Multi-system GPS receivers have been around for some time now, and they're quite cheap: http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/5...
mm-level accuracy is more difficult, but even the accuracy specs of those cheap modules is usually rather conservative.
Multi-system GPS receivers have been around for some time now, and they're quite cheap: http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/5...
mm-level accuracy is more difficult, but even the accuracy specs of those cheap modules is usually rather conservative.