Getting north in Cesium for billboard

I have two maps, a 3D and a 2D Map in my app, The user can move both camera, but the current position and rotation of the 3D camera must be reflected in the 2D map (see Screenshot)

I made it draw on the 2D map using a billboard, and rotate using the rotation property and assigning to it the heading of the camera :

{
      image: 'assets/images/entity/camera.png',
      position: this.camera.position,
      rotation: this.camera.heading,
      id: this.id
 }

but it appear that depending on the place of the globe, the rotation as to be different. For example in the above example I have to do:

-this.camera.heading + 90

but in other place of the world, the calculation is different.

How can I make a formula that makes it match everywhere around the globe ?