During my years there, often I was in isolation (because I like it) and could spend time (lots of it!) to do some basic navigation skills testing. Using only a time tick from a passing C/O, a navigator's quick table (S-tables), and the angle of the sun as it shown through the holes in my steel window, I calculated my position on Earth to within a couple miles. Why? For the fun of it. The challenge. Using an 8 1/2 x 11 sheet of paper for an accurate ruler, and trigonometry, I could get the sun's angles down quite accurately. What was giving me the error was the C/O's watches which were accurate to the minute, not the second. That got me to within a couple hundred miles. Doing the calculations (based on unreliable times) 30 or so times allows for a statistical reduction in the circular error probability which eventually got me to within those couple miles.
Now do you see why I enjoyed my life in prison? :)
So, don't feel bad for me.