### Best way to determine the speed relative to the ground?

Hello! I'm in a group at Chalmers University of Technology that are working on a self stabilized quadcopter with position hold, see picture.

We've got a little problem: We need to know the speed relative to the ground in x, y and z. What's the best way to handle this?

THANKS!

#### Replies

• Thanks everyone for your input!

We have decided to control our quadcopter with position (not velocity) using GPS, an absolute pressure sensor and a magnetometer.

• While the absolute error for GPS chips can be quite big, their drift in the short term (minutes) is very small. So for distances relative to the start point, they are pretty good. You need to be outdoors and have very good sat readings.

That said, you're reqs are probably pretty tight and for testing you will need a system that will work indoors (for testing at least). Then it is pretty much image flow that you need to use, like the parrot quad copter (at CES). They use a downward facing camera and process the image into, I think, 16 sectors. They then find edges or other distinctive features in the image and compare XY positions of those from frame to frame to determine orientation and movement.

Works indoors (using horrible carpet with nasty designs) and outdoors. Needs a fair bit of processing power, but small arm boards should do it. http://designsomething.org/leopardboard/default.aspx

Look up optical flow in google

Great looking platform you have. Any info on specs, etc

Cheers

Diarmuid
• That's a pretty good idea! But how good will the measurements of the speed be, calculated by GPS?
• There's a couple of ways to go about this. The simplest is just to differentiate the latitude and longitude given by GPS, and that'll give you your x and y. GPS altitude isn't really accurate enough to give a z, so for that you need an airspeed, from something like the pressure sensor sold on this site. Then just compare the airspeed, which is sqrt(x2 + y2 + z2) to the groundspeed, which is just sqrt(x2 + y2), and from the difference between the two you can get your z velocity.

### Activity

RT @TinkerGen_: "The Tinkergen MARK (\$199) is my new favorite starter robocar. It’s got everything — computer vision, deep learning, sensor…
Monday
RT @roboton_io: Join our FREE Sumo Competition 🤖🏆 👉 https://roboton.io/ranking/vsc2020 #sumo #robot #edtech #competition #games4ed https://t.co/WOx…
Nov 16
First impressions of Tinkergen MARK robocar https://ift.tt/36IeZHc
Nov 16
Our review of the @TinkerGen_ MARK robocar, which is the best on the market right now https://diyrobocars.com/2020/11/15/first-impressions-of-tinkergen-mark-robocar/ https://t.co/ENIlU5SfZ2
Nov 15
RT @Ingmar_Stapel: I have now explained the OpenBot project in great detail on my blog with 12 articles step by step. I hope you enjoy read…
Nov 15
Nov 15
RT @a1k0n: Got a new chassis for outdoor races (hobbyking Quantum Vandal) but I totally didn't expect that it might cause problems for my g…
Nov 11
First impressions of the Intel OpenBot https://ift.tt/36qkVV4
Nov 10
Nov 9