As a rule, devices that transmit or receive data on the electromagnetic spectrum may cause interference. Portable radio, TV, Bluetooth, Wi-Fi, and GPS are all potential sources of interference. The other day I was flying and the DME readings were all over the place. I remembered that my cell phone was left on. I turned it off and the DME eventually settled down. Please, unless you're flying a glider or light sport aircraft, keep your GPS off. If you really want to know your speed, ask before or after the flight. The crew will gladly entertain such questions.