The GPS signals are very weak compared to, e.g., a cell-site transmitter. In spite of a "strong" signal, cellular coverage problems are well known. And on top of that, the GPS satellites are about 11,000 nautical miles away. Needless to say, GPS radio receivers must be very sensitive. Antenna design is also a very important aspect of GPS receiver function.

GPS-equipped PLBs contain other radio transmitters, e.g., the 403 MHz beacon and a 121.5 MHz homing signal, as well as the highly sensitive GPS receiver. In addition to these "intentional radiators," other electronics in the PLBs generate "unintentional (radio) radiation," or radio-frequency noise.

This noise and the transmitted signals can interfere with the operation of the internal GPS receiver. Similar behavior can be observed in broadcast radio receivers placed next to computers (especially AM radios).

Consequently, in a variety of circumstances, it can be advantageous to utilize an external GPS unit. Such circumstances include, for example, the relative quaility (gain and pattern) of the GPS antenna between the two devices, and an ability to physically separate a GPS receiver from the electrical noise of the PLB.

Such potential benefits of an external GPS must be balanced with the additional complexity and failure modes of such a system, e.g., breaking the cable.

/Neal