First in Space

The Optical Transient Detector (OTD) on the MicroLab-1 satellite was the first lightning imager in space. As an engineering prototype for LIS, it paved the way for the space-based lightning research that culminated in GLM on the R-series GOES satellites.

MicroLab-1 was an imaging microsatellite launched into a 740 km low Earth orbit in Apr 1995. It carried 2 instruments for characterizing the Earth-atmosphere system. The GPS/Meteorology (GPS/MET) instrument was a proof-of-concept demonstration instrument that used radio occultation to generate soundings of atmospheric properties along the limb of the Earth. The primary payload on Microlab-1, however, was the Optical Transient Detector (OTD) - the first lightning imager in space.

The primary goals of OTD were to demonstrate the optical lightning detection technology and refine the global maps of thunderstorm activity. OTD was a flight-qualified engineering prototype for the future Lightning Imaging Sensor (LIS) that would later be deployed on the TRMM satellite and on the International Space Station. The higher orbit of the MicroLab-1 satellite meant that the OTD pixels were larger (~10 km across), and there were also differences in the glint rejection and flash clustering codes between OTD and LIS.

Before ISS-LIS, I used OTD to examine lightning characteristics in the mid-to-high latitudes. Like ISS-LIS, OTD lacked the coincident cloud measurements that were available with the LIS deployment on the TRMM satellite. This means that cloud interactions are not well captured in the OTD data, and one must be careful with attributing optical signatures to different types of lightning discharges.