This page describes various onboard external devices called a "Data logger", that can be connected to the Paparazzi autopilot.
Such a Data Logger device can be used to store telemetry data to a storage medium on board the aircraft. Such a storage medium can be an SD card or a Flash chip. For now logging is not done directly by the autopilot itself as the usage of a file system as well as possible lag you might get with a SD storage speed does not fit well with the real-time nature of Autopilots.
Note that not all loggers accept SD HighCapacity(SDHC) cards as storage
One would naturally first ask; why on on-board logger when we have data over the air?
The answer to this is multi-fold; Sometimes it is not possible to log everything over the air because of speed, lag and datavolume limitations of data send wireless over the air. Or one needs to log when there is no telemetry at all in long distance flight. Or one want to log all the RAW IMU data to debug specific scenarios.
To solve this issue, there are various loggers that can be connected to your Paparazzi Autopilot board.
Available Data logger options
There are numerous logging solutions, both with their pro's and con's; here the options:
A more unified method of storing data on the logger, info available here