3 followers Follow

Knowing the drone's actual height above terrain

I am very new to flying drones and still very much in the process of learning. On my third or fourth flight, I was using Map Pilot, flying at an altitude of 335 ft with terrain awareness enabled. My Mavic Pro was about 2,000 feet out and only a quarter of a mile into its mission when out of nowhere, I hear a the sound of a small plane approaching. I panicked and immediately hit the return to home button on the controller. The drone was at 865 feet above the home point and immediately started climbing, reaching 913 feet, though it was all down hill to the home point. I knew this would happen as I hadn't yet reached the summit of the highest hill in the survey area. I used the left stick to manually bring the vehicle down on its way back, but was overly conservative as I was afraid that I'd get too low and run into the trees. I ended up with the Mavic at 700 feet AGL when I reached flat land at the base of the hill, which was 500 feet from the home point, and had brought it down to 500 feet by the time it was overhead. Luckily the plane, which was flying pretty low, passed a little past the area my drone was flying, but the incident sure gave me a scare.
I found the following FAA rules governing low flying aircraft:
Title ·14, ·Code of Federal Regulations, Section 91.119 of the General Operating and Flight Rules which specifically prohibits low-flying aircraft.
·91.119 Minimum safe altitudes; general
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes;
(a) ·Anywhere. ·An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) ·Over congested areas. ·Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2.000 feet of the aircraft.
(c) ·Over other than congested areas.
An altitude of 500 feet above the surface except over open water or sparsely populated areas. In that case, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) ·Helicopters. ·Helicopters may be operated at less than the minimums prescribed In paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with routes or altitudes specifically prescribed for helicopters by the Administrator.
I interpret paragraph (c) as meaning that a plane can fly lower than 500 feet over water or sparsely populated areas, which is scary because I'd think it's where probably the majority of UAV flights take place. But even 500 feet is only a hundred feet higher than 400 feet!
Looking back, I think the most prudent thing to do in that situation, instead of returning to home, would have been to temporarily stop the mission and hover, bringing the drone down to a hundred feet or so above the tree tops and wait for the plane to pass. If I had known the Mavic's actual height above the terrain as it was descending, I would have been able to do this.
My question is, in the case of a paused or cancelled mission, or on the way back to the home point from a terrain aware mission, would it be possible, either graphically or numerically, to display the drone's actual height above the ground based on SRTM or whatever elevation data is being used? This would allow the pilot to have the drone "hunker down" at a safer altitude until an aircraft passed, or for manually lowering the craft to a safe altitude when returning from a terrain aware flight, which using the default return to home behavior, may be many hundreds of feet higher than what is safe or legal.
Bruce Young

Please sign in to leave a comment.



Hi Bruce,

Just a couple of comments on what happened.

First and foremost, you are in control and relied upon to fly the craft in a safe and legal manner, even when it is flying autonomously.  I'm not saying you weren't.  Just emphasizing.  RTH is an option, but when you think there may be a chance that doing that may lead to unexpected results, like when you don't know if it may even fly closer to the manned aircraft, it may not be the best option.  

For example, you did have the model within your VLOS, correct?  You could have taken manual control and using the down facing camera, lower the drone towards the ground until you were able to identify and determine the manned aircraft was no factor to your flight.  The FAA fully expects you to get out of the way and even terminate the flight (possibly crashing into trees or terrain) in order to ensure separation.

Manned airmen are of course supposed to follow the rules too.  But that point is somewhat irrelevant to what you need to do to stay safe and legal. I'm glad everything went okay and you now have this great experience to learn from.



Dave Pitman 0 votes

Once you have initiated the RTH it is fully under the aircraft's control at that point. It will return home to the home point at either the highest point in the terrain aware path or 40 meters higher than that value depending on what you chose at the prompt before takeoff. The extra 40 meters is a safety factor for dealing with taller terrain. There is no way to make the RTH actually terrain aware itself. We could potentially add a readout of the AGL value during flight though but assumes you still have a live telemetry connection. 

You can pause a flight, take note of the current altitude, manually fly the aircraft downward to a safe height and then after the aircraft has moved out of the area ascend back to the original height before continuing the mission. The part about returning to the original altitude is the important part here.

Zane 0 votes

Dave, thanks for the advice. I will certainly do everything in my power to keep out of the way of low flying manned aircraft.

Zane, I certainly feel a readout of the drone's current height above the terrain, especially during the RTH trip from a terrain aware mission, would be very useful in helping the pilot safely keep the drone at a legal altitude by allowing the use of the control sticks to force the craft to fly lower, instead of letting it autonomously return at its default altitude.

I have a question about your statement that the drone: "... will return home to the home point at either the highest point in the terrain aware path or 40 meters higher than that value depending on what you chose at the prompt before takeoff." Is the highest point you mention the actual elevation of the terrain, or the elevation PLUS the altitude that you have chosen to fly the mission at? As shown in the image below illustrating the flight I abandoned because of an approaching aircraft, the highest point in the terrain aware path was 649 feet according to SRTM data, and an extra 40 meters would make 780 ft. (The home point was 7 feet above MSL.) The flight was abandoned when the drone was at 858 ft above the home point. Upon abandoning, the craft almost immediately started climbing, but I think I started pulling the left stick back before it began returning to the home point. Since the abandonment point was already higher than 780 feet, this makes me think that the RTH altitude is either the highest point in the terrain aware path, or that height plus 40 meters, PLUS the predetermined mission altitude. Is that correct? I hope my question isn't too confusing!


Bruce Young 0 votes