Characterizing Jitter in the Hyperspectral Thermal Imager (HyTI) Satellite

Core Concepts

A novel low-cost optical metrology system is used to characterize the jitter in the Hyperspectral Thermal Imager (HyTI) satellite, a 6U cubesat, due to its reaction wheels and cryocooler. The system can identify modal frequencies and attribute them to specific vibratory sources, enabling optimization of the satellite's design to meet stringent pointing requirements.

Abstract

The Hyperspectral Thermal Imager (HyTI) is a 6U cubesat mission that aims to obtain high spatial, spectral, and temporal resolution long-wave infrared images of Earth's surface. To meet the mission's science requirements, the satellite's pointing accuracy must not exceed 0.014 mrad (approximately 2.89 arcseconds) over the 0.5 ms integration time due to jitter.
The authors present a novel low-cost optical metrology system to characterize the jitter in HyTI. The system uses a laser, a mirror mounted on the satellite, and a lateral effect position sensor to measure small deflections of the laser beam, which are then used to calculate the satellite's jitter.
The authors conducted a series of experiments by incrementally adding vibrational sources, such as the reaction wheels and cryocooler, and measuring the resulting jitter. They analyzed the power spectral density (PSD) plots to identify modal frequencies and attribute them to specific vibratory sources. The results show that the jitter from the reaction wheels meets the HyTI system requirements within 3σ.
The key highlights and insights from the experiments are:

Configuration 1 (air bearing off, bus electronics on): Established a baseline for the ambient environment in the cleanroom, identifying modal frequencies likely due to the environment.

Configuration 2 (air bearing off, cryocooler and payload on): Identified modal frequencies at multiples of the cryocooler's operating frequency of 60 Hz, as well as some additional frequencies.

Configuration 3 (air bearing on, all other components off): Showed that the air bearing introduced high-amplitude oscillations, leading to noisy data and a modal frequency at 120 Hz due to the fluorescent lighting.

Configuration 4 (air bearing on, ADCS electronics on): Demonstrated that the ADCS electronics did not introduce any additional modal frequencies.

Configuration 5 (air bearing on, x-direction reaction wheel on): The 3σ frame rate jitter value was smaller than the HyTI mission requirement for the integration time jitter.

Configuration 6 (air bearing on, z-direction reaction wheel on): Showed that the addition of angular momentum along the optical axis helped stabilize the system, reducing the frame rate jitter.

Configuration 7 (air bearing on, x and z-direction reaction wheels on): Identified two new modal frequencies at 209 and 368 Hz, demonstrating the benefits of investigating the system dynamics with a fully integrated setup.

The authors also discuss the limitations of their approach, including the impact of the air bearing imbalance, the added mass and inertia of the support jig, and the need for a higher-frequency ADC to directly measure the integration time jitter.
Overall, this work presents a novel and cost-effective method for characterizing jitter in small satellite systems, which can inform the design and optimization of future missions to meet stringent pointing requirements.

Stats

The variance for the frame rate jitter in Configuration 5 was 0.35 arcsec^2.
The variance for the frame rate jitter in Configuration 6 was 0.021 arcsec^2.
The variance for the frame rate jitter in Configuration 7 was 0.019 arcsec^2.

Quotes

"The 3σ frame rate jitter value is smaller than the HyTI mission requirement for the integration time jitter."
"Cleaner results will allow us to apply the technique from configurations 1 and 2 to estimate the integration time jitter for configurations using the air bearing."
"Better characterizing the damping properties of the clamp will allow us to adjust the results presented here to be more representative of the actual satellite's response."

Key Insights Distilled From

Jitter Characterization of the HyTI Satellite

by Chase Urasak... at arxiv.org 04-25-2024

https://arxiv.org/pdf/2404.15575.pdf

Deeper Inquiries

How could the authors further improve the metrology system to directly measure the integration time jitter, rather than estimating it from the frame rate jitter?

To directly measure the integration time jitter, the authors could enhance the metrology system by implementing a higher sampling frequency for the lateral effect position sensor. By increasing the sampling rate of the sensor to at least double the integration time frequency of 2000 Hz, they would be able to capture the high-frequency variations in the laser beam position accurately. This would allow for a more precise measurement of the integration time jitter without relying on estimates from the frame rate jitter. Additionally, incorporating a more sensitive and stable sensor with higher resolution could help in detecting smaller displacements and reducing noise in the measurements. Calibration of the system to ensure accuracy and consistency in the measurements is also crucial for direct measurement of integration time jitter.

What other types of small satellite systems could benefit from this jitter characterization approach, and how would the methodology need to be adapted for different use cases?

Various small satellite systems, such as Earth observation satellites, communication satellites, and scientific research satellites, could benefit from this jitter characterization approach. For Earth observation satellites, precise pointing accuracy is essential for capturing high-resolution images of the Earth's surface. Communication satellites require stable pointing to maintain connectivity with ground stations. Scientific research satellites, like those studying celestial bodies or atmospheric phenomena, rely on accurate pointing for data collection.
The methodology would need to be adapted based on the specific requirements of each satellite system. For communication satellites, the focus may be on lower-frequency vibrations from onboard equipment, requiring adjustments in the frequency range of analysis. Scientific research satellites may need to account for external factors like solar radiation pressure or gravitational forces, which could introduce additional sources of jitter. Earth observation satellites might require a more extensive study of modal frequencies to optimize imaging capabilities. Customizing the metrology system and analysis techniques to suit the unique characteristics and operational needs of each satellite system is crucial for effective jitter characterization.

How could the insights gained from this study on the modal frequencies and their sources be used to inform the structural design and component placement of future small satellite missions?

The insights on modal frequencies and their sources obtained from this study can provide valuable information for optimizing the structural design and component placement of future small satellite missions. By understanding the specific frequencies at which vibrations occur and their sources, engineers can design satellite structures to minimize resonant frequencies that could amplify vibrations.
For structural design, the modal frequencies can guide the selection of materials and damping mechanisms to reduce vibrations and improve stability. Component placement can be optimized to minimize interference with critical systems and reduce the transmission of vibrations. By strategically locating components with known vibrational sources, engineers can mitigate the impact of jitter on sensitive payloads and ensure the overall performance of the satellite system.
Overall, leveraging the insights from modal frequency analysis can lead to more robust and reliable small satellite designs, enhancing mission success and data quality.