Is it possible to get close to centimetre accuracy without using ground stations like DGPS? Would you get finer resolution if a GPS receiver also used an atomi

Is it possible to get close to centimetre accuracy without using ground stations like DGPS? Would you get finer resolution if a GPS receiver also used an atomic clock? I thought this was interesting - https://www.gps.gov/systems/gps/performance/accuracy/#difference https://www.gps.gov/systems/gps/performance/accuracy/#differ... the difference between military GPS and civilian GPS sounds like it uses 2 frequencies.