Portable GPS limits
After I got my PPL, I picked up a portable Garmin GPS to help with navigation and awareness, and it has been wonderful in those respects.
Now that I'm working on my IFR training, it has been repeatedly emphasized that only approved, FIXED installations of GPS can be used for IFR.
I'd like to understand exactly why this is. In my limited VFR experience, I've never seen my portable GPS be inaccurate or otherwise misleading in the slightest.
I can certainly understand the bureaucratic, paper-pusher reasons why they would want to exclude a device that is frequently mounted/unmounted, and may not have a clear view of the sky.
But are there any actual, technically accurate, considerations that make a portable GPS unsafe for IFR flight?
If my GPS has limitations (technical, not legal) that I don't know about, I think I should know about them, even in VFR flight. If this is purely a question of certification/inspection, and not technical short-comings, I'd feel better knowing that as well.
(to state what should be obvious: No, I am NOT asking to use a portable for IFR flight. I'm seeking to understand the practical and legal limitations of my equipment.)