If you're in a bomber and you're guarding a sat and you get this message it means you're too far away. Simple as that. Get closer to the sat and you won't have this issue. I make sure I don't leave the sphere even when I'm killing sat attackers. Best way to figure it out is the light streams, once they start to fade you've gone too far up or down. You have to use your head to figure out the other axis.
Sorry, that's incorrect. I'm practically touching the surface, laying mines behind vanes, LoS underneath the B sat in mesa. I still get the message in the text buffer. I make random orbits or slow circles to LoS gunships, always right next to the surface.