I found a way to make an artificial horizon:
Create a second view, that circles around a ball.
The faces of the ball face inwards.
Code:
temp.X = -35;
temp.Y = 0;
temp.Z = 0;
VEC_ROTATE(temp.X,player.PAN);
VEC_ADD(temp.X,pos_1.X);
vec_set(camera2.x,temp.x);

pos_2 is the center of the ball.
(The ball is deep under the terrain)