Nav systems
I believe they had triple inertial nav systems, and one captain said he had never had an error in the inertial nav system in ten years of DC10 flying.
I have great respect for the head of the navigation section who stood up in court and explained that the co-ordinates had been changed.
The crew were, I believe navigating by reference to the inertial nav system. Not visually.
Of course they were discussing what they could see. That was the purpose of the flight.