What is a datalog GPS receiver and how does it work?
A datalog GPS receiver is a device, which - as long as it is switched on - continuously receives and logs (stores) data concerning its current position.
The operating mode of a datalog GPS receiver is based on the principle of the global satellite-based positioning system (GPS).

GPS
GPS (Global Positioning System) uses satellites that permanently emit their changing position and time. On the basis of this information global positioning can be generated.

GPS receivers
Based on the information emitted by the satellites GPS receivers generate their own position. Therefore they need the signals of at least three satellites.

Datalog GPS receivers
Contrary to a "pure" GPS receiver the datalog GPS receivers store the received data to an internal memory. Being switched on, datalog GPS receivers start writing the position, date, and time into a log file. This logged data can be read and saved on e.g. a PC. So the data is also available for other applications like locr GPS Photo.

Note: Different datalog GPS receivers create log files in different formats. How these files can be used with locr GPS Photo is explained here: "Which GPS datalogger can I use for automatic geotagging anyway?"