||Instrument: LIS : Lightning Imaging Sensor|
Earth Remote Sensing Instruments
Instrument Class: Passive Remote Sensing
Instrument Type: Spectrometers/Radiometers
Instrument Subtype: Imaging Spectrometers/Radiometers
Wavelength Keyword: Near Infrared
Spectral/Frequency Resolution: 0.777 micrometers with 5-km spatial resolution and 2-ms temporal resolution over an imaging area of 600 km × 600 km.
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LIS is designed to investigate the global incidence of lightning, its correlation with convective rainfall, and its relationship with the global electric circuit. Conceptually, LIS is a simple device, consisting of a staring imager optimized to locate both intra cloud and cloud-to-ground lightning with storm-scale resolution over a large region of surface on Earth, to mark the time of occurrence, and to measure the radiant energy. It will monitor individual storms within the field of view (FOV) for 80 seconds, long enough to estimate the lightning flash rate. Location of lightning flashes is determined to within 5 km over a 600-km × 600-km FOV.
The LIS design uses an expanded optics wide-FOV lens, combined with a narrow-band interference filter that focuses the image on a small, high-speed, charge-coupled-device focal plane. The signal is read out from the focal plane into a real-time data processor for event detection and data compression. The particular characteristics of the sensor design result from the requirement to detect weak lightning signals during the day when the background illumination, produced by sunlight reflecting from the tops of clouds, is much brighter than the illumination produced by the lightning.
A combination of four methods is used to take advantage of the significant differences in the temporal, spatial, and spectral characteristics between the lightning signal and the background noise. First, spatial filtering is used to match the instantaneous FOV of each detector element in the LIS focal-plane array to the typical cloud-top area illuminated by a lightning event (about 5 km). Second, spectral filtering is applied, using a narrow-band interference filter centered about the strong hypoiodite ion (OI) emission multiplet in the lightning spectrum at 777.4 nm. Third, temporal filtering is applied. The lightning pulse duration is of the order of 400 microseconds, whereas the background illumination tends to be constant on a time scale of seconds. The lightning signal-to-noise ratio improves as the integration time approaches the pulse duration. Accordingly, an integration time of 2 ms is chosen to minimize pulse splitting between successive frames and to maximize lightning delectability. Finally, a modified frame-to-frame background subtraction is used to remove the slowly varying background signal from the raw data coming off the LIS focal plane. If, after background removal, the signal for a given pixel exceeds a specified threshold, that pixel is considered to contain a lightning event.
LIS investigations are striving to understand processes related to, and underlying, lightning phenomena in the Earth atmosphere system. These processes include the amount, distribution, and structure of deep convection on a global scale, and the coupling between atmospheric dynamics and energetics as related to the global distribution of lightning activity. The investigations will contribute to a number of important EOS mission objectives, including cloud characterization and hydrologic-cycle studies. Lightning activity is closely coupled to storm convection, dynamics, and microphysics, and can be correlated to the global rates, amounts, and distribution of convective precipitation, to the release and transport of latent heat, and to the chemical cycles of carbon, sulfur, and nitrogen. LIS standard products include intensities, times of occurrence, and locations of lightning events. The performance of LIS has exceeded specifications and has been returning unprecedented data on lightning activity. LIS is enabling investigators to quantify relationships between lightning, convection, and ice production.
NASA Earth Science Reference Handbook [ Mission: TRMM ]
Data Rate: 500 frames per second
Instrument Start Date: 1997-11-30
Instrument Owner: USA/NASA