Landing point designator – a head-up display
In the LPD, the engineers had devised a simple but powerful and ingenious way to tell the commander where P64 was taking them. It was as basic 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 vertical lines carefully scribed onto the inner and outer panes of the commander’s forward-facing w indow.
The lines calibrated the commander’s line of sight, as measured from a line directly forwards from his eye, downwards in degrees. To use it properly, he 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, at least, defined a downrange coordinate. Simultaneously, the computer would yaw the LM left or right so the line itself defined a lateral coordinate. The combination of the two pointed to 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 w here it was aimed. “Hey, there it is!” he called excitedly as he recognised an arc of craters and, just before them, the Snowman. “There it is! Son-of-a-Gun! Right dow n the middle of the road!”
The commander’s window in a LM simulator with the LPD scribe marks clearly visible on the window panes. (Courtesy Frank O’Brien)
“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.”