I recorded a dual navex using gps with a view to using it as a debrief tool.. I was disappointed to see that the graphical representation of the flight failed to capture those aspects of the student's flying that gave me cause for concern, such as poor height and heading control, especially when attention was divided between tasks such as flying straight and level and conducting checks. With sufficient resolution those aspects might have been caught but it's all smeared (filtered) out in the picture displayed by whatever software is being used to display the data the picture and the sample rate is insufficient. Now, a student who comes back 20 mins late wih 2 gallons less than plog fuel...that's much more of a debrief tool.