Near Loss in Tall Grass

UAV completely hidden in the tall grass

UAV completely hidden in the tall grass

This weekend I went out with a friend to fly my quadcopter UAV.  It had been several weeks since I started working on the lighting project for my UAV and I’d not had opportunity to do any flying since. I still don’t have the lighting relaying the control board status, but I do have the new lights installed, with both of the right arms wrapped in green and both of the left arms wrapped in red.

I’ve been flying the Ardupilot firmware 3.0.1 since I put the APM into my quadcopter. In general I’ve been happy with the solution, but I’ve had problems with it’s loiter function and sometimes return to launch causes the copter to take off in odd directions. I do not have the optical flow sensor installed on my unit and so the loiter function is purely based on GPS signals.

I’ve been reading the release notes on the release candidates for the Ardupilot firmware 3.1 and decided that since I didn’t know why my 3.0.1 copter wasn’t flying well, I might as well live dangerously and see if the 3.1rc4 did any better.

I checked the box in Mission Planner that I wanted to use the beta software, it refreshed itself and offered to let me load the 3.1rc4 firmware. After doing that, I went through the initial setup and calibration of the compass, accelerometer, and throttle settings. Then we went to try to fly the copter.

Initially things seemed very good. The new firmware rotates the motors when they are armed, which is a nice safety and status indication. The takeoff seemed smoother than most of my takeoffs have been, with the initial trim seeming to be perfect. I could release the right stick entirely and the copter was hovering in place with throttle control alone. Changing the switch to loiter mode also had it holding in the right position.

Problems seemed to happen after about five to seven minutes of flying. The return to launch and loiter functions seemed to become significantly less stable with erratic horizontal movements.

During the first flight with the new firmware after several minutes of comfortable flying I put the controller into loiter mode and handed the transmitter to Brett so he could see how it flew. I noticed that I’d obviously not configured the firmware to correctly monitor the battery power because Mission Planner was reporting 0.00v. Things continued well for a couple of minutes but then he started to have problems controlling it, RTL was not bringing it back towards us, and loiter was not causing it to hold position. He told me that he was making the decision to land where he was in the field and I confirmed that was the best thing to do. I have no problem walking a distance through a field to find a copter, but would not want to lose the copter in the trees.

Mission Planner Display of Flight

Mission Planner Display of Flight

Measuring some of the distances using the ruler in Google Earth show my copter was 160 meters away from us at the farthest southwest point, and only 20 meters from the trees. When Brett set it down, it was about 120 meters from where we were standing. We were flying visually, I have no cameras installed on my UAV right now.

I thought I knew where the copter had gone down, and so told Brett to stand in place and I’d go retrieve it. After walking a large distance and not seeing any sign of the device I turned around to see if Brett could point me in the right direction. He pointed me in one direction but I was still not having any luck. Communication was made worse because I’d left my phone back where we were flying from and was now standing over 150 meters away.

I realized that carrying the transmitter would have been a good idea because I could arm the motors and possibly hear them thrashing the grass, or even attempt to take off the unit to be able to zero in on it’s location. I went back and retrieved the transmitter, and Brett decided he could assist me from the air with his HexCopter. He’s set up with First Person Video and also has a GoPro Hero3+ Black on a brush-less gymbal mount. He was confident that he’d be able to quickly find my brightly lit QuadCopter from the air.

I went back out walking around the field, and he soon joined me virtually via his UAV. The video of our searching is on youtube. It was an overcast day and the grass had never lost the moisture the morning fog left it coated with. It’s possible to see the tracks where I walked because of the darkness where the grass has been disturbed.

After all of the manual searching, we were not able to find the copter. Starting up my computer again I was able to reestablish communication with the copter and read it’s reported GPS coordinates.  Brett entered the coordinates into an app on his iPhone and it took us to within 15 feet of the device and I was able to find it by the noise of the rotors attempting to move the grass when I tried to raise the throttle. The grass was thick enough that my motors would still not spin, and not much noise was made.  Even standing and looking down on the unit, it was easy to lose it in the grass.

I have overlayed the Brett’s search path and my flight path in Google Earth and it appears that he never went exactly over the top of the copter. Here are two different angles on the complete flight profiles.

Path of two UAVs with an overhead perspective

Path of two UAVs with an overhead perspective

Path of two UAVs showing the elevation

Path of two UAVs showing the elevation perspective

The path of my UAV switches colors each time the mode is switched between flight modes. I have configured Stabilize, Loiter, and Return to Launch. The relative horizontal speed of travel is indicated by the distance between the points on each line. Closer points indicate slower movement. Brett’s search path generally moves slowly and all of the points are close together. He nearly exhausted his battery in approximately 8 minutes of searching.

I have spent a lot of time sailing and one of the things leaned is a proper man overboard drill. During a proper man overboard drill it is one person’s responsibility to do nothing but keep pointing at the person overboard. Holding an arm outstretched pointing makes it much easier to keep track of exactly where the person is. To someone who has never done a drill it seems like it would be easy to keep track of a person or relocate them, but it is not. Losing a UAV in a field is not the life and death situation that losing a person overboard in cold water may be, but it is a reminder that you should have a plan for proper recovery when things go wrong.

I should have:

  • Read the GPS coordinates from the ground station immediately after the UAV landed.
  • Had Brett point at the location the device disappeared into the grass.
  • Carried a communication device into the field with me so that Brett could have relayed directions to me without shouting across over 100 meters.
  • Carried the transmitter with me on my initial trip to recover the UAV.

Considering that nothing was lost but time, it was a good day of flying.

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