The power of BTS is definitely bigger than the Iridium handset. The effective coverage maybe 3km, which means there is not guarantee of service, but it doesn't means that you won't be able to pick up signals. In fact, cell phone location signals has been picked up by towers even thought no service is provided. You should have heard of the stories about missing hikers being located because of their cell phone registered to the BTS, even thought they could not make phone calls.