Session
Session IX: Advanced Technologies I
Location
Utah State University, Logan, UT
Abstract
Pathfinder-3 (PTD-3) spacecraft is the third vehicle of the NASA Pathfinder Technology Demonstrator (PTD) series, which are a collection of 6U CubeSats launched in Low Earth Orbit (LEO) to demonstrate innovative payload capabilities. The payloads are hosted on commercially developed satellites designed and manufactured by Terran Orbital Corporation (TOC) with a goal to support a wide array of technology demonstration missions through a flexible architecture that can be tailored for custom needs. PTD-3 hosts a high data rate laser communications payload that does not include its own pointing acquisition and control system, and, therefore, is dependent on accurate bus pointing to establish and maintain the space-to-ground (S2G) link for optical communication transmission.
Traditional CubeSat attitude control sensors (i.e. star trackers, gyros) are too coarse to achieve the pointing accuracy and bias requirements of less than 6.2 arcsec (30 µrad) and 3.1 arcsec (15 µrad), respectively. Thus, direct measurements of line-of-sight error from the laser communication payload is provided as feedback into the bus attitude control loop to achieve the pointing accuracy required for the mission. Standard bus attitude control without payload feedback, using star trackers, gyros, and reaction wheels, is implemented to achieve initial acquisition of the ground optical terminal. After acquisition, payload line-of-sight error measurements serve as the source of attitude control feedback for the bus to achieve finer pointing accuracy.
This paper presents the design of the spacecraft with a focus on the pointing control architecture, design drivers, preliminary performance predications, and performance evaluation of the on-orbit system. Key design and analysis topics impacting pointing performance centralized around payload-to-bus frame misalignments (both thermal and mechanical), high frequency-induced reaction wheel jitter in the presence of spacecraft flexible modes and mitigation strategies, reaction wheel zero crossings, and the role of TLE induced ephemeris propagation error. The discussion concludes with demonstrations of on-orbit pointing accuracy achieving approximately 0.75 arcsec (4.0 µrad) when payload feedback is in the loop. To the authors’ knowledge, this is among the best CubeSat demonstrated bus pointing achieved while ground tracking. The sub-arcsecond accuracy is accomplished via a low-cost CubeSat architecture (no multi-stage pointing loops with gimbals, fine steering mirrors, etc.) that can be immediately applied to support other similar laser communication systems or observation payloads capable of providing the spacecraft with low noise small angle attitude error measurements.
From Design to Orbit: Engineering Sub-Arcsecond CubeSat Pointing Performance on Pathfinder-3
Utah State University, Logan, UT
Pathfinder-3 (PTD-3) spacecraft is the third vehicle of the NASA Pathfinder Technology Demonstrator (PTD) series, which are a collection of 6U CubeSats launched in Low Earth Orbit (LEO) to demonstrate innovative payload capabilities. The payloads are hosted on commercially developed satellites designed and manufactured by Terran Orbital Corporation (TOC) with a goal to support a wide array of technology demonstration missions through a flexible architecture that can be tailored for custom needs. PTD-3 hosts a high data rate laser communications payload that does not include its own pointing acquisition and control system, and, therefore, is dependent on accurate bus pointing to establish and maintain the space-to-ground (S2G) link for optical communication transmission.
Traditional CubeSat attitude control sensors (i.e. star trackers, gyros) are too coarse to achieve the pointing accuracy and bias requirements of less than 6.2 arcsec (30 µrad) and 3.1 arcsec (15 µrad), respectively. Thus, direct measurements of line-of-sight error from the laser communication payload is provided as feedback into the bus attitude control loop to achieve the pointing accuracy required for the mission. Standard bus attitude control without payload feedback, using star trackers, gyros, and reaction wheels, is implemented to achieve initial acquisition of the ground optical terminal. After acquisition, payload line-of-sight error measurements serve as the source of attitude control feedback for the bus to achieve finer pointing accuracy.
This paper presents the design of the spacecraft with a focus on the pointing control architecture, design drivers, preliminary performance predications, and performance evaluation of the on-orbit system. Key design and analysis topics impacting pointing performance centralized around payload-to-bus frame misalignments (both thermal and mechanical), high frequency-induced reaction wheel jitter in the presence of spacecraft flexible modes and mitigation strategies, reaction wheel zero crossings, and the role of TLE induced ephemeris propagation error. The discussion concludes with demonstrations of on-orbit pointing accuracy achieving approximately 0.75 arcsec (4.0 µrad) when payload feedback is in the loop. To the authors’ knowledge, this is among the best CubeSat demonstrated bus pointing achieved while ground tracking. The sub-arcsecond accuracy is accomplished via a low-cost CubeSat architecture (no multi-stage pointing loops with gimbals, fine steering mirrors, etc.) that can be immediately applied to support other similar laser communication systems or observation payloads capable of providing the spacecraft with low noise small angle attitude error measurements.
