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A tachograph is a device fitted to a vehicle that automatically records its speed and distance, together with the driver's activity selected from a choice of modes. The drive mode is activated automatically when the vehicle is in motion, and modern tachograph heads usually default to the other work mode upon coming to rest. The rest and availability modes can be manually selected by the driver whilst stationary.
A tachograph system comprises a sender unit mounted to the vehicle gearbox, the tachograph head and a recording medium. Tachograph heads are of either analogue or digital types. All relevant vehicles manufactured in the EU since 1 May 2006 must be fitted with digital tachograph heads. The recording medium for analogue heads are wax coated paper discs, and for digital heads there are two recording mediums: internal memory (which can be read out with one of a variety of download devices into a so-called .ddd file) and digital driver cards containing a microchip with flash memory. Digital driver cards store data in a format that can later also be read out as a .ddd file. These files – both those read from internal memory with a download device, and those read from the driver cards – can be imported into tachograph analysis/archival software.
Drivers and their employers are legally required[where?] to accurately record their activities, retain the records (files from internal memory and from driver cards must both be retained) and produce them on demand to transport authorities who are in charge of enforcing regulations governing drivers' working hours.[citation needed]
They are also used in the maritime world, for example through the Central Commission for Navigation on the Rhine.