The High-Spectral-Resolution Lidar (HSRL) Pathfinder Mission concept is designed to provide HSRL measurements at 532 nm and elastic backscatter lidar measurements at 1064 nm. The instrument is based on Clio, the HSRL that was descoped from NASA's Atmosphere Observing System (AOS) mission due to cost constraints. The NASA Langley Research Center (LaRC) developed the HSRL Pathfinder concept as an example of a lower-cost mission to advance the technology and demonstrate the measurement capability originally planned for AOS. Cost savings are achieved via a Class-D instrument development approach and some reductions in performance from the original Clio design. Despite these changes, the HSRL Pathfinder Mission promises to provide valuable observations for advancing studies of aerosol and cloud radiative effects, cloud microphysics, aerosol-cloud interaction, aerosol transport and speciation, and air quality. The design also enables scientifically important observations of depth-resolved ocean subsurface optical properties, snow water equivalent, and seasonal sea ice, making HSRL Pathfinder a truly multifunctional lidar mission.

Space-based light detection and ranging (LiDAR) sensors have provided valuable insight into the global, vertical distribution of aerosol and cloud layers in Earth's atmosphere, and, more recently, of the distribution of phytoplankton in the ocean. However, the photodetectors in these sensors lack the performance necessary to capture the vertical structure of cloud tops and ocean phytoplankton to a fidelity sufficient for advancing our understanding of the global water cycle and ocean carbon cycle, respectively. Recent advancements in high-performance single photon avalanche diode (SPAD) arrays promise to enable these measurements, while also offering a sensitivity that will allow significant reductions in laser power and telescope size, with associated sensor-level size, weight, and power (SWaP) savings. To harness the unique benefits of SPADs for these measurements, we propose to develop a large-format array of photon counting SPADs with <10 ns dead time, along with readout integrated circuitry that sums and bins (histograms) photon counts in real time to the desired temporal resolution for the target application. The feasibility of this approach has been investigated with a small-scale 8 X 8 SPAD array proof of concept hardware demonstration developed at Politecnico di Milano, with promising initial results. Progress is reported on designs that will allow scaling the array and readout integrated circuit electronics to the requisite of 128 x 128 size in a chip-scale, low power, photodetector ideal for LiDAR remote sensing of the atmosphere and ocean from SWaP-constrained platforms. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)

Following on the success of the NASA-CNES CALIPSO/CALIOP space-based cloud-aerosol lidar, which is now approaching its 14th year of continuous operation, NASA Langley Research Center has been advancing space-based lidar mission concepts and related technologies to address future cloud, aerosol and ocean science objectives identified in the 2017 Decadal Survey for Earth Science and Applications from Space. Recently, scientists at NASA Langley have been able to use data from CALIOP to retrieve global ocean biomass, which is a measurement that was never originally envisioned for this instrument, and represents a new scientific frontier for space-based lidar. CALIOP’s measurements complement ocean color records, extending ocean retrievals into nighttime and to high latitudes, and creating a more complete picture of global ocean biomass. Due to detector bandwidth limitations, however, CALIOP, as with ocean color, only obtains column-averaged measurements that are biased towards the surface. To address this limitation, Politecnico di Milano is developing a lidar detector based on a single photon avalanche diode (SPAD) array. This detector is expected to achieve a depth resolution of <1 m in the ocean, and have a detection efficiency and dynamic range that far exceeds that of the analog-readout, PMT-based receiver on CALIOP. Naturally, these same characteristics will also benefit cloud and aerosol retrievals, making SPAD arrays an excellent detector candidate for future space-based atmosphere and ocean lidar missions. Here, we describe potential advantages of fast (~THz) photon counting SPAD arrays, and how they are being developed for this application.