AgOpenGPS represents the pinnacle of technology and is capable of ±2.5 cm (RTK) accuracy. This requires an RTK correction signal. The great news is that you are not required to pay for expensive annual subscriptions! You can use the free state or community networks available in many countries (e.g., Centipede network in France/Europe, SAPOS in Germany, ASG-EUPOS in Poland). If there are no free base stations within a 25-30 km radius of your area, the most cost-effective solution is to purchase your own RTK Base Station PoE (500 EUR one-time fee, no annual fee!).