Third, mutual interference between satellites restricted the total number of satellites to five. These drawbacks pretty much eliminated aviation applications and severely limited land-based applications. Four satellites are spaced in orbital planes 45 degrees apart. This is an international system for location of distress beacons on 121.5, 243.0, and 406 MHz from ships and airplanes. Six satellites are spaced in orbital planes 30 degrees apart.
Frequencies are different from the civil system.
The 150 MHz signals are very easy to receive on a scanner or VHF-receiver. This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. This FAQ chapter was last updated in February 1998.