it's a while back...
...but if I remember correctly, the gps signal is put together out of a p code and a c/a code, where we, as civilian users can only utilise the c/a code. Military users (i.e. smart bombs etc.) lock onto the p-code giving them a higher level of accuracy. Selective Availability was used to further degrade the c/a code, but was switched off (as mentioned before..).
please correct me if I'm wrong!