Landsat 8: TIRS SSM Encoder Current Anomaly Resolution

This paper will document the Thermal Infrared Sensor (TIRS) anomaly, the Anomaly Review Board (ARB) investigation, and the ultimate return to “new nominal” operations using an Alternate ConOps (ACO). The paper will include samples of the anomalous data, details of the ARB investigations, the ambient ground tests, the ACO paradigm and differences between the nominal and ACO. Launched in February 2013, Landsat 8 (L8) became the latest in the United States Geological Survey (USGS) series of Earth Observing satellites. Unfortunately, in November 2014 TIRS began displaying abnormal current behavior. The Flight Operations Team (FOT) were challenged, as this abnormal behavior was serious enough that if it could not be resolved with an alternative concept of operations, the probability was extremely high that TIRS A-side would become inoperable. In November 2014, telemetry trending data for the TIRS Scene Select Mechanism (SSM) encoder −12V current monitor (A-side) showed out-of-family behavior that started to approach the yellow HIGH limit. Further investigation showed this behavior had actually started as early as June 2013. However the SSM performance (which relies on a stable encoder) was never affected. During the initial investigation it was noted that by periodically cycling the encoder power, the current level would recover but never quite return to post-launch levels. As the current trend was unpredictable but continuously increasing, continuing nominal operations on A-side was considered too risky. TIRS was switched over to B-side for the Mechanism Control Electronics (MCE) and Focal Plane Electronics (FPE) in early March 2015. While the Anomaly Review Board (ARB) could not be certain of a proximate cause, the team's initial assessment was that the most likely cause was Conductive Anodic Filaments (CAFs) within the layers of the encoder printed circuit board (PCB). Various recommendations were made for B-side operations (e.g. changing the trending paradigm such that TIRS could meet science requirements with the encoder turned off or minimally used). Tests were conducted to understand SSM drift without positive feedback control. Approximately 5 months after resuming nominal operations on the TIRS B-side, indications of an anomalous current were observed in the +12V MCE current (~August 2015). The ARB reconvened to resume investigations. Nominal Ops were suspended in November 2015, and an Alternative ConOps (ACO) was tested on B-side, and started on B-side in December 2015. This ACO leaves the Thermal Infrared Sensor (TIRS) Scene Select Mechanism (SSM) encoder in an OFF mode, but being switched ON every two weeks for calibration activities. During the ACO mode of operation for TIRS, ambient tests on the flight spare SSM electronics were initiated in an attempt to discover the root cause for the anomaly. These tests ultimately led to the discovery of cracked and cracking capacitors in the encoder circuit, most likely thermally-induced upon installation of a susceptible lot of parts. The conditions of these capacitors has implications for mitigation of similar risks for currently flying and future missions.