The performance of a radiometric unmanned aerial system (UAS) composed of a directional gamma spectrometer, the Advanced Radiation Detector for Unmanned Aerial Vehicle Operations (ARDUO), and a medium-lift helicopter airframe was demonstrated. The ARDUO's properties of the full-width half maximum energy resolution, linearity, gain stability, intrinsic photopeak efficiency, sheet source sensitivity and directional response were characterized in laboratory experiments. Small-scale trials were conducted over a single point source and two point sources of Cs137 at a 27m separation. The single point source strength was measured at 175±39MBq, which is within error of the true source strength of 161MBq. The directional information was shown to improve source locating by differentiating the two point sources. A large-scale `L'-shaped distributed source trial using La140 was completed. The total activity distributed in the area was estimated at 12±1GBq. The directional information was shown to improve constraint of the `L'-shaped distribution area.