Session
Session IV: Advanced Technology 2-Enterprise
Location
Salt Palace Convention Center, Salt Lake City, UT
Abstract
The GHGSat-D spacecraft, a precursor demonstration to GHGSat’s commercial constellation, was launched on June 21 2016 into a polar Sun-synchronous orbit with altitude 507 km and local time of descending node 10:30. The spacecraft successfully demonstrated GHGSat’s novel payload’s ability to measure and monitor precise greenhouse gas emissions from low Earth orbit. Such measurements were enabled by custom target tracking maneuvers allowing its imaging spectrometer to pan across ground targets of interest at prescribed rates. In July 2017 one of its three reaction wheels exhibited signs of high friction, resulting in its discontinued use. With only two functional reaction wheels and a set of three orthogonal air-core magnetorquers, the spacecraft control authority was greatly reduced and this created a challenge to continue nominal operations. This prompted development of new flight control software and operational strategies to allow the spacecraft to continue to fulfill its mission until the end of its orbital lifetime when it passively de-orbited due to the influence of atmospheric drag on December 9, 2024. A particular challenge to overcome was that magnetorquers were designed only for momentum unloading, not for direct attitude control; the magnetorquers had less than 1% the torque capability of the reaction wheels. In this paper we describe the flight implementation of the control laws to enable under-actuated operations, and demonstrate how well they worked in practice with several case studies supported by on-orbit telemetry. We examine both nominal attitude modes (e.g., nadir-tracking) and payload target tracking modes. We also discuss operational aspects such as target setup methodology – predicting the expected control authority as a function of target location on the Earth, and customizing the momentum control set points in order to ensure the highest chance of success for a given observation. With the new strategies in place, close to 80% of planned target tracking observations could be performed successfully, greatly extending the life and utility of the mission by an additional 7 years.
Document Type
Event
Under-Actuated Target Tracking Performance - Flight Results From the GHGSat-D Spacecraft
Salt Palace Convention Center, Salt Lake City, UT
The GHGSat-D spacecraft, a precursor demonstration to GHGSat’s commercial constellation, was launched on June 21 2016 into a polar Sun-synchronous orbit with altitude 507 km and local time of descending node 10:30. The spacecraft successfully demonstrated GHGSat’s novel payload’s ability to measure and monitor precise greenhouse gas emissions from low Earth orbit. Such measurements were enabled by custom target tracking maneuvers allowing its imaging spectrometer to pan across ground targets of interest at prescribed rates. In July 2017 one of its three reaction wheels exhibited signs of high friction, resulting in its discontinued use. With only two functional reaction wheels and a set of three orthogonal air-core magnetorquers, the spacecraft control authority was greatly reduced and this created a challenge to continue nominal operations. This prompted development of new flight control software and operational strategies to allow the spacecraft to continue to fulfill its mission until the end of its orbital lifetime when it passively de-orbited due to the influence of atmospheric drag on December 9, 2024. A particular challenge to overcome was that magnetorquers were designed only for momentum unloading, not for direct attitude control; the magnetorquers had less than 1% the torque capability of the reaction wheels. In this paper we describe the flight implementation of the control laws to enable under-actuated operations, and demonstrate how well they worked in practice with several case studies supported by on-orbit telemetry. We examine both nominal attitude modes (e.g., nadir-tracking) and payload target tracking modes. We also discuss operational aspects such as target setup methodology – predicting the expected control authority as a function of target location on the Earth, and customizing the momentum control set points in order to ensure the highest chance of success for a given observation. With the new strategies in place, close to 80% of planned target tracking observations could be performed successfully, greatly extending the life and utility of the mission by an additional 7 years.
