Firstly, your nav version is shown on the top right corner of the Settings screen

You probably have a 3-1/xx where xx is the version number

If you have 3-1/xx or 4-1/xx you do NOT need to buy a Trimble 9GPS receiver) as the receiver is built into the nav

Go into the nav service mode and see what it says about the GPS signal and report back here what you find.

Service mode:
Go into Settings
Press and hold MENU and wait
Service menu appears, go to GPS menu
Report on the GPS status information that you see