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Since about two years, autonomous guidance systems for agricultural machines are commercially available and their usage in practice is increasing. The technology is generally based on the Global Positioning System (GPS) with differential correction (DGPS) or Real Time Kinematic (RTK) DGPS. Inertial guidance technology is often used for dead reckoning and roll and pitch compensation. Until now, only limited information is available on the accuracy in practical use under typical and often difficult agricultural field conditions (side slope, wheel slip).

The tested system (AGRO NAV., GEO TEC electronics GmbH, Germany) was implemented on a standard tractor (MF 4255) and utilized RTK DGPS and an Inertial Measurement Unit (IMU) for navigation. Besides the steering, the system controlled also the engine, the clutch, the power take-off (PTO) and the whole hydraulic system (three-point linkage and remote valves).

To investigate the basic accuracy, two independent geodetic RTK DGPS rovers have been used as reference system. The measured accuracy in following a track (on a road) was about 25 mm with a maximum deviation of 100 mm. Field tests during winter wheat seeding showed a similar accuracy. But the downhill drift of the implement and the downhill yawing of the rear of the tractor resulted in deviations of up to 240 mm from path to path.

This system showed the great potential of RTK DGPS based auto guidance technology. Although equipped with a very efficient inertial system, limitations could be detected while working on side hill / cross slopes with three point mounted implements caused by yawing.