Start off by saying NAT tracks and INS (using the lat/long MARKED ON THE RAMP) as a start point...
Then say it uses a Gyro and an accelerometer for knowing where it is and how fast its going!
If you mention GPS make sure your ready for him to ask the problems with it...
1. sattelite coverage.
2. in civil terms its accurate to 100m in mil. terms 2m.
3. the U.S can take it away at anytime for mil. use in a war.
Say these and believe me, he won't ask anymore simply because he wont know anymore!!
This is straight out of the Intro course Nav and Navaids lessons.