Even if the GPS satellite constellation is 100% functional and your receiver is working perfectly, the receiver may not give you usable "Desired Course" data; it may tell you your destination is at a Bearing of 137 degrees, but you may have no way of determining "Which way is 137 degrees?".
I've run into this problem in areas where there hasn't been a forest fire in a century, and I have to climb over all the long-fallen trees and crawl under the almost-fallen trees and I can't get up to the 2 mph the receiver requires to generate a "Go that way" arrow.
That’s a very good point. GPS units do have their limits even in the best of circumstances.