This is irrelevant info for the topic, but Differential GPS doesn't get you much these days. We used to have a subscription for FM corrections in MN and it was expensive, but that was before SA was turned off. We usually just post-processed uncorrected data back at the lab, which is more accurate than live corrections anyway, and is free. Out here you can use the USCG beacons though, which are free, but with SA off, it isn't much of an improvement.
CMT Z33 System
Here is the unit we were using, though it is not technically "millimeter" accuracy, 1cm realtime with post-process static corrected data at 5mm + 1ppm is good enough for most people! The backpack makes it a little inconvenient for the EPA stream surveys we were doing but there's some really killer featuring options with ArcInfo with these units. Unfortunately they use Winblows. So yes you can get very accurate with GPS, despite what the naysayer above says. It just takes a little while and is fairly worthless for avalanche rescue.
Back to avalanche beacons...