This paper discusses a novel drone-based external radar calibration technique, which utilizes a metal sphere hanging from a drone. The advantages of drone-based calibration are introduced and applied to a dual intermediate-frequency frequency-modulation continuous-wave (FMCW) radar system, which was developed to detect a low radar cross section (RCS) target like a drone with high sensitivity. The developed radar system operates in 150-MHz bandwidth with a frequency sweeping time of 500 mu m at Ku-band frequencies. A 75-Omega coaxial cable was utilized in the radar system to physically separate the transmitter (Tx) and receiver (Rx) modules for better isolation. The physical separation with a coaxial cable helps in reducing Tx leakage signal effect, which is a critical concern of typical FMCW radar system, and also it helps the proposed radar system to have higher sensitivity. The designed radar system is a wired system that is fully synchronized with an internal reference clock generator. Offset frequency and other compensation factors were calculated for the calibration of the radar system using a big size metal sphere hanging from a drone. We observed that the theoretical calculations using the standard radar equation and the measurement results on calibration are in good agreement with each other. Furthermore, the RCS value of a small size drone is also investigated using the calibrated radar system. The RCS value estimated for the drone was about -22 dBsm.