the units are.
http://www.bendixking.com/static/cat...ls.jsp?pid=120
and
http://www.trimble.com/cugr.html
And 300 hours is about 4 months flying for me. I would say on average the signal will drop out maybe 2-3 times a week in nice wx. And if you get stuck in or near heavy rain. ie viz getting down to less than 2k in RA or huge wet clouds for a couple of grand above. If your above the tops it very rarely drops out but does if you go near hot danger areas sometimes.
As for the avionics engineers. They will all be licensed to work on public transport aircraft because thats what I fly, so I presume they know what they are doing before the CAA will let them loose.
MJ