@ammanfrommars ...using pencils creates carbon dust, which over time builds up, possibly on vital electronics. As carbon is conductive, this is not deemed to be a good thing.
@Richard. It is quite easy to jam GPS with relatively low-powered, low-tech transmitters, but it is not a good military tactic. Not only do you deny positioning to your own forces you instantly become a 'hot' target. It is a then simple for the enemy to locate (and target) you. They simply fire a missile at, or bomb, the beacon you have conveniently provided. As such 'jammers' tend to be short-lived. If you want to jam, and live till lunchtime, you have to keep your transmitter constantly moving (preferably at high speed/altitude).
Standard military tactics often include not broadcasting your exact position to your enemy.