Your a man after my own heart . I spent a lot of hours trying to capture the videos below with my
MA-2. You are correct in assuming that it is possible to fly in the tunnel without GPS but you will be in ATTI mode . My experience doing this is that your drone will act like a helium balloon. It will drift sideways unless your apply deliberate forward motion with the sticks .Slow but steady is my advice and land when you have travelled the required distance . It wont hover and will drift due to the slightest breeze or even its own downwash .This is caused both by the lack of gps, and the darkness ,causing the downward vision (responsible for stable hovering) system not to function.
Also like flying at night the lack of light will render the collision sensors useless so be prepared like me to sacrifice a couple of sets of propellors . I had no issues with connectivity inside the tunnels but be very very gentle with the stick inputs.
