Mostly used lightning detection method in the past was Magnetic Direction Finding (MDF) method. Method based on detection of lightning electromagnetic wave direction. Later, Time Of Arrival (TOA) method was developed. TOA method measures time difference between lightning and lightning detection by sensor. Based on that time, distance of lightning form the sensor is determined. Modern sensors nowadays uses combined method for lightning detection. With combined method time and location of lightning strike is accurately determined.
Magnetic Direction Finding (MDF) method
To determine lightning strike location with MDF method at least two sensors are needed. Sensors detect azimuth (angle between north and direction towards lightning strike) between the sensor and lightning. That is the way to "draw" line through the sensor and lightning hit point. With at least two lines (from two or more sensors) intersection of those lines is calculated. Intersection represents the exact location of lightning. With the use of several sensors the lighting strike position offset is reduced.
Above picture shows determination of the direction from sensor to lightning. Electric field caused by lightning, causes the magnetic field (vector marked with B). The magnetic field is always perpendicular to electric field, so with the determination of magnetic field, lightning direction is determined. With two or more sensors the exact location of lightning can be calculated.
Time Of Arrival (TOA) method
To determine exact location of lightning strike with TOA method at least three sensors are needed. Each sensor is GPS synchronized, so each sensor knows the exact time in UTC and its location. Accuracy of time detection is about 300 nanoseconds. In TOA method, time differences between sensors and lightning are compared. Central analyzer calculate lightning location. Each sensor gives a hyperbola, which represents possible lightning locations.
Lightning strike location determined with three sensors, is accurate only, if it is somewhere inside of triangle determined with those three sensors. Each par of sensors determine one hyperbola. Three sensor gives three hyperbola which intersects in one point.
If the lightning location is outside of three sensors triangle, two possible lightning strike locations are given. In that case at least one additional sensor is needed to determine exact location of the lightning.
Combined method
New generation of sensors uses TOA and MDF method. This method is also named Combined method. Those sensor provides azimuth between sensor and lightning hit point and exact UTC time. With those sensors exact time and location of lightning is determined.
This method has no problems that are specific to a particular method. With using Combined method only two sensors are enough to determine lightning position. More sensors contributes to accuracy of lightning time and position.First direction vectors are used for lightning position evaluation, then with the time differences the exact lightning location is determined.