The National Lightning Detection Network (NLDN) is comprised of 59 LPATS-III time-of-arrival sensors and 47 IMPACT sensors that provide both time-of-arrival and direction-finding information. The NLDN sensors are responsive to VLF EM radiation (similar to the LASA response), sensing the radiation associated with return strokes and intracloud strokes. In addition to generally weaker radiation from intracloud strokes, the NLDN processing is designed to filter against intracloud activity. This network of sensors provides primarily cloud-to-ground lightning detection across the coterminous United States (CONUS).
The NLDN data sets used for the comparisons were not standard NLDN data products, but were reprocessed from raw data using relaxed event criteria (`loosened criteria') to maximize detection of intracloud discharges and distant/weak cloud-to-ground discharges. The 1999 data were processed with somewhat more strict criteria than the 1998 data. The standard NLDN data provide 80-90% detection efficiency of cloud-to-ground strokes with currents of greater than 5 kA within the CONUS. These events are located with an accuracy of 500 m [Cummins et al., 1998a]. The `loosened criteria' data used for comparison in this paper may not meet these quality-control criteria. Their uncertainty has not been characterized.