In conventional tillage systems, uniform depth tillage is applied over the entire field; however, the depth of hardpan layer varies greatly within the field. Site-specific tillage, which modifies physical properties of soil only where the tillage is needed for crop growth, could achieve significant savings in fuel consumption and drawbar power requirements. Site-specific tillage can be implemented either with (1) a pre-tillage map technology, or (2) a real-time sensor. The pre-tillage map technology would be a two-step process in which a sensor such as a soil cone penetrometer would be employed to develop maps showing the existence of hardpan and its depth. This map would then be used in the site-specific tillage equipment control system to control tillage implement location and depth. The real-time sensor would provide a one-step system to control tillage implement location and depth. In this research, a GPS-based variable-depth tillage implement was developed and evaluated. While the tractor is moving, a laptop receives the location of the tractor through the GPS, and then based on the soil compaction map, the controller dictates the depth of tillage implement. The variable-depth tillage implement was first calibrated in the workshop. The response time of the electro-hydraulic system was assessed. Then, the variable-depth tillage implement was evaluated under field conditions. The time required for the implement to reach the set depth under two ground speeds (2.6 and 3.6 km h-1) was determined. Stress analysis of the designed tillage implement, using a finite-element method, showed that the implement benefited from enough strength. The response time was increased linearly with depth, i.e., 3.7 cm depth increase per second. The accuracy and precision of the control system for changing the depth were 1.2 and 0.83%, respectively. The results showed that by considering the response time of the system and the travel speed of the tractor, the control system can be activated such that the variable-depth tillage implement reaches to the desired depth by the time it approaches the next expected depth changing zone.