A portable communication device has a first system for receiving signals from GPS satellites and a second system for communication with multiple satellites of a low orbit communications system. The device is arranged, at a time of required transmission, to monitor the elevation and azimuth of the GPS satellites in view so as to generate an elevation mask of where the sky is open at the time and to commence a required transmission only if the calculation prediction indicates that at least one of the satellites of the low earth orbit communication satellite system is in view.