A system for determining the locations of nearby lightning strikes from electric-field and acoustic measurements has been developed and built. The system includes at least three receivers, each equipped with an antenna and a microphone. For each strike, the system measures the difference between the times of arrival of the electric-field and sonic (thunder) pulses at each receiver, computes the distance of the strike from the time difference and the speed of sound (about 320 m/s), then uses the distances to determine the location of the strike. The basic concept of this system is thus a variant and extension of the time-honored concept of estimating the distance of a lightning strike from the difference between the times of arrival of the visible flash and the audible thunder.
"Nearby" as used here signifies that the receivers and the lightning strikes of interest are located at distances of the order of 1 km from each other. Older lightning-location systems cover observation areas with radii of the order of 30 miles (48 km), and typically locate lightning strikes with errors of the order of 0.5 km or more; thus, the older systems do not locate strikes accurately enough for purposes of assessing actual or potential damage by lightning to specific structures and pieces of equipment. The developmental system locates strikes to within errors of the order of a meter.
Unlike in some other systems, there is no need for expensive, high-speed waveform digitizers. Instead, the leading edge of the electric-field pulse detected in each receiver is used to start a timer, and the leading edge of the thunder pulse arriving at the same receiver is used to stop the timer. A microcontroller in each receiver transmits the time thus measured to a processing station, where the times are converted to distances that are used to compute the location of the lightning strike (see figure).
The accuracies achievable by older systems and by this system can be compared via consideration of the time-measurement problem. Given that the applicable speed in this system is the speed of sound, even a timing error as large as 1 ms in this system would result in a distance error of only about 0.3 m. However, in a typical older system based on the speed of light, the allowable timing error to achieve such accuracy in distance would be only about 1 ns.
Thus far, only one receiver has been tested. It has been shown to be capable of measuring the distances to lightning strikes. Optionally, the fully developed system could contain more than the minimum of three receivers needed to determine the location of a lightning strike unambiguously under ideal conditions. The additional receivers could be used to enhance accuracy under nonideal conditions. By use of a suitable algorithm, the partly redundant data from more than three receivers could be used to resolve uncertainties introduced by wind and by echoes from nearby objects. The wind problem is especially important because in the presence of wind, the equal propagation-time distance contour around each receiver becomes distorted from a circle.
This work was done by Pedro J. Medelius, formerly of I-NET, Inc., for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Technology Programs and Commercialization Office
Kennedy Space Center
Refer to KSC-11992.