The fundamental requirement for GLM is to detect the occurrence of both cloud-to-ground and intra-cloud lightning flashes with a high detection efficiency and a low false alarm rate, 70% and 5% respectively at end of instrument life. The OTD and LIS have demonstrated the efficacy of the optical detection approach, which discriminates the optical pulses from the high-background daytime environment of sun-lit clouds. In order to optimize the GLM design for low cost and low risk, sophisticated performance modeling is required.
Because lightning is a transient event, the GLM instrument is quite different from the typical remote sensing environmental instrument. In addition to radiometric modeling that provides the usual signal-to-noise ratio, analysis of the ôreceiver operating characteristics (ROC) is required to determine the probability of detection or false alarm. These performance parameters are customarily used in certain military sensors.
Our approach to the analysis is twofold: a performance model has been generated that provides moderate fidelity results and allows rapid trade-off of sensor characteristics, and another tool provides high-fidelity simulation of particular special cases.
For both methods, a detailed understanding of the lightning optical pulse characteristics is required. Statistical models of the amplitude, temporal and spatial distribution of the optical emissions have been generated from the observations described in the literature. The particular detection algorithm has also been integrated into the detection analysis. Noise sources, including scene shot noise and sensor noise are included in the moderate fidelity model.
In the high-fidelity simulation, solar glint; clutter leakage due to spacecraft jitter and edge effects; and the radiation environment from cosmic ray and solar particle radiation (based on BATC detailed models) have also been included. A Monte Carlo approach is used to incorporate radiation hits and lightning events. Finally, the high-fidelity simulation includes the effects of the varied and high radiance background by the inclusion of actual scenes from MODIS and other data.