How do they sense lightening?

The weather forecast said there were several thousand lightening strikes yesterday.  How do they know the exact number? 

We have a triangulation method called ATDNET which detects and monitors all lightning strikes around the world.

It's not completely accurate - sometimes it will 'detect' a lightning strike in a place that cannot have had one (i.e. where there's no clouds!), but it's pretty good.

See here:
http://www.metoffice.gov.uk/learning/learn-about-the-weather/thunder-and-lightning/lightning
and
https://www.wmo.int/pages/prog/www/IMOP/publications/IOM-94-TECO2006/2(7)_Nash_UK.pdf

Map of all the lightning strikes detected in Europe right now:
http://www.metoffice.gov.uk/public/weather/observation/map/#?tab=map&map=Lightning&fcTime=1469071800&zoom=5&lon=-1.11&lat=53.24

Cool , I wanted to know how the sensors worked, that link told me.  (Low frequency radio waves crammed sferics)

Thanks!