Dragon ISS Docking Autopilot in Go

Autopilot written in Go and executed as WebAssembly for docking the SpaceX Dragon capsule in the official simulator.

Screencast of the autopilot in action

Why

I never used Go and WebAssembly together before. This seemed like a nice and small excercise and I also got to refresh controller algorithms.

Using this autopilot

• Compile the autopilot and serve the files.

go run ./cmd/autopilot-server

• Open the SpaceX ISS docking simulator: https://iss-sim.spacex.com
• Open the developer console of your browser (eg. right click > Inspect in Chrome)
• Paste the following code into the console to load the autopilot:

const s = document.createElement("script");
s.setAttribute("src", "http://localhost:8000/loader.js?t="+ new Date().getTime());
document.body.appendChild(s);

If you want to make modifications to autopilot.go, restart the autopilot-server, refresh the simulator page and paste the above JavaScript code into the console again.

Controller algorithm

The main controller algorithm is the correct() method in autopilot.go. At first the current rate is calculated using the previous time and offset and dampened over some cycles DampingCycles.

Then a correction factor Correction is used to calculate the target rate. This target rate is limited based on the offset and RateFactor but kept between RateMin and RateMax.

The difference between target and current rate is then added to a clicks accumulator. Full clicks are then subtracted from the accumulator and returned from the function.

Play around with the ios configuration to get different controller behavior.

Built by @mbertschler