In December 2014, an increase in the current of an electronics component (encoder) that measures the orientation of the Scene Select Mechanism (SSM) of the Landsat 8 Thermal Infrared Sensor (TIRS) triggered an anomaly (Figure 1). Level 1 processing of TIRS data was suspended while investigations into the cause of the anomaly were conducted. In March 2015, the Landsat 8 mission Flight Operations Team (FOT) switched the TIRS to the redundant/backup electronics to prevent potential damage to the sensor. Thermal data processing resumed in May 2015.
Figure 1. Cut away of TIRS showing the SSM in relationship to the other components of the sensor.
Initially detected in the fall of 2014, the telemetry data for the TIRS SSM position encoder -12V supply showed increased current. Examination of the data indicated that the current may have begun rising as early as July 2013, one month after commissioning was complete (however, performance of the SSM encoder was not affected at that time). In December 2014, the encoder current reached the “yellow limit” threshold, set by the FOT as a trigger to power off the encoder. (Figure 2)
Figure 2. Increase of the -12V current. Far right of plot shows when the encoder was powered off and tested.
The SSM Encoder
The encoder converts angular positions to digital codes representing the orientation of the SSM. The angular position of the SSM is used to point the mirror precisely to any of three primary SSM positions: Earth imaging, black body calibration, and deep space calibration. The angular position of the SSM is used also during data processing to properly position TIRS pixels onto the Earth’s surface. Operationally, the angular position of the SSM is provided as ancillary data to the TIRS Level 0 data processing stream.
On December 19, 2014, the FOT suspended nominal calibration collections and suspended processing of newly acquired TIRS data while a team from the USGS and NASA established an Anomaly Review Board (ARB) to analyze the problem, determine the root cause, and conduct on-orbit tests.
The TIRS was developed with a redundant set of electronics that allow it to be switched from side A (primary) and side B (redundant/backup) if the primary electronics became inoperable. Therefore, when the encoder current reached the “yellow limit” threshold, operations on the side A electronics were stopped.
The ARB concluded that the TIRS should be switched to the backup electronics, and the change was made March 2, 2015. Following an abbreviated commissioning period to assess the side B configuration, the TIRS returned to routine imaging operations on March 7, 2015. On April 30, 2015, the USGS EROS began reprocessing TIRS data acquired during the anomaly period, and completed by June 18, 2015.
How The Primary (side A) Electronics Failed
Investigations have determined that the increased -12V current was most likely due to a resistive short in the encoder’s printed circuit board. While the root cause is not certain, the most viable cause is that Conductive Anodic Filaments (CAF), which may grow along the fibers within composite circuit boards. CAFs can occur when the right combination of gaps or voids in the circuit board allow for a buildup of material connecting a cathode and anode.
Further details and updates to the SSM mitigation can be found in Appendix A of the Landsat 8 Data Users Handbook.