Modeling of intensified high dynamic star tracker

Citation

Yan, Jinyun, Jie Jiang, and Guangjun Zhang. “Modeling of intensified high dynamic star tracker.” Optics Express, vol. 25, no. 2, 23 Jan. 2017, pp. 927–48.

Keywords

• intensified high dynamic star tracker (IHDST)
• quantum transfer model
• centroiding accuracy
• dynamic performance

Brief

An intensified high dynamic star tracker (IHDST) uses an image intensifier to significantly improve star detection sensitivity during short exposure times, thereby achieving high dynamic performance. The sources describe the IHDST's imaging process, modeling it as a compound stochastic process for quantum transfer and using the convolution of point spread functions for spatial spreading. They also analyze the factors influencing IHDST star locating accuracy, primarily exposure time and gain control voltage, and propose a working parameter optimization strategy to enhance accuracy and dynamic performance.

Summary

This article presents a novel imaging model for an intensified high dynamic star tracker (IHDST), a device designed to determine a spacecraft's orientation with high accuracy even at high angular velocities. The model is particularly important because traditional star trackers struggle in high-dynamic scenarios where the spacecraft's rotation causes star images to smear across image pixels, reducing accuracy.

Here's a breakdown of the article's key points:

• The Need for IHDST: Traditional star trackers face limitations in high-dynamic situations because longer exposure times, needed to gather enough light from faint stars, result in motion blur. IHDST addresses this limitation by incorporating an image intensifier. This component amplifies the incoming starlight, enabling the use of shorter exposure times without compromising sensitivity. Consequently, IHDSTs can achieve accurate star tracking even when the spacecraft is rotating rapidly.
• Modeling the IHDST: The authors construct a comprehensive model of the IHDST's imaging process, accounting for both the quantum transfer of photons and electrons within the intensifier, and the spatial spreading of light as it traverses the system's components.

1. Quantum Transfer: This aspect of the model treats the conversion of photons to electrons and their subsequent multiplication as a compound stochastic process. This approach accurately captures the inherent randomness in these processes and allows the researchers to derive the probability distribution of the output signal—a crucial aspect overlooked in previous models.
2. Spatial Spreading: The model characterizes spatial spreading, which blurs the star image, as a series of convolutions involving the point spread functions of the optical lens, electron lenses within the intensifier, and the fiber optic taper that couples the intensifier to the image sensor.

• Analyzing Star Locating Accuracy: A key factor in determining a spacecraft's attitude is accurately pinpointing the center of the star image, a process called centroiding. The sources use the developed imaging model, along with a Monte Carlo simulation method, to analyze how the accuracy of star centroiding is affected by:

1. Exposure Time: Longer exposure times lead to more light captured (higher signal) but also increase motion blur. Conversely, shorter exposures minimize blur but might result in weak signals.
2. Gain Control Voltage: This parameter controls the image intensifier's gain. Higher gain amplifies the signal, improving detectability, but can also amplify noise.

• Optimizing IHDST Parameters: Through their analysis, the authors demonstrate a strategy for selecting optimal exposure times and gain control voltage settings for different star magnitudes and angular velocities. This strategy aims to minimize centroiding error and ensure accurate and stable attitude output from the IHDST even under high dynamic conditions.
• Validating the Model: The accuracy of the proposed imaging model and the effectiveness of the parameter optimization strategy are validated through a combination of laboratory experiments and a night sky experiment. The results confirm the model's ability to accurately predict the IHDST's imaging characteristics.

The authors' work provides a significant contribution to the field of star tracker technology. The developed imaging model, with its emphasis on both quantum transfer and spatial spreading, offers a more precise representation of the IHDST's behavior compared to previous models. This enhanced understanding, coupled with the proposed parameter optimization strategy, paves the way for developing IHDSTs capable of delivering highly accurate attitude information even when operating in challenging high-dynamic environments.

Origin: https://opg.optica.org/oe/fulltext.cfm?uri=oe-25-2-927&id=357303