Can one build a GPS reciever chip that uses the signals from GPS, GLONASS, EU and the Chinese navigation satellites?
In theory, there should then, assuming the EU/Chinese navigation systems have an equal amount of satellites as GPS/GLONASS, be always 24 or more satellites in view of the user - which should be enough to provide millimetre-precision positioning in real time.
I have a friend who build an automatic lawnmower as his pet project at uni,and he used two GPS receivers to get <10cm accurancy which allowed the lawnmower to make automatic turns around the garden.
In theory, there should then, assuming the EU/Chinese navigation systems have an equal amount of satellites as GPS/GLONASS, be always 24 or more satellites in view of the user - which should be enough to provide millimetre-precision positioning in real time.