UAV - Unpiloted Aerial Vehicle
Flight Crew Last 90 Days 60; Flight Crew Total 200; Flight Crew Type 200
Confusion; Situational Awareness
Airspace Violation All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action
Chart Or Publication; Procedure
I was flying an unmanned aircraft which has an autopilot which was programmed to fly a grid pattern nearby within Visual Line of Site (VLOS). I was monitoring the flight when a helicopter appeared in the area. I took action by triggering the autopilot's RTL (Return to Launch location). The aircraft automatically flew back to above where I launched it from and circled once; still at it's cruise altitude. It then began its landing sequence which consists of flying out while maintaining cruise altitude then turning around and starting a 10 degree pitch approach. The landing is automatic. Meanwhile the helicopter tracked my aircraft during this process and followed it to the ground where the helicopter then landed. The helicopter pilots then exited their aircraft and approached me; turns out they were Sheriffs! They claimed I flew over them by 20 feet and that they estimated my altitude to be 550 AGL.Altitude information between unmanned aircraft and manned aircraft are different. Unmanned aircraft reference the ground where manned aircraft reference a barometric pressure and set to calibrated to MSL. Disparity in altitudes are bound to occur using different reference points. Judging spatial distance between two objects from a d=ground observer's perspective is very difficult and inaccurate at best. Abundance of altitude buffer should be practiced by all. I'm surprised that with the abundance of drones in today's society that the helicopter pilot would be flying anywhere this low for his own safety's sake.
An Unmanned Aircraft System (UAS) pilot reported his aircraft and a law enforcement helicopter had a near miss at 550 feet even though his UAS computerized flight plan's altitude restriction was 400 feet. The QNH was 30.40 at the time.