Perhaps an old non-WAAS GPS???
I have a Garmin 196 in my airplane, but the GPS systems we deal with in agriculture have <1meter precision with WAAS. We've had them for 3-4 years now.
We have one very high end system that uses "RTK", real time kinematic, to get < 1 inch precision using a local differential.
Taking down GPS would basically wipe out the modern surveying industry. I'd bet most of these guys would have to re-train on how to use the old equipment. If they still have it.
It would take out way more than that. See:
http://www.freerepublic.com/focus/f-news/1302340/posts?page=93#93