The first generation consisted of four direction finders, which receive the electromagnetic disturbances (sferics) generated in the lightning channel and test whether they are due to a cloud-to-ground stroke. Data of accepted strokes are sent to the central unit, which locates a stroke whenever at least two simultaneous direction messages arrive. Real-time located data are stored and sent forward for other uses. The system covered the southern half of Finland.
In 1997, the system was replaced by a new one, consisting of five sensors and a central unit. A sensor, called IMPACT, is basically a direction finder equipped with a satellite navigation (GPS) receiver, which gives a very accurate timing for each lightning-stroke observation. This enables a combined use of direction-finding and time-of-arrival methods of lightning location, which inreases the accuracy substantially. The sensor is also much more sensitive than its ancestor direction finder. The network extends farther to the north, now covering also most of Lapland. Summer 1998 was the first full season of the new system. In the 2000's, successors of the IMPACT sensors have been developed which still are based on the same principles. Since 2002, a co-operation with Norway and Sweden, called NORDLIS, has been used to share real-time sensor data to improve the system performance.
Yearly summaries of located lightning are published in the FMI report series, first Geophysical Publications and later Reports under the title Lightning Observations in Finland, YYYY (in Finnish and English). It usually appears before the end of the year, and the latest issues are available electronically.
The results of the old location system from 1987-97 can, with appropriate coefficients, be corrected to correspond to the stroke numbers of the new system. Also, the results of a flash-counter network in 1960-1988 has been matched with the location data (with some remaining uncertainties). The mean flash density in 1987-2008 in Finland is estimated to be 39 flashes per one hundred square kilometres. The highest flash density during this period was 109 in 1972, and the lowest, 11 in 1996.
Because the IMPACT system uses two independent methods for locating the strokes, triangulation and times of arrival, it is able to estimate its location accuracy. Most of the location errors within the country are near 1 km.
The rate of lightning-caused deaths in Finland is now about one per two years; in the beginning of the 20th century it was about ten per year.