And then there's RTK (I like to talk about https://centipede.fr https://centipede.fr ) that measures the offset between GPS signals and litteral ground truth an

And then there's RTK (I like to talk about https://centipede.fr https://centipede.fr ) that measures the offset between GPS signals and litteral ground truth and proadcasts it, enabling centimeter-level precision. The more systems, the merrier, even if they aren't all accurate, you can use them to increase precision (kalman anyone?). Someone mentioned multi band. You can also use signals from multiple systems at different frequencies.