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One of John's collaborators on the more theoretical side, Craig "the theory guy" Stevenson, is in town, so John fires up his base-station computer to run a short demo. He was up late last night getting the code ready, and managed to get his new cooperative block-lifting code algorithm to work. As the computer comes up, John hooks in the quadquads' s batteries and confirms their normal startup beeps and bloops.
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The quads jump to life and Craig unconsciously steps back. John smilies as he sees his friend watch their work in action. The test goes well until the third task when one of the helicopters refuses to move. It slowly lowers itself to the ground and remains there, idling. John looks glances at Craig who seems surprised but doesn't know what is wrong. John looks at his phone and sees that data is still moving on all of the channels. He takes a closer look at the "QUAD2_COMMAND" channel, but valid commands are clearly being sent.
A few weeks ago, John had some integration trouble, so he created a "debug" mode for the system. Might as well try it, he says to himself and a few taps later he is on the command channel of the Robo-Monitor app. A quick tap sends the "debug" command, and the other quad lands. A flood of new data appears on the screen and John drills down into the QUAD2 subsystems. On-board sensing seems good... battery voltage is nominal... internal temperatures are valid... embedded processors are running... plenty of free RAM... wait... looks like position isn't correct. John picks up his ill ailing craft and notices that velocities remain zero.
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As John is thinking, Craig starts pacing and steps on a small silver ball. He asks John, "what's this," showing John a small silver the ball. "That's a Vicon marker... where did you find it?"
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"Well, only if it came off of this one. Let's check."
"Hey, looks like it must have gotten bumped off when it picked up that last block. That would definitely do it."
Ten minutes of marker-repair later, John switches back to his app. He punches "normal operation" command to move out of debug mode and the quads jump back to life. "It's always something," says Craig. "This is That's why I work in simulation."
...