galaxy flyer
I don't think the different coordinate system of GLONASS is a great problem--it is "only" some geometric transformation arithmetic. Since I am told you can do all theGPS maths on a single chip these days, so what is a little bit more!
What is probably a greater difference between the two systems is the signal structure.Civil GPS (so far) uses single carrier frequency with multiple digital moduation (pseudo-random-numbers) adequate for all the constellation,(this is clever stuff invented by a guy called Gold). On other hand GLONASS has different carrier frequency for each satellite.
I think I have read somewhere that at leas one US manufacturer is developing a combined receiver