Locating lightning strikes, which in essence are radio signal emitters, can be done in various ways and two common methods are Direction Finding (DF) and signal Arrival Time Difference (ATD, TOA or TDoA). Microsferics lightning detection network (MicroLDN) uses the latter method whereas in older networks, e.g. StrikeStar, the less accurate DF method is used.
TDoA requires that every station in the network is on a common precise time base. In other words, the local clock on each of the participating stations must not deviate from any of the other stations by more than a few microseconds. The reason for this is that the process of locating a signal transmitter, in this case a lightning discharge which emits an electromagnetic wave, is by means of looking at the time when the lightning signal was received at each station.
Electromagnetic waves travel at the speed of light, 299 792 458 meters/second. If we translate this to distance traveled during a microsecond we find that it is approximately 303 meters (994 feet) per microsecond. This number tells us that for each microsecond deviation in a TDoA locating network time base, any position estimations will be wrong by at least 303 meters.
Obtaining such an exact time base across a network of independent nodes can only be achieved by equipping each node with specialized electronics that include a GPS disciplined clock.
TDoA locating requires a minimum of four sensors to resolve ambiguities in computed emitter locations. More than four sensors will yield more accurate locating results. The illustration below demonstrates in a simplified manner how TDoA locating works. In this example we have the minimum required four lightning detectors, named SENSOR 1 to 4.
A lightning discharge occurs at an unknown time and location, symbolized by the bolt icon. T1 to T4 arrows represents the time it takes for the lightning signal to reach the respective sensor. These arrival times will not be equal because the distance from each sensor to the lightning discharge is different.
The circles around each sensor are representative of the signal travel time from point of origin to receiver. A larger circle means the signal had to travel for a longer time and vice versa. For a single station the circle will therefore indicate the distance to the lightning while direction will be unknown. When we combine this information from all four stations alongside their respective known geographical location, we can computationally figure out where all four circles intersect. The intersection point is the location of the lightning discharge. By using the now known position of the lightning discharge and the signal time of arrival at each site, we can also find the exact time when the lightning event occurred.
MicroLDN uses the Boltek range of directional lightning detectors (StormTracker/LD-250/LD-350) as base. To this our specially designed electronics module must be added. This so called TOA kit extends a station's capability to include operating on a network common and very precise time base, geographical position reporting and lightning signal processing and data transmission.