Here's a URL with some informed discussion, links to FAA, Boeing, emission tests on the Garmin GPS III etc.
<a href="http://www.gpsinformation.net/airgps/gpsrfi.htm" target="_blank">http://www.gpsinformation.net/airgps/gpsrfi.htm</a>
Interesting read, notwithstanding PIC jurisdiction, which is not in question.