It's pretty hard.

This assumes the existing radio links can handle the extra bandwidth. I don't see a WiFi card, so they may use radios with low-baud serial connections. Maybe the radios are good enough that doubling the radio communications is fine. Now, all the new hardware means extra draw (and potentially extra batteries) on each quadrotor. They might fly with this extra weight, but it won't be nearly as long.

Also, indoor flying means no GPS. In this case, using accelerometers means something is dead-reckoning the position of each quadrotor. There's no more space/power for processing juice, so the external system must do this. Now the external system must take this localized data from each quadrotor and place it in some global coordinate system (how does it do that?). It will use dead-reckoning to determine the position of each quadrotor. This means the system is accumulating errors over time, for each quadrotor. At least sailors could eventually correct their position with stars - there is no real global correction with these sensors. They could hypothetically do radio range estimation on the radio signals of the quadrotors, but who knows what kind of accuracy they will get.

Now I remember why I left robotics :-)

In one of their papers they state that they use Zigbee modules for a 56kbit link.

> Also, indoor flying means no GPS.

Correct. However, indoor positioning tech will likely come of age in the next 12-18 months driven by indoor mapping and navigation. Once that becomes commoditized, it'll make positioning a little aircraft like this in the x,y plane of a floorplan doable without dead-reckoning. From there, you could probably do some crude lidar to measure the distances above/below the craft to the ceiling/floor.

Actually ... that would be a very difficult step to perform. The room they fly in has a very expensive, sophisticated, high-speed motion capture system that gives a global picture of the placement of each vehicle. The data is then feed to a bank of computers for real-time processing. There's a reason they only show videos in that one room.

The data from individual sensors would be very noisy.

I completely disagree. None of those sensors would likely give anywhere near the accuracy, frame rate, or context of the current system. Also, there is the issue of where the computation is done. It seems obvious to me that the intended goal is a decentralized system, where every vehicle is autonomous and creates an internal model of its own surroundings. As best I can tell, in the current system all the computation and dispatching so instructions is done by a single centralized external computer (the one the external cameras are attached to).

Personally, I've operated accelerometers and gyros at 200 Hz after some low-pass filtering, which is really all you need for this application. I'm sure there are sensors that operate at even higher frequencies if need be. The GPS module doesn't have to be updated too fast (maybe 10 Hz?) because it only serves to correct for horizontal drift.

I run the 100 Hz flight control loop for my tricopter on an ATmega1280 clocked at 16 MHz. A _lot_ more could be done with something like the Beagleboard, including generating a 3D map of the surroundings if I mounted a Kinect on it.

I want to agree about the decentralized operation because there would be so many wireless signals that could interfere, but those guys seem to be controlling 20 quadrotors without problems. I know XBees can be configured to communicate on different channels, but decentralized operation will most likely require something beefier like wifi (and that, I don't know how to configure to work over so many different channels).

Even in best case scenario, I'm not sure GPS would be accurate enough, as it's only accurate down to few centimeters, you'd probably need more if you'd like to perform that kind of acrobatics as shown in the video. And what about the height? You could of course use GPS, but apparently (http://users.erols.com/dlwilson/gpsvert.htm), average consumer devices have error of few meters. Maybe we could use laser range finders pointed at the ground, but what if the quadcopter tilts? Or what if another quadcopter flies underneath it? Of course it's not unsolvable, but it'd probably require more computing power in the quadcopter.

What do you think?

I think LIDAR is the most likely solution. With enough resolution, you could get a reasonable 3D map of the vehicle's surroundings with a very accurate relative model of where the vehicle is located.

The wireless becomes the issue then, as the network effect quickly takes over. Sensing at high speed with bad sensors and low power/computation is really tough.