A phone's own GPS is good to a few meters. To measure to the centimeter, you need three things together.
1. An RTK GNSS receiver
An external receiver with RTK, connected to the phone over Bluetooth LE or Wi-Fi. Multi-band receivers (L1 + L2 or L1 + L5) fix fastest. See Choosing a GNSS receiver and Connect a GNSS receiver.
2. An NTRIP correction account
RTK works by comparing the receiver's signals with those of a base station at a known position, within about 20–30 km of you. An NTRIP correction service streams that data to you. You can get an account from:
- A government or university network (CORS). Many countries, states and provinces run one, often free after registration.
- A commercial RTK service, by monthly or yearly subscription, usually with wider coverage.
- Your own base station, or a community caster nearby.
The service gives you a caster address, port, username and password to enter in the app. See Set up RTK corrections (NTRIP).
3. Internet on the phone while you measure
The app receives the corrections over the phone's internet connection, mobile data or Wi-Fi, and passes them on to the receiver. They have to keep arriving for as long as you measure: when they stop, the receiver soon drops back from centimeter level. In the field that usually means mobile data with coverage where you work.
The phone's internet isn't needed if the receiver gets corrections by itself, for example over radio from your own base station, or through its own SIM card.
Then wait for the fix
With all three in place, the receiver usually reaches Centimeter-level (RTK fixed) within a minute or two under open sky, and every point keeps the receiver's own accuracy estimate. See Fix levels and accuracy, or, if it doesn't fix, Why isn't RTK fixing?