TDoA swarm with the Loco Positioning system

We have spent most of our time working on the two way ranging in the Loco Positioning system lately, mainly on features that are not directly related to the actual ranging but such as making it easier to configure and upgrade the anchors. As a result we have not exercised the TDoA mode in a while, so on our Fun Friday we wanted to play a bit with that and try to fly a small swarm with some sort of coordinated autonomous flight.


We have a Loco Positioning system set up in the basement, we call it the flight lab to make it sound more fancy! The setup has been 6 anchors with three anchors in the ceiling and three on the floor, configured as triangles pointing in opposite directions. When using two way ranging that is fine as the positioning works pretty well outside the volume defined by the anchors. For TDoA on the other hand, the accuracy of the estimated position degrades rapidly when you go outside the convex hull. We decided to add two more anchors (to a total of eight) and arrange them as the corners of a box. A few hours and mounting/cabling later we were ready to try it out.

We modified the swarmSequence script to suite my (limited) space and set it up to fly four Crazyflies in a square, moving them to the next corner of the square every 5 seconds. Next problem was to find 4 working Crazyflies and Loco Positioning decks. We have a few Crazyflies lying around but a fair number of them have been modified in one way or another but finally we had the hardware we needed and could run the script. After a couple of failed tries (out of battery and such) we shot this video

Lessons learned

So what did we learn from this exercise? Adding two more anchors and changing anchor positions improved the positioning significantly. We have seen earlier that TDoA is less accurate than two way ranging, but better anchor positions reduces the problem. We could also fly the swarm using our example python script (not using ROS) without too much work and trouble, even though the TDoA mode still is very experimental.

In this flight we used the stock controller and just moved the set point to the next desired position for each copter. We are really looking forward to try out the improved controller and trajectory planning that we showed at Fosdem in combination with the TDoA mode, we think it will improve the performance a lot!

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