In orbit performance of Si avalanche photodiode single photon counting modules (SPCM) in the Geoscience Laser Altimeter System on ICESat

Si avalanche photodiode (APD) single photon counting modules (SPCMs) are used in the Geoscience Laser Altimeter System (GLAS) on Ice, Cloud, and land Elevation Satellite (ICESat), currently in orbit measuring Earth surface elevation and atmosphere backscattering. These SPCMs are used to measure cloud and aerosol backscattering to the GLAS laser light at 532-nm wavelength, with quantum efficiencies of 60 to 70% and maximum count rates greater than 13 millions/s. The performance of the SPCMs has been monitored since ICESat launch on January 12, 2003. There has been no measurable change in the quantum efficiency when comparing the average photon count rates in response to the background light from the sunlit Earth. The linearity and the afterpulsing, seen from the cloud and surface backscattering profiles have been the same as those during ground testing. The detector dark counts rates monitored while the spacecraft was in the dark side of the Earth have increased linearly at about 55.5 counts/s per day due to space radiation damage, which is a little lower than what we expected based on the ground testing and sufficiently low to provide useful atmosphere measurements through the end of the ICESat mission. The radiation damage appeared to be slightly dependent of the device temperature. There was also a distinct increase in the dark counts during the solar storm in 28-31 October 2003. These SPCMs have been in orbit for almost four years to date. The accumulated operating time has reached to over 380 days (9150 hours). These SPCMs have provided unprecedented receiver sensitivity and clarity in atmosphere backscattering measurements from space.