Changing GPS Frequency

I am currently conducting research on UASs that requires me to change the frequency at which the GPS updates. I have looked through the development page at but I may have overlooked a section.

I am using the released version of ArduPlane. I would like to sample the GPS at 50Hz and I see in the code of ArduPlane.pde that there is a function that supposedly updates at that frequency but I cannot find it in any of the logs or datalogs.

Any and all help would be much appreciated.

the gps code is in ardupilot/libraries/AP_GPS
GPS device is supposed to be already configured, not so much chipset detection and config uploading in the code.