI), which operates with a collimated beam to replicate real sky observations. This system enables static and dynamic testing, simulating real star patterns based on provided attitude quaternions and body rotation rates. Open-loop testing with such simulators ensures that a star tracker can correctly process images, identify stars, and compute attitude data in a controlled environment.
Closed-Loop Testing: Dynamic Performance Validation
Closed-loop testing extends open-loop evaluations by incorporating the star tracker into a feedback system that simulates realistic spacecraft dynamics. This method allows engineers to observe how the tracker responds to real-time attitude changes, verifying its ability to maintain accurate orientation during motion.
JPL’s SFS has been validated through closed-loop tests on air-bearing platforms, where a star tracker was subjected to simulated spacecraft maneuvers. The star tracker’s measured attitude was compared against known reference orientations, confirming its tracking stability and response accuracy. This validation process demonstrated that the simulator could replicate real-sky conditions with a high degree of fidelity.
A separate closed-loop test conducted at DLR integrated an optical simulator with a real-time space vehicle simulation to synchronize simulated sky images with vehicle dynamics. By feeding real-time attitude data to the optical simulator, the system adjusted the projected star field accordingly, allowing for continuous verification of star tracker outputs. One key test evaluated the turnaround time (the interval between image capture and attitude computation) which was found to be well within the required 0.5-second limit for real-time operation.
Simulating Night-Sky Conditions for Validation
One of the key challenges in ground-based star tracker testing is replicating the vast dynamic range of real celestial environments. Both the JPL and DLR simulators address this by utilizing star catalogs such as the Hipparcos database to generate accurate star positions and magnitudes. However, due to the limitations of digital displays, absolute brightness values must be calibrated to ensure that simulated stars match real-world conditions. Engineers use Gaussian-based intensity modeling and manual fine-tuning to align the simulator’s output with real-sky observations.
To further enhance realism, JPL conducted a night-sky validation test at the Table Mountain Observatory (TMO), comparing actual celestial observations against simulator-generated star fields. The results confirmed that the simulator’s projected images closely matched real astronomical data, solidifying its reliability for pre-flight testing.
Similarly, DLR's OSI system was tested for real-time response accuracy, measuring delays between simulated image updates and star tracker outputs. The system achieved a response time of approximately 90 milliseconds, confirming that it could support real-time hardware-in-the-loop (HIL) testing with minimal latency. Additionally, alignment calibration procedures ensured that simulated star positions remained consistent across repeated tests.
Star trackers have long been essential tools for spacecraft attitude determination, offering high-precision orientation data by analyzing star positions. However, advancements in optical sensor technology have enabled these systems to serve a dual-use function, not only guiding spacecraft but also contributing to Space Domain Awareness (SDA). SDA is critical for monitoring and tracking objects in Earth's orbit, particularly in congested environments such as Low Earth Orbit (LEO) and Geostationary Orbit (GEO). A new generation of star trackers is being developed to enhance SDA capabilities while maintaining efficient Size, Weight, Power, and Cost (SWaP-C) characteristics.
Star Trackers as SDA Sensors
The fundamental principle behind SDA is similar to traditional star tracking, both rely on optical sensors to detect and track objects against a reference star catalog. However, instead of focusing solely on celestial navigation, SDA-equipped star trackers can identify and monitor Resident Space Objects (RSOs), such as satellites, debris, and potential threats. These new star trackers offer simultaneous star tracking and full-frame imaging at 10 frames per second, enabling real-time observation of space objects .
Key features of dual-use star trackers for SDA include:
In-Space Testing & Operational Validation
To validate the dual-use functionality of star trackers, the U.S. Space Force (USSF) is conducting in-space testing. The star tracker is scheduled to fly as a hosted payload on the Tetra-3 satellite, which is expected to launch into GEO orbit. This test aims to verify the Technical Readiness Level (TRL) of the star tracker, advancing it from TRL-5 to TRL-7 through demonstration in a relevant space environment .
During testing, the star tracker will undergo:
The star tracker’s SDA functionality will be tested in two orientations:
Data collected will be transmitted to ground-based processing systems, where advanced image processing algorithms will be used to identify and characterize space objects .
Future Implications for Space Operations
Integrating SDA functionality into star trackers represents a significant leap in space situational awareness. By leveraging existing spacecraft navigation systems for dual-use monitoring, operators can enhance orbital safety and security without requiring additional payloads. This is particularly important for military and defense applications, where real-time knowledge of potential threats or adversary satellites is crucial.
Furthermore, the ability to detect and track RSOs in real-time will:
Star trackers remain fundamental to satellite attitude determination, and advancements in testing methodologies and expanded applications continue to enhance their capabilities. The development of starfield simulators has addressed key challenges in ground-based verification, allowing for more rigorous testing in both open and closed loop environments. These improvements contribute to the overall reliability of satellite navigation, ensuring accurate performance before launch.
Beyond their primary function, star trackers are now being adapted for Space Domain Awareness, demonstrating their potential to detect and track objects in orbit. By leveraging full-frame imaging and advanced processing techniques, dual-use star trackers offer a cost-effective solution for monitoring space activity while maintaining their role in spacecraft orientation. As space traffic increases and mission demands grow, these innovations will play an increasingly important role in both operational efficiency and space security.
Discover more about Star Trackers in the Attitude Determination and Control Systems (ACDS) category of the SmallSat Catalog. The SmallSat Catalog is a curated digital portal for the smallsat industry, showcasing hundreds of products and services from across the industry. As a one-stop shop for nanosatellite and small satellite missions, the SmallSat Catalog provides everything a mission builder needs to plan a successful smallsat mission.
To learn more about innovations in advanced deployable solar arrays in small satellites, please explore the following research works on this topic.
Samaan, M. A., Steffes, S. R., & Theil, S. (2011). Star tracker real-time hardware in the loop testing using optical star simulator. Spaceflight Mechanics, 140.
Rufino, G., Accardo, D., Grassi, M., Fasano, G., Renga, A., & Tancredi, U. (2013). Real‐Time Hardware‐in‐the‐Loop Tests of Star Tracker Algorithms. International Journal of Aerospace Engineering, 2013(1), 505720.
Filipe, N., Jones-Wilson, L., Mohan, S., Lo, K., & Jones-Wilson, W. (2017). Miniaturized star tracker stimulator for closed-loop testing of cubesats. Journal of Guidance, Control, and Dynamics, 40(12), 3239-3246.
Plotke, E., Lai, P. C., Chan, A., Ewart, R. M., Miller, K., & Griesbach, J. (2021, September). Dual use star tracker and space domain awareness sensor in-space test. In Proceedings of the Advanced Maui Optical and Space Surveillance Technologies Conference.
