Differential drag controllers have been examined in the context of a full system simulation of a target/chaser pair of spacecraft in low Earth orbit. An Extended Kalman Filter has been designed to process measurement sets from GPS receivers on the target and chaser spacecraft. The estimated state from the Kalman Filter is used in a differential drag control algorithm to determine the appropriate control action. Modifications are made to the standard differential drag control algorithms to reduce unnecessary actuations in the presence of errors in the dynamic modeling, control actuation, and incoming measurements. Detailed explanations of the algorithms, dynamic models, and derivations for both the Kalman Filter and the differential drag control laws are presented. Multiple test cases are used to validate the controller performance under a variety of initial conditions. In these simulations, the differential drag control algorithms successfully maneuver the chaser spacecraft from the initial conditions to a final state with instantaneous time-average position (relative to the target spacecraft) of not more than 10 meters in the radial and in-track directions. Modifications to the standard control algorithms ensure that extraneous control actuations are minimized. An optimization algorithm is used determine the time-optimal differential drag control history, and the results are compared to the standard control logic and modified control logic. Based on the optimization results, it is recommended that a system employing differential drag control (especially those with limited computational resources) should use the modified control logic, as it provides a standardized methodology that can be followed in any mission. / text