2. ISS LIS instrument and data structure
The ISS LIS measurement concept is based on time-differenced geolocated images at 777.4-nm wavelength [Christian et al. , 1989], an oxygen-absorption line that enables detection of lightning signals at cloud top during both day and night. The instrument consists of narrowband-filtered optics coupled to a 128 x 128 pixel focal plane. At the ~400-km altitude of the ISS, this provides nadir pixel resolution of ~4 km. The data processing follows Mach et al. [2007]. The first component of the data structure is events, which correspond to individual pixels that exceed the background level by a given threshold during a particular frame (2 ms). Adjacent events are clustered into groups when they occur within the same frame. Groups are then clustered into flashes when they occur close in time and space to one another. ISS LIS also provides quantitative information about optical energy output from lightning.
The ISS LIS instrument heritage spans multiple decades. The National Aeronautics and Space Administration (NASA) Marshall Space Flight Center (MSFC) - along with support from other science, academic, and commercial partners - developed a unique space-based lightning detection instrument, the Optical Transient Detector (OTD). The instrument design concept began in earnest in the 1980s with high-altitude aircraft measurements of lightning optical signatures at cloud top [Christian et al., 1983 ]. These measurements helped determine optimal space-based instrument requirements (e.g., what sensitivity would be required to detect lightning under both day and night conditions). The OTD was then engineered from these requirements in the early 1990s, and was launched aboard the MicroLab-1 satellite in 1995 from Vandenberg Air Force Base. It provided the first global-scale lightning climatology for its operational lifetime (1995-2000) [Christian et al. , 2003].
The OTD was essentially a prototype design for the LIS concept developed as part of the NASA Earth Observing System (EOS). The LIS was selected as an EOS instrument to fly on both a polar platform and the ISS, then known as Space Station Freedom. However, LIS instead was moved to the Tropical Rainfall Measuring Mission (TRMM) instrument complement because it had strong synergies with the core TRMM science instruments [Kummerow et al. , 1998]. TRMM LIS was launched in November of 1997 and ended its mission with the deorbiting of the TRMM satellite in April of 2015, a very successful 17+ years of operational lifetime.
Building on this long and dynamic heritage, the flight spare instrument for TRMM LIS was adapted to work on the ISS platform, resulting in ISS LIS. ISS LIS was part of the 5th Space Test Program - Houston mission (STP-H5), which is sponsored by the US Department of Defense (DoD). ISS LIS is currently located on the Express Logistics Carrier 1 (ELC-1) module of the ISS (Fig. 1). Given TRMM LIS’s long operating period, ISS LIS is expected to be capable of operating for many years into the future. However, this is subject to the NASA senior review process for long-term missions.
Relative to TRMM LIS, ISS LIS extends the program of record for global coverage of low-latitude lightning (± 38°). Relative to OTD (which was in a 70° inclined orbit), ISS LIS extends the program of record for higher latitude lightning (up to ± 55°). And relative to geostationary lightning sensors, ISS LIS enables fully global-scale coverage as well as cross-platform validation. ISS LIS also has better spatial resolution (4 km) than either OTD or current geostationary sensors. Thus, ISS LIS is a unique and complementary Earth-observing instrument that provides near-realtime observations of lightning, including global-scale coverage to high latitudes (observing areas that produce an estimated 98% of all global lightning) throughout the diurnal and seasonal cycles.
In terms of NASA EOS Data and Information System (EOSDIS) Data Processing Levels, the ISS LIS science data are Level 2 (i.e., derived geophysical variable), while the background data are Level 1B (i.e., reconstructed instrument data processed to sensor units). The data are available in the TRMM LIS heritage Hierarchical Data Format Version 4 (HDF4) as well as the modern Network Common Data Format Version 4 (netCDF4), with corresponding browse images in Graphics Interchange Format (GIF).
ISS LIS products are generated and distributed by the NASA Global Hydrology Resource Center (GHRC) Distributed Active Archive Center (DAAC; Figs. 1 and 2) and can be discovered via the NASA Earthdata Search tool [Earthdata , 2020], the GHRC Hydrology Data Search Tool [HyDRO , 2020], and the GHRC website for ISS LIS [ISS LIS , 2020].
The LIS measurement concept was recently adapted to work from geostationary orbit [Goodman et al. , 2013; Rudlosky et al. , 2018]. This enables operational applications using continuous spaceborne lightning observations over a hemispheric field of view (FOV) [Bruning et al. , 2019].