Flight planning in difficult terrain

Hi – I have been flying IRIS+ for a few months, mainly to get an understanding of UAV’s as tools for environmental survey and monitoring as well as of course being cool exciting technology. As an ecologist and GIS user I am imagining lots of opportunities for working at the end of a RC transmitter. I have made many orthophotos that have proved the tools work and could be the basis for habitat classification etc.. I now have a definite task that I’m looking for some advice with.

I am involved with a coastal sheep grazing project in order to re-establish heath on what are currently bracken covered sea slope and cliffs. I hope to establish monitoring with both regular and IR photo monitoring. I would hope to repeat this twice yearly for at least 5 years with analysis done with a variety of GIS tools. I am just about finished in building the kit, just waiting for some photographic filters in the post. I am using a pair of identical Canon cameras running CHDK on a hard working IRIS+ drone.

Here is a link to the area I am working on - HERE - which is 1.5km of difficult to access north facing slope immediately adjacent to the sea. My plan is to have maybe 20 x 10m diameter areas that I make repeated very low level (high resolution) photographical + IR survey to monitor the change in vegetation composition.

The challenges are that my take off point is higher than the areas I want to survey so everything would require planning in negative altitude, the cliffs are very steep in places so may require altering the altitude whilst within the survey area and I am uncertain of the accuracy of the elevation information I have (taken from contour data). There are also the usual battery time issues so I would expect to be looking at maybe 5 independent flights.

Does anyone know about using absolute altitude (as opposed to relative) in flight planning and about the use of sonar in maintaining elevation? Would it be possible to rely on sonar to maintain constant 6m altitude in such terrain? Also do people have experience of adding missions together to perform complex missions where speed would vary dramatically eg 1.5m/s low altitude surveys over small areas then jumping at normal speed to the next survey site? I am hoping to achieve this without scripting if at all possible.

Twin camera mount:3691202968?profile=original3691202891?profile=original

Thanks in advance for your thoughts

David Tipping  

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  • I could be wrong about the landings in RTL for hillsides?  I filmed another one today and nudged it a bit (so it didn't RTL in the trees)  All was going well and out of nowhere it darted into the hillside & did the hoky poky.  It's darted off twice on me now on those tight landings, which makes me think the sats are blocked and a few feet off the ground it freaks. 

    Maybe RTL to originally line it up and then full manual the rest of the way so that doesn't happen?  

  • Distributor

    check out this sensor,

    it is much better than sonar and cheaper than the one from LightWare:



    • Neat. And looking at the spec sheet, <30g installed (+UBEC)? Anyone put one of these on an IRIS+? Perhaps use the front mount and still stay out of the way of the gimbal?

    • How many units can you attach? I would like to protect front,back and sides too. 

    • Distributor

      currently the code doesn't support obstacle avoidance, only distance to the ground (so one sensor, looking straight down)

      I created a video a few days ago, it shows how the copter flies with such a sensor


    • Felix!  

      Multi Thanks!  I've been niggling at LidarLite for a few months.


      Your video will help me get it completed.

      1- I've got a UBEC to power LidarLite, and the resistors.  Using the UBEC to power the Pixhawk power rail, should I hook up the UBEC before or after the 3DR power module in order to keep noise down?  (My DiscoPro has open 5v power points that were used to power the Naza, but they're after the Power Module and I want to be sure to isolate the Lidar.)  

      My ESCs are connected to the power rail, too. All 3 wires. Problem?

      2- I'm probably going to mount the LidarLite at a forward angle, to give the MR time to react in fast forward flight. (Photos in the linked thread.)  Problem?

      3- How is LidarLite going to function over water?  Smooth flat water that bounces a Lidar signal 45 degrees away into space?  (I'll test over a mirror.)

      Again, thank you for the help.  Now I just need Lidar to work in 'Auto' mode.


    • Distributor


      1. Yes, you do have to provide 5V to the output rail of the Pixhawk, because the 5V coming from the Power Module are not connected to the output rail. You can use a ESC with built-in BEC or a standalone uBEC. I personally would not recommend to connect the 5V (red cable) of the ESCs if they have a built-in BEC, cause in this case you connect 4 (in the case of a quadcopter) independent BECs together. I have seen cases where ESCs did shut down because they did overheat (due to the BECs).

      So, if you use a separate uBEC (that's what I personally prefer the most), then you remove all the 5V cables from the ESCs (only connect signal and ground). If you want to use the BEC of one of the ESCs, then leave the 5V connected from ONE ESC, and remove the red cable from the others (you can pull it out of the connector if you lift the small plastic clip securing it - afterwards make sure to insulate it to prevent any short circuit).

      2. I was thinking of trying the same - but actually have doubts that it will deliver good results. You should keep in mind, that if you mount it with an angle, and fly backwards - then the sensor is pointed even more up, leading to false readings. Furthermore if you are in Loiter and have wind blowing from behind (so kind of flying backwards again), then the same happens.

      I am actually going to test now, how much the angle of the copter (during flight) affects the readings and thereby performance of the sensor, cause a tilt of the vehicle (for example during forward flight) will tilt the sensor and therefore create a different range reading.

      3. If the signal is reflected to the side (nothing coming back), then the sensor will not manage to read the distance. Have a look here:



    • Hey Felix,

      I can report success in getting LidarLite to work on my bench using the tips you provided.  Mission Planner had to be rebooted a few times for the connections to sort themselves out, but on the 3rd reboot (MP unplugged, UBEC unplugged, cold everything.) Wah Lah! I had 'Sonar Range'.

      In testing I had the Pixhawk powered by USB hooked up to Mission Planner.  If I unplugged the UBEC (No Power to the rail), I was scolded with, "BAD Lidar health!"  So gettting 5v to/from the rail is critical.

      I hypothesize the following results if a 35 degree forward tilt is used in mounting.

      If I mounted at a 45 degree forward angle, and went backward for 45 degrees, the MR would drop to the ground.  Right?


      Lidar Tilt.jpg

    • That would be a great application for the "follow me" mode!

    • Distributor

      it works for modes which keep/control the altitude of the copter (AltHold, Loiter, etc.)

      Usually it uses the barometric pressure sensor for those modes - if you install a rangefinder sensor, then it uses the rangefinder up to the maximum distance which the sensor can measure (40 meter for the LidarLite, bit over 7 meter for a sonar sensor), if above it uses the barometric pressure sensor.


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