There are no published studies investigating the optimal sensor or sensor suite to be used for formation flight for drag reduction. This project will develop a sensors requirement document indicating what is needed of a vortex sensor to enable formation flight for drag reduction including survey report of available sensors, their strengths, limitations, and how they match the requirements. In simulation studies, vortex induced rolling moment is nominally used to sense the vortex structure. Other sensors suggested have been fuel flow, yawing moment, pressure sensors, LIDAR and others. These sensors using rolling moment, yawing moment, or similar measurements have finite range and are only effective when near the vortex core. The performance of extremum seeking control techniques, which can optimize drag reduction in formation flight, is dependent on the accuracy and resolution of the chosen vortex sensor. The project will determine the sensor or combination of sensors which will enable real-time sensing of the wing-tip vortex of an aircraft that will enable formation flight for drag reduction. The program will develop two sensor functions, one that will coarsely sense the wing-tip vortex location from a long range and one that will accurately sense the vortex structure from a near range. The unknown sensor or sensors must be light weight, inexpensive, and possess adequate accuracy. The ultimate solution may be a collection of disparate sensors whose outputs are combined through an optimal sensor fusion technique.