CALIPSO Laser Performance and Anomaly Report

Abstract

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission was a partnership between NASA and the French Space Agency, CNES. It operated from June 2006 to June 2023, providing critical profile measurements of atmospheric aerosols and clouds needed to better understand their roles in air quality, weather, and climate. The core instrument was an atmospheric lidar featuring a primary and backup laser transmitter for a nominal 3-year mission lifetime. Remarkably, the system operated for 17 years. The primary laser delivered over 1.6 billion shots before being powered off in February 2009, and the backup laser generated over 8.5 billion shots, far exceeding the initial 2-billion-shot requirement. Throughout the mission, the laser system performed above expectations and satisfied all Level 1 measurement requirements. Both lasers experienced a gradual, expected decay in total energy output (~6% for the primary unit over 30 months, ~17% for the backup unit over about 14 years) due to the occasional loss of diode arrays and their normal aging. Despite pre-launch concerns regarding potential catastrophic diode failure from gold-indium solder contamination, the diode arrays proved resilient, suffering only manageable, incremental diode bar losses. While highly successful, the lasers experienced off-nominal behaviors primarily driven by two factors: radiation exposure and the slow depressurization of the lasers’ canisters. • Pressure Leaks and Corona Discharges: Both lasers experienced slow pressure leaks from their sealed canisters. As pressure dropped, the risk of high-voltage corona discharges (arcing) at the laser's Q-switch increased. The primary laser was deactivated in February 2009 because of significant impacts to routine operations. The backup laser eventually suffered from similar low-pressure conditions, resulting in intermittent low-energy pulses in its later years. The primary laser was restarted in a near-vacuum at the end of science operations in June 2023 to verify its status. During its brief period of operation, no very low-energy pulses were observed. • Anomalous Laser Shots: The project categorized off-nominal laser shots into three types: Type 1: A discrete, slightly lower-than-average energy shot followed by a series of shots with slightly greater-than-average energy shots. These events were believed to be caused by radiation-induced Single Event Upsets (SEUs) affecting the diode driver circuitry in radiation-prone regions such as the South Atlantic Anomaly (SAA). Type 2: Small to extreme low-energy (as low as zero-energy) shots caused by corona discharges across the Q-switch high-voltage terminals operating in a low-pressure LOM environment. These anomalous shots were also highly correlated with radiation-prone regions and could be induced at pressures above the theoretical corona threshold by enhanced energetic ion concentrations. Type 3: High-energy pulses immediately following Type 2 shots that were believed to be inadvertently caused by the temperature control circuit to the Q-switch reacting to a preceding corona event. • Unexplained Energy and Diode Voltage Trends 2018-2023: During much of the mission, laser energy at both wavelengths decreased at a rate that was consistent with an occasional loss of diode bar arrays and the gradual degradation of diode emitters. The diode voltage trends also tracked the energy trends. After 2017, the laser energy trends revealed different behaviors between the two channels, and the diode voltage trends increased in a manner that was inconsistent with the earlier trend relationships. The payload’s autonomous Fault Detection, Isolation, and Recovery (FDIR) system was fundamental to the mission's longevity, routinely detecting unexpected performance issues and, if necessary, placing the system into safe operational states to prevent damage. The confidence gained with the FDIR system enabled adjustments to the laser system, allowing for extended operations with the backup laser at pressures far below operational limits defined before launch. To address impacts to the science data quality from the increasing frequency of low-energy shots later in the mission, the integrated project team developed innovative data screening and algorithmic mitigation strategies. CALIPSO’s extensive pre-launch testing and its resilient design were key to its historic run. For future spaceborne lidar systems, the report recommends implementing enhanced insulation of high-voltage terminals to a laser Q-switch to prevent corona discharges and the addition of an upstream photodiode sensor to better monitor and diagnose the laser energy conversion efficiency.