If you own an AM radio, you own a lightning sensor.
Lightning either transmits a signal, or leaves a signature, depending on how you look at it (or when, rather). Associated with every lightning flash there is a low frequency radio signal, the creation of a momentary electrical field, generation of a magnetic field, and for reasons that are not really well understood a release of X-ray and Gamma radiation, and, depending on your proximity a brilliant flash of light and quite a lot of noise. All of which we have the technology to sense.
When your radio makes those occasional “scrcrchchc” sounds of static, what you may be hearing is the radio signal associated with a flash of lightning occurring from a distant thunderstorm. A signal which in the right atmospheric conditions and especially at night time can propagate vast distances. Up to six thousand miles even!
The XkyFire network of sensors “listens” continuously for the radio burst, and ‘feels’ for the electrical and magnetic fields that are created with every strike. Each event is reported by the sensing antennas second by second to a data collection server (a computer with a database <DB>) with a time stamp derived from GPS receivers co-located with each antenna. The GPS receivers report time that is accurate to a nanosecond, a vital element of accurately plotting strikes simultaneously reported by multiple antennas.
Custom applications running on the data collection server interrogate the DB at regular intervals to identify strikes with coincidental timing. In any instance where the timing is seen to be within 3/10000ths of a second, the system labels that as a correlated strike and assigns it a unique identification number.
The actual final determination of the strikes location is deduced from its “time of arrival” at each antenna.
From the image above, you can see that when a few sensors are used together they will generate a mathematical solution based on arcs, and where those arcs meet a small polygon is formed. The more sensors used, the smaller the resulting polygon. Radio signals travel at the speed of light, and the speed of light is a quantity that is well understood. In a nanosecond, for example, light travels about thirty feet. (Same reason you normally see a possible position error with your personal GPS receiver of +/- 30 feet!)
The XkyFire system “imagines” a polygon that is no more than thirty feet across, then calculates the center of that area and “says” that is where the strike must have occurred, plus or minus fifteen feet. If you were actually standing that close to a strike you probably wouldn’t be thinking of the calculation error, so at this point we say ‘good enough’ and put it onto the map.
The XkyFire network is programmed to use not less than four sensors with the calculation of each calculation for every lightning strike reported to the DB. We could do this with three, but with somewhat diminished accuracy. Often, twelve or fourteen detectors are used in the calculations.
It is important to note, too, that we are plotting the most energetic portion of a strike, and that we only plot + Cloud-Ground strikes. While positive cloud-ground strikes are usually nearly vertical, they are never perfectly so. Therefore, the position the strike grounds at might be slightly different than its reported position, depending on where the most energetic portion of the strike was sensed to be at. Also, purely atmospheric lightning, which is between ninety-two and ninety-six percent of all lightning that can be sensed, is specifically left out of the XkyFire calculations. The data is there, but XkyFire discards it. It is not dangerous to us earthlings, and really clutters up the map with storm information that is unlikely to be of any danger to ground facilities, or people.
It is not possible to predict exactly where, or when, the next flash of lightning will occur. But, the XkyFire system tells you an awful lot in real-time about where it has just recently hit. It is if from that information that one can infer the risk to yourself and your facilities in the coming minutes and hours. Different than a regional odds forecast, XkyFire tells you where there is imminent storm activity.
And it is all done by telling the time.
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