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].