In order to attain the weight saving of a Mars observation airplane, we are planning to supply energy by microwave. This paper presents the direction detection of the airplane. An array antenna on the ground receives a pilot signal from the airplane, and detects the direction of the airplane by phase differences of antenna elements. Quadrature detection is used for phase difference detection. For weight saving, we utilize the second harmonics radiated from rectennas on the airplane as a pilot signal. After checking the accuracy of direction detection under power transmission, we conducted direction detection experiments utilizing second harmonics radiated from a rectenna array on the small-sized airplane. At nearly all of the points, we could detect the azimuth angle of the airplane, but we couldn't detect at some points. We explained this is because multi pilot signals are radiated from a rectenna array, which behaved as phased array.