The initial idea was simple: recreate this project, but with the cheaper ESP8266 Wifi-module. The ESP would catch the rise and set time of the ISS for a given location from open-notify.org , and turn some leds on when it's overhead. This was easily done but the API didn't tell anything about visibility or elevation of the overpass.

After I saw the video from Grady Hillhouse, I decided to implement the SGP4 model. I wasn't sure if the ESP was capabel of doing all those calculations, but after some tinkering around I got it working. Without any additional features the ESP can calculate around 300 satellite positions per second (at 160MHz).

With the information I found on Celestrak, I was able to write an algoritme for predicting overpasses. Calculating 10 overpasses takes around 1-2 seconds. After that I added some code to determine if the satellite is in the earth shadow or not. You can't see a satellite if it's eclipsed. So letting the user know if it's visible or not, can prevent a nightly walk outside to see the ISS passing by.

If you want to know the details on how to predict overpasses, I suggest reading the articles on Celestrak. I won't be able to explain it better.

On the hardware side, there isn't much to see. You just need an ESP8266 with a power source and some WS2812b connected to GPIO2. I first used a level shifter between the LEDs and ESP because I wasn't sure if it was stable enough. But after trying without, I removed them.