1. Go to options and turn on "Use on-screen display".
2. Freeplay on Surtur, No track. (any drone works)
3. Fly to the south and find the stone circle.
4. Once there figure out which stone gates(forget about the middle one)make North, south, east, and west.
5. you are going to have to fly into these gates in the correct order(you can reset if you get lost), the order you fly is N E S W S E W N.
6. Once done you should hear Morse code being played from the center of the circle. Watch and listen to what happens.
How I found out:
there is 3 radio towers at the top of the mountains in Surtur, each playing a different Morse code.
1. STONE CIRCLE
2. N E S W S E W N
3. COMPASS
if you are wondering in the steam achievement there is Morse code saying RADIO TOWER.