Maintaining Software for Active Missions: a Case Study of Chandra's CTI Problem

During early flight operation, the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer (ACIS) suffered degradation of charge transfer inefficiency (CTI) due to radiation induced charge trapping. The ACIS front-illuminated detector CTI has significantly degraded over time with respect to the intrinsic energy resolution of the chips. In this presentation we use the degradation of CTI in the ACIS camera as an example of how software was adapted to fix a significant and unforeseen post-launch problem in on-board hardware. We will discuss the impact of CTI on events, RMF and gain files. Then, we will discuss the software updates necessary to alleviate the degradation effects and to improve spectral resolution. Finally, we will highlight other ACIS hardware problems and their corresponding software solutions.