Philip...
If it helps you, you can download 3.1.3 here -> http://appldnld.apple.com.edgesuite....8_Restore.ipsw
And Assisted GPS still requires satellite to get a decent fix...
Like every GPS, tree cover, heavy thunder cloud etc will block a fix.
In other areas you can also get what is known as Canyoning... Common in city streets or valleys.. This can throw even a high end GPS out by over 100km (occasionally)

