Does someone already use a dual frequency smartphone ?
Might also be worth a look at using an SDR/gnuradio. Seems to need a reasonable SDR RF front end though, but have seen some discussion using cheap USB dongles…
I already use a dual-frequency receiver (u-blox F9P) with the centipede RTK network.
For HAS, 20 cm accuracy is the convergence target in less than 300 seconds.
The target seems to be with Galileo only and float convergence algorithm. With GPS+Galileo (without iono correction, not available in HAS Phase 1) and optimized algorithm, one may go down to 3 cm (horizontal) in about 2 mn.
It’s surprisingly difficult to get good quality GPS devices. Most of them don’t support the full accuracy of the GPS/Gallileo services available.
You end up having to spend hundreds on dedicated GPS hardware that is impractical to use.
Around 2000, we paid hundreds of euros/dollars for an outdoor GPS like Magellan Meridian for myself. Even in 2015, I bough my Etrex 30 for 200 €.
It costed my about 400 € to build my ublox F9P RTK datalogger (incl. antenna) following this project: