Originally Posted by doobierw
A GPS unit is a receive only device and does not transmit any signal which would possibly interfere with electronics on the aircraft...
Being receive only is not a guarantee of no emissions. Many types of receivers, especially those using heterodyne circuitry, emit in the process of receiving.
That's why no TV's or radios are allowed to be used.
Whether these signals cause ( or can) problems is another matter, but a 'receive only' device is not necessarily totally passive.