Abstract:
A real-time, multi-sensor local-area lightning detection network system. The system uses waveform indicative of electrostatic field changes with respect to time is generated at each of N locations due to a cloud-to-ground lightning strike occurring in the vicinity of the N locations. Each waveform is integrated to generate a corresponding electric field associated with a corresponding one of the locations. A mathematical relationship is used to determine a ground surface location of the lightning strike, height of the lightning strike, and charge per unit length of the lightning strike using each electric field generated during integration of the waveforms.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by employees of the Department of the Navy and the Department of Commerce, and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to lightning detection systems, and more particularly to a method and system for determining information about cloud-to-ground lightning strikes. 
     BACKGROUND OF THE INVENTION 
     Information about cloud-to-ground lightning strikes is important for a variety of safety and research reasons. For example, it is known that intense downdrafts or microbursts follow lightning-producing updrafts. Precise knowledge of lightning strikes, accordingly, can serve as a predictor for the locations of possible microbursts. This type of information, for example, would provide safer air traffic control in order to protect planes from such microbursts. Lightning strike information is typically generated using lightning detection systems that use magnetic field sensors to detect a radiation component or an induction component of a lightning strike. 
     In terms of scientific research, any additional knowledge about a lightning strike could prove beneficial in existing or future applications requiring knowledge about severe weather conditions. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method and system for determining real-time information about cloud-to-ground lightning strikes as line segments by calculating changes in the electrostatic field. 
     Another object of the present invention is to provide a method and system for determining a precise ground location of a cloud-to-ground lightning strike in the near-field based on specific assumptions. 
     A further object of the present invention is to provide a method and system where each solution for each station has about tens of meters (or less) of uncertainty compared to conventional technology, which typically has hundreds of meters of uncertainty, and thus a significant reduction. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a method and system are provided for determining real-time information by treating cloud-to-ground lightning strikes as line segments. A waveform indicative of the derivative of the electrostatic field with respect to time is generated at each of N locations due to a cloud-to-ground lightning strike occurring in the vicinity of the N locations. In the present invention, there must be at least four such locations. Each waveform is integrated to generate a corresponding electric field associated with a corresponding one of the locations. A mathematical relationship is used to determine a ground surface location of the lightning strike, height of the lightning strike, and charge per unit length of the lightning strike using each electric field generated by integration of the waveforms. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the exemplary embodiments and to the drawings, where corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  is a schematic view of the system for determining cloud-to-ground lightning information in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a flow diagram of the general method used to determine cloud-to-ground information in accordance with the present invention; and 
         FIG. 3  is a block diagram of the system components at a lightning monitoring station in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and more particularly to  FIG. 1 , an overview of a system for determining real-time lightning information with improved accuracy in accordance with the present invention is illustrated. As will be described herein, the information will describe the ground strike location  200  of a cloud-to-ground lightning strike  100 , the vertical extent or height of lightning strike  100 , and the electrical charge per unit (i.e., height) of lightning strike  100 . All of this information is obtained by the same processing system/method. Accordingly, the present invention advances the lightning detection state-of-the-art by providing comprehensive real-time data about an electrostatic field component of a lightning strike  100  as a line segment in the near field as opposed to the mere detection or location thereof using magnetic field sensors to detect a radiation component or an inductive component of a lightning strike or, alternatively, simply using an electrostatic field to measure amplitude and infer distance. 
     In general, the present invention monitor changes in a local electrostatic field with respect to time at each of a plurality of spaced-apart stations located on or near a ground surface  300  (e.g., the plane of the paper used to illustrate  FIG. 1 ). For reasons that will be explained further below, in an exemplary embodiment, at least four stations are used in the present invention. More stations may be used to increase reliability, that is, the margin of error decreases with the use of more stations. Accordingly,  FIG. 1  illustrates stations  12 ,  14 ,  16  and  18  in a spaced-apart arrangement. In an exemplary embodiment, no station is less than one kilometer from another station in order to be effective. Stations  12 - 18 , that is, first structures, may be located on ground surface  300  or a relatively short distance above ground surface  300  (e.g., on a pedestal, building, cell tower, etc.) without departing from the scope of the present invention, provided each such station can monitor local electrostatic field changes. The local electrostatic field changes monitored (e.g., continuously or during storm periods) by stations  12 - 18  are indicated in  FIG. 1  by dE 12 /dt, dE 14 /dt, dE 16 /dt and dE 18 /dt, respectively. 
     In the illustrated example, the distance between each of stations  12 - 18  to ground strike location  200  is indicated by a respective dashed line  22 ,  24 ,  26  and  28  where the distance “D” of each line is such that D 22 &lt;D 24 &lt;D 26 &lt;D 28 . When lightning strike  100  occurs, each of the stations  12 - 18  measures an electrostatic component of the lightning strike so that the local (near-field) electrostatic field monitored at each of the stations  12 - 18  may experience a spike or peak as evidenced in each of the waveforms indicative of the corresponding field change dE/dt at each station. Since electrostatic field amplitude decreases with distance from a lightning strike, the peak amplitude of each of the waveforms dE 12 /dt, dE 14 /dt, dE 16 /dt and dE 18 /dt is in correspondence with the distance between the respective station and-ground strike location  200 . This characteristic is apparent in each of the dE/dt waveforms in the exemplary embodiment shown in  FIG. 1  where the peak amplitude at station  12  is greatest and the peak amplitude at station  18  is smallest. This amplitude difference is used in the present invention to provide information about lightning strike  100 . 
     To accurately characterize lightning strike  100 , it is best to use only the portion of each dE/dt that is related to lightning strike  100 . The “relevant portion” (as it will be referred to hereinafter) of each dE/dt waveform related to lightning strike  100  includes a brief portion of the waveform both before and after the occurrence of a dE/dt waveform peak. To select the relevant portion, a threshold criteria is applied to each monitored dE/dt waveform so that just the peak region (i.e., waveform data to include the waveform peak and brief periods before and after the peak) of the monitored electrostatic field changes is processed. Such identified peak region referred to as thresholding/windowing is indicated in  FIG. 1  by the vertical dashed lines on each dE/dt waveform where the waveform information between the dashed lines contains information related to lightning strike  100 . Accordingly, the stations or first structures  12 - 18  generate a respective waveform indicative of electrostatic field changes with respect to time at each station location. 
     The relevant portion of each dE/dt waveform (i.e., between the vertical dashed lines) from stations  12 - 18  is integrated by a processor  20  sometimes-referred to as a “second structure,” that is, in an exemplary embodiment, remotely located with respect to stations  12 - 18 . In such a case, the relevant portion of each dE/dt waveform may be transmitted over a wired or wireless transmission system (not shown) to a (remotely located) processor  20 , such that the stations  12 - 18  are coupled to the processor or second structure  20 , as indicated by respective transmission arrows  13 ,  15 ,  17  and  19 . The processor  20 , in part, may perform an integration function as well as a function to solve simultaneous equations. Alternatively, in a different exemplary embodiment, and without departing from the scope of the present invention, each station  12 - 18  may include its own processor component not shown), which is separate from the controller  38  and different from processor  20 , in order to integrate the relevant portion of its dE/dt waveform where the results of such integration may then be transmitted to the (remotely located) processor  20 , which may, in part, function to solve simultaneous equations relating to determining a ground surface location of the lightning strike, height of the lightning strike, charge per unit length of the lightning strike and other related information. 
     Integrating the relevant portion of a dE/dt waveform yields an electric field measurement “E” at the particular one of stations  12 - 18  due to lightning strike  100 . The resulting four electric field measurements (e.g., E 12 , E 14 , E 16 , and E 18  in the illustrated example) are processed substantially simultaneously to provide coordinates of ground strike location  200  relative to stations  12 - 18 , the vertical extent or height of lightning strike  100 , and the electric charge per unit length (i.e., height) of lightning strike  100 . Accordingly, E is calculated essentially independent of time of arrival of the electrostatic signal at a station  12 - 18 . In general, the electric field in E N  of an N-th station may be defined as follows: 
               E   N     =       q     2   ⁢           ⁢   π   ⁢           ⁢   ɛ       *     (       1     d   N       -     1         d   N   2     +     z   2             )             
where d N =√{square root over (x N   2 +y N   2 )}.
 
     In this electric field relationship x N  and y N  are the coordinates of ground strike location  200  relative to the N-th station, z is the height of lightning strike  100  where the lightning strike may be treated as a line segment not a point charge, q is the electric charge per unit length (height) of lightning strike  100 , and ε is the permittivity of the ambient atmosphere through which lightning strike  100  propagates. The above electric field relationship is developed based on the following exemplary assumptions:
         lightning strike propagates through a “channel” defined by a vertical line, that is, the lightning strike is treated as a line segment not a conventional point charge,   the charge q per unit length is constant along the lightning “channel”,   there is no branching of the lightning “channel”, and   only the first stroke-to-ground is used.       

     In an exemplary embodiment, processor  20  may implement any multiple-equation/multiple-unknown methodology to solve, for example, simultaneously, at least the four E N  equations having four unknowns (i.e., x N , y N , z, q). The choice of a particular solution technique is within the skill in the art and is not a limitation of the present invention. The generalized method of the present invention is illustrated in the flow diagram presented in  FIG. 2 . Stations  1  through N will perform the same processing as described above. By way of example in an exemplary embodiment, each station monitors (step  50 ) electrostatic field changes dE/dt locally, that is, an electronic component in the near field compared to conventional technology that may use magnetic field sensors to detect a radiation component and/or an induction component of a lightning strike. When a lightning strike occurs, a thresholding technique (step  52 ) is applied to the locally-monitored dE/dt waveform to select the relevant portion thereof, that is, relevant portion of dE/dt. In an exemplary embodiment, integration (step  54 ) of the relevant portion of each dE/dt waveform may be performed as part of the particular station&#39;s  12 - 18  processing function, for example, as shown in  FIG. 2 . In another exemplary embodiment, integration may be performed by processor  20 , which is separate and may be remotely located from the stations  12 - 18 , or, in another exemplary embodiment, more stations  12 -N. Since, in an exemplary embodiment, there are four unknowns in the electric field relationships, data from four stations is required in the present invention so that a simultaneous solution technique can be applied (step  56 ). Based on this system, each solution for each station  12 - 18  has about tens of meters (or less) of uncertainty compared to conventional technology, which has hundreds of meters of uncertainty, and thus a significant reduction. 
     The hardware required at a monitoring station may vary depending on system design. In an exemplary embodiment, one possible station design is illustrated in  FIG. 3  where an electrostatic field sensor  30  is used to “continuously” sense the local electrostatic field. In an exemplary embodiment, the electrostatic field sensor  30  may be a short modified whip antenna. As used here, “continuously” refers to any time period of interest (e.g., all the time, during the summer/thunderstorm months, when a storm is approaching, etc.). As used here, “locally” refers to “at each stations&#39;s electric field sensor,” that is, the antenna at each station installation, so “locally” is not a distance. Further, the output of sensor  30  is supplied to a trigger circuit  32  that, in turn, is supplied with a threshold level indicative of a lightning strike in the vicinity of the sensor  30 . In an exemplary embodiment, the trigger circuit  32  may be a comparator with an adjustable trigger level and digital trigger output. In different exemplary embodiments, the threshold criteria may be based on the rate of change (or slope of dE/dt), or may be based on a particular peak amplitude of dE/dt without departing from the scope of the present invention. In either exemplary embodiment, a lightning event is indicated when the threshold level is achieved. The output of sensor  30  via the trigger circuit  32  is passed to an analog-to-digital (A/D) converter  34  for digitization. In an exemplary embodiment, the A/D converter  34  may be a single channel with 16 bit resolution and 10 microseconds time resolution. The entire dE/dt waveform or just the relevant portion thereof due to a lightning strike is digitized by the A/D converter  34 . In an exemplary embodiment, the digitized waveform data may be recorded and stored “on station”  12 - 18  in a memory unit  36 , for example, in an exemplary embodiment, a flashcard memory  36 . The A/D converter  34  and the memory unit  36  are sometimes jointly referred to as a “monitoring structure.” Control of such data storage, as well as control of A/D converter  34  and the threshold level supplied to trigger circuit  32 , may be provided by an “on station” controller  38 . In an exemplary embodiment, the controller  38  may be enabled to record 500 readings of an lightning electric field signal where 100 readings are pre-trigger and 400 readings are post trigger. A transmitter  40  is used to relay, generally, just the digitized, relevant portion of the dE/dt waveform data to a remotely located processor  20 . 
     Since the present invention determines ground strike location  200  relative to each particular station, absolute geographic coordinates of ground strike location  200  may be readily determined if the absolute geographic coordinates of each station are known. Accordingly, in an exemplary embodiment, each station may also include a GPS location unit  42  for providing a GPS location of the station, and more particularly, the location of the electric field sensor, that is, antenna, as well as providing a date and a time, for example, in an exemplary embodiment, a time accurate to 1 milliseconds. This information may be provided to the controller  38  for final transmission to the remotely located processor (e.g., processor  20 ) for determining coincidences between the reading of all stations. The GPS is used to locate the station and, more specifically, the antenna. The GPS is also used to synchronize the timing circuitry (clocks) in each station. The GPS location may be known in advance or determined by “on station” GPS location unit/electronics  42  (not shown in detail) as would be well understood in the art. In an exemplary embodiment, the GPS location unit may provide a computed location of each station  12 - 18  with an accuracy within 1 meter of the actual location of each station. 
     The advantages of the present invention are numerous. Precise lightning strike location, vertical extent, and charge intensity are simultaneously determined. Based on this system, each solution for each station  12 - 18  has about tens of meters (or less) of uncertainty compared to conventional technology, which has hundreds of meters of uncertainty, and thus a significant reduction. Such cloud-to-ground lightning information is of value to variety of safety and research applications. 
     Although the invention has been described relative to specific embodiments thereof, there are numerous variations and modifications that may be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described. 
     Finally, any numerical parameters set forth in the specification and attached claims are approximations (for example, by using the term “about”) that may vary depending upon the desired properties sought to be obtained by the present invention. 
     At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of significant digits and by applying ordinary rounding.