Precision Pointing of IBEX-Lo Observations

TL;DR

This study validates the precision of IBEX-Lo instrument pointing using star sensor data, confirming no systematic corrections are needed for accurate interstellar gas flow measurements, and demonstrating the star sensor's utility for spacecraft orientation.

Contribution

The paper demonstrates that the IBEX-Lo star sensor provides accurate, independent spacecraft pointing data, confirming no systematic boresight correction is necessary for interstellar gas flow analysis.

Findings

Star sensor data agrees with star catalog positions within uncertainties.

No significant deviations found between star sensor and ACS pointing.

Star sensor can independently determine spacecraft spin axis and correct spin phase.

Abstract

Post-launch boresight of the IBEX-Lo instrument onboard the Interstellar Boundary Explorer (IBEX) is determined based on IBEX-Lo Star Sensor observations. Accurate information on the boresight of the neutral gas camera is essential for precise determination of interstellar gas flow parameters. Utilizing spin-phase information from the spacecraft attitude control system (ACS), positions of stars observed by the Star Sensor during two years of IBEX measurements were analyzed and compared with positions obtained from a star catalog. No statistically significant differences were observed beyond those expected from the pre-launch uncertainty in the Star Sensor mounting. Based on the star observations and their positions in the spacecraft reference system, pointing of the IBEX satellite spin axis was determined and compared with the pointing obtained from the ACS. Again, no statistically significant deviations were observed. We conclude that no systematic correction for boresight geometry is needed in the analysis of IBEX-Lo observations to determine neutral interstellar gas flow properties. A stack-up of uncertainties in attitude knowledge shows that the instantaneous IBEX-Lo pointing is determined to within $\sim 0.1\degr$ in both spin angle and elevation using either the Star Sensor or the ACS. Further, the Star Sensor can be used to independently determine the spacecraft spin axis. Thus, Star Sensor data can be used reliably to correct the spin phase when the Star Tracker (used by the ACS) is disabled by bright objects in its field-of-view. The Star Sensor can also determine the spin axis during most orbits and thus provides redundancy for the Star Tracker.