Maintaining Software for Active Missions: A Case Study of Chandra's Instrumentation Over Time

During eleven plus years of operation, ample knowledge has been gained regarding the Chandra X-ray Observatory's scientific instruments and how they are performing over time. In this paper we will summarize the significant software changes related to the performance and knowledge gained about the observatory's instrumentation specifically targeting the last five years of the mission. With this knowledge, numerous upgrades to the Chandra processing software have taken place to correct issues that have developed on orbit (ACIS CTI effects, LETG/ACIS rotation), to correct for hardware issues (HRC timing), and for our better understanding of the evolution of the instrumentation (temporal gain shifts in both instruments). In addition, we will discuss the challenges in maintaining software when the calibrations for different operating modes are incrementally made available. This includes challenges in maintaining an archive with a mixture of data products with different calibrations applied. This paper roughly covers the era of the 3rd reprocessing of the Chandra archive (Repro-3).