I have a project that has a PLC based control system on a buoy in the ocean. The system is collecting data all of the time which is periodically transmitted back to shore via a marine grade Ethernet based comm link. The system is powered by a 24V battery bank so this link won't be up all of the time so we can conserve power. The PLC will control power to the comm link device. I would like to use something that could be signaled from shore about 5-6 miles away which could output something to my PLC system which I would use as the "wake up" call. Does anyone know of such a device or system? Thanks.