HR: 1330h
AN: AE22A-1109    [PDF]
TI: Accuracy of the Lightning Mapping Array
AU: * Thomas, R J
EM: thomas@nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Krehbiel, P
EM: krehbiel@ibus.nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Rison, W
EM: rison@ee.nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Hunyady, S
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Winn, W
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Hamlin, T
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AU: Harlin, J
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801
AB: We have conducted a detailed study of the accuracy of the Lightning Mapping Array in which experimentally determined values of the location uncertainties are compared with simple geometric formulations of the errors and with linear covariance estimates of the uncertainties from the least-square solution procedure. Observations of a sounding balloon that carried a GPS receiver and pulsed VHF transmitter show that the mapping system is able to locate sources inside the periphery of the network with a horizontal accuracy of about 10 m rms and, when the sources are higher than 1-2 km above ground, with a vertical accuracy of 30-50 m rms. The vertical accuracy is determined primarily by the nearest station contributing to the solution; the accuracy approaches that of the horizontal locations when the source is directly above the close station and the uncertainty increases by a factor of 2-3 or more for sources located between stations or at low altitude. For sources outside the network, the locations and their uncertainties are best described in a network-centered spherical coordinate frame. The radial distance $R$ of the source is determined primarily by stations transverse to the arrival direction, which measure the radius of curvature of the incoming wavefront. A simple geometric model shows that the radial location error increases as $(R/D)^2$, where $D$ is the network diameter. The vertical location error is dominated by the uncertainty of the source's elevation angle, which is determined primarily by stations along the arrival direction, and increases as $R^2/(zD)$ where $z$ is the source height above the network tangent plane. The azimuthal or transverse position of the source is most accurately determined, with the uncertainties increasing only as $(R/D)$. The location uncertainties are all proportional to $c\Delta t$, where $\Delta t$ is the rms uncertainty of the time-of-arrival measurements. $\Delta t$ can be accurately determined from the statistics of the reduced chi-squared values of the solutions; the effective value of $\Delta t$ was 50 ns rms for the LMA network operated during STEPS 2000. Most of the lightning sources (typically 80%) are located by only 6 or 7 stations of the network (6 being the minimum number required by the processing) and therefore have somewhat greater uncertainties than if they were located by the full complement of measurement stations. The uncertainties of the system were also investigated by determining the scatter of airplane tracks detected by the LMA when the airplanes were flying through ice crystal clouds (anvils or cirrus). For both the aircraft and sounding balloon data, the measured uncertainties agreed well with those predicted by the simple geometric models and by the covariance error estimates. When zoomed in on, the aircraft and balloon tracks exhibited characteristic `sawtooth' patterns that we believe are caused by the gradual accumulation and successive correction of systematic timing errors at each measurement station. The effect of these errors is comparable to the other timing uncertainties and are included in the overall error determinations. We should be able to reduce this contribution to the timing errors with small improvements to the system.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3360 Remote sensing
DE: 3394 Instruments and techniques
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting