Sorry, I forgot. Another two pins are taken by a crystal. Could I get rid of it and use the GPS module's provided time to correct the inaccuracy of it's internal oscillator?.

The other device on my system is a GPS module, and it uses serial (RX-TX) :(

@PeterJ It is true that the time of flight is almost 0. However, the more beacons I use, and the more times I make the measurement, the more precise my measurement will on average be. I've also corrected my post: 0.5m is the ideal scenario, a multi-metre precision is acceptable.