The typical minimum signal strength required to register on the BGAN network is as follows:
HNS 9201 49 dBHz
Thrane & Thrane   EXPLORER 300, 500, 700  46 dBHz
Nera WorldPro 1000/1010 41 dBHz
If you are connected to BGAN LaunchPad, check the signal strength on the status bar.  You can also use the LEDs or the display on the terminal to monitor signal strength. The amount and type of information provided depends on the terminal.