LF帯センサネットワークを用いた雷放電位置標定におけるビームフォーマ法の適用

摘要

電磁波を用いた雷放電位置標定手法は到達時間差法(Time of arrival location technique ：TOA)と干渉法に大 別される.到達時間差法は最も一般的な標定手法のーつで，センサネットワークを構築する各センサに記 録されたパルスのピーク時間や相互相関を用いて，電磁波到来方向推定を各センサ間で行い電磁波放射源 位置を推定する手法である.到達時間差法は孤立したパルスに対しては有効であるー方，バースト状の信 号に対しては各センサ間でピークの検出，同定が難しいため，時間差の推定が困難である.また，干渉法は，記録された電磁波形の位相情報を基に到来方向推定を行う手法である.位相情報を用いる利点として は，バースト状の信号に対しても，ピーク検出をする必要がないことが挙げられる.%We propose a new lightning location method using a digital beamforming method for a network of lightning location sensors in Low Frequency (LF) band. The lightning location system in LF bands, which we has been developing, is called "Broadband Observation network for Lightning and Thunderstorm (BOLT)". The BOLT produces 3-D images of lightning discharges using time-of-arrival estimation method (TOA) or interferometry method using the BOLT. The two radiation source location methods assume that the radiation source is a point source. While, distributed radiation source are estimated by a beamforming method. We applied a beam former method, which is the most basic method for steering a beam in a direction by uniform phase shift, to observation results of a return stroke, a Preliminary breakdown (PB) and Narrow bipolar event (NBE) recorded by the BOLT. In the result, the distributions of radiation power in the horizontal direction denoted the same tendency in all events, but those in the vertical direction were different in each event. The return stroke showed a wide distribution over 9 km. this is consistent with the findings that the return stroke has along discharge path toward the vertical direction. On the other hand, the distribution of radiation power toward vertical direction was 2-3 km in case of using the data of PB and NBE. The same result was obtained by the simulation using a point wave source. Therefore, as for the two phenomena, the distribution of radiation power is nearly the same as that obtained from a point wave source. It is considered that the discharge path is relatively short (several km) because these discharge phenomena occur in the clouds. The results indicate the fact that the distributions of radiation power toward the vertical direction for both PB and NBE are relatively short. It is concluded that the beam former method is useful to estimate the distribution of radiation power for lightning discharges.