The electronic field unit SDR31 by Sokkia

The SDR31 was a light and compact field unit. The side-mounted  measurement trigger buttons virtually allowed for the one-handed operation of the unit in case of standard measurements. With 640 KB or 1 MB of RAM, the SDR31 with DR-DOS operating system offered sufficient memory for data and the demanding SDR expert software.

The Sokkia SDR31 was sold in Germany from 1991 to 2003. Spare parts are no longer available.

The current successor model is the Archer.