In the LPD, the computer's programmers had devised a simple but powerful way to tell the commander where P64 was taking them. It was as simple a device as you could hope to find in a high-tech spacecraft, though its operation depended on what was then one of the world's most sophisticated small computers. It consisted of nothing more than lines carefully scribed onto the inner and outer panes of the commander's forward-facing window that marked his line of sight, measured from a line directly forwards, downwards in degrees.
To use it properly, the commander merely positioned himself in such a way that the two sets of lines were perfectly superimposed, which meant that he was in the proper position and their sight lines were valid. As the computer flew the LM to a landing, it displayed an angle on the DSKY that represented the line of sight to the expected landing site. The commander looked past the markings towards the surface and noted the terrain in front of him that coincided with the stated angle. That was the designated landing site. This lightweight but elegant solution also allowed him to redesignate the landing site by nudging his hand controller left, right, back or forward, and P64 would then aim the LM for the new target.
Immediately Conrad had his angle, he looked out his window to see where it was aimed. ''Hey, there it is!'' he called excitedly as he recognised the Snowman. ''There it is! Son-of-a-Gun! Right down the middle of the road!''
''Outstanding!'' said Bean who then began feeding LPD angles to his commander. ''42 degrees, Pete.''
''Hey, it's targeted right for the centre of the crater!'' enthused Conrad. ''I can't believe it!''
"Amazing!" agreed Bean. "Fantastic! 42 degrees, babe."
After the mission, Conrad talked about this moment when their plans for an accurate landing came good. "For the first couple of seconds, I had no recognition of where we were, although the visibility was excellent. It was almost like a black-and-white painting. The shadows were extremely black, illustrating the craters; and, all of a sudden, when I oriented myself down about the 40-degree line in the LPD, our five-crater chain and the Snowman stood out like a sore thumb."
Whereas the first three landings had been on open, if rugged sites, the approach taken by Falcon on Apollo 15 took the LM between a pair of mountains. This made the experience of landing somewhat different, especially during P64's regime.
"Falcon, Houston," said Ed Mitchell in mission control. "We expect you may be a little south of the site. 3,000 feet.'' By that, he meant that their flight path, travelling east to west, seemed to be taking them to a point 1 kilometre south of their intended target. When they started P64, Scott would have to steer to the right to get them back on track.
"Okay. Coming up on 8,000," said Irwin as they passed 2,500 metres altitude. Even before P64, he had begun to concentrate on keeping his commander up to date. This was one of two events that biased Scott's estimation of where they were going to land. The other concerned what he and Irwin saw out to the left prior to P64. "I looked out the window, and I could see Mount Hadley Delta. We seemed to be floating across Hadley Delta and my impression at the time was that we were way long because I could see the mountain out the window and we were still probably 10,000 to 11,000 feet high.'' Scott then approached pitchover thinking he was going to land long and south, which was worrying because several kilometres to the west of the intended landing point was Hadley Rille, and he didn't want to come down in its canyon. Actually, they were at about 2,750 metres, and the 3,350-metre mountain was towering 600 metres above them.
In later years, Irwin discussed the moment of pitchover. "We're not looking down as we come over the mountains,'' he chuckled. "We're looking straight up until we get down to around 6,000 feet [1,800 metres] and we pitch forward about 30 degrees and, at that point, we could look forward and see where we were. We could see the mountains. I was startled because, out the window, I could see Mount Hadley Delta which towered about 6,000 or 7,000 feet above us. And we never had that type of presentation in the simulator.'' Landing simulations had used a small TV camera flying over a plaster model of the surface to present the crew with the view they would get out of their windows. It included the impressive canyon called Hadley Rille that would be in front of them, but not the mountains to either side of the approach track.
"When we pitched over,'' continued Irwin, "I could see the mountain that towered above us out Dave's window. I'm sure it startled Dave, too, because we wanted to know, you know, were we coming in to the right place? Fortunately, the rille was there and it was such a beautiful landmark that we knew we were coming in to the right area. But we'd never had that side view in any of our simulations. It was just the front view. A level plain with the canyon. And it would have been very impressive to be able to look out as we were skimming over the mountains with about 6,000-foot terrain clearance. At that speed it would have been really spectacular, like a low-level pass as we came over the mountains down into the valley.''
''7,000 feet. P64!'' called Irwin as they passed 2,150 metres altitude.
''We have LPD,'' said Irwin as an angle appeared on the DSKY.
Scott finally saw where he was going. His impression of landing long had been wrong, but mission control's southerly estimation was correct. ''LPD. Coming right,'' he said as he began a series of redesignations to move the computer's targeting north to where they had planned to touch down.
To deal with the mountainous terrain around the Hadley landing site, planners had made a change to the approach phase of the descent. Instead of making a shallow approach of only about a 12-degree angle to the ground as the previous missions had done, Falcon came in on a much steeper 25-degree approach path.
''Four-zero,'' called Irwin as he began feeding Scott with constant updates of the LPD angle on the window and their altitude. ''5,000 feet. 39. 39. 38. 39.''
''Okay. I got a good spot,'' said Scott once he had decided where he was going to set it down.
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