Analysis of the Texas A&M impact lightning detector to determine detection efficiency and site error corrections

Abstract

The Texas A&M IMProved Accuracy from Combined Technology (IMPACT) lightning detector has been analyzed for 1997 and 1998-99 to determine detection characteristics. These characteristics include stroke detection efficiency as a function of range and azimuth, site errors, signal strength, attenuation coefficient, and peak current. Cloud-to-ground (CG) return stroke measurements from the IMPACT detector were compared to National Lightning Detection Network (NLDN) measurements to determine these detection characteristics. Because the IMPACT detector's sensitivity was increased in 1998, was determined that a higher stroke detection efficiency occurred in 1998-99. The IMPACT sensor's signal strength amplitudes were also affected by the increased sensitivity. The sensor's range-normalized signal strengths (RNSS) were 1.6 times higher in 1998-99 than in 1997. These were corrected using a calibration factor determined statistically from IMPACT and NLDN RNSS values. Results from peak current analyses show that the increased sensitivity caused a higher percentage of weak signals to be detected. Electromagnetic Gelds from CG lightning channels are attenuated with distance. Attenuation coefficients are a function of geographical area. A coefficient of 1.14 was determined for the area 1000 km within the IMPACT detector location. A site error correction curve was used to correct for azimuthal errors in bearings to CG stroke locations. Errors were reduced to less than 4° in most cases. The IMPACT detector uses a crossed-loop magnetic field antenna to determine stroke location bearings and magnetic field amplitudes. Analyzing sisal strengths revealed that strokes in the north-south direction had amplitudes approximately 20% higher than strokes in the east-west direction. This is a result of the north-south loop of the antenna being more sensitive than the east-west loop. This condition also affected the detection efficiency of the IMPACT sensor as a function of range. Efficiency minima in the north and south directions predicted by theory are not present.