Apollo: The Race To The Moon

by Catherine Bly Cox

Then there was the difficulty with the PGNS (Primary Guidance and Navigation System, pronounced “pings” because sometimes the acronym was informally spelled PNGS). In the descent, if the PGNS found that the trajectory was deviating from nominal, it was supposed to keep balancing two competing desirable states of affairs: landing at the assigned target, and landing softly. During the simulations, the PGNS, given a choice between the two, had an unnerving tendency to decide to get to the target, come what may. “On occasion,” Kranz said, “if you were far enough off in your navigation state, [the PGNS] would point the LEM directly at the target and try to approach it in pretty much the same fashion that you would fire a projectile.” This made for an extremely hard landing, the kind that was euphemistically called dinging the spacecraft, less euphemistically known as a fatal crash.