An 
automatic steering system and method are provided for a vehicle including an hydraulic primary 
steering system. The 
automatic steering system includes a guidance module with a 
GPS receiver and a 
microprocessor adapted to process and store 
GPS data defining travel paths, which can be associated with a cultivated field in an agricultural vehicle application. An 
automatic steering module is connected to the guidance module and to a steering valve control block, which provides pressurized 
hydraulic fluid in parallel with the vehicle's primary hydrostatic 
steering system. The automatic 
steering system utilizes a constant factor, such as steering rate, for 
predictability and simplicity in the operation of the automatic steering 
system. A 
feedback loop from the vehicle hydrostatic steering system uses the vehicle's actual turning rate for comparison with a desired turning rate. The system is adapted for original equipment installation and 
retrofitting on vehicles, such as farm tractors, with various primary hydrostatic steering system configurations. An automatic steering method includes the steps of: initializing the system; adjusting the steering with an hydraulic valve to provide a constant steering rate; providing feedback corresponding to the vehicle's actual turning rate and combining the feedback with other input signals to provide automatic steering.