Getting the height right

The spacecraft’s design had assumed that the windows would face forward during the final approach to give the crew a view of their landing site, and that it would pitch into this attitude from a windows-up attitude. If the window’s were facing up, and the landing radar had to face downwards, then its antenna had to be mounted on the base of the descent stage on the side opposite the windows.

“Intrepid, Houston,” called Carr to Conrad and Bean. "You’re looking good at three [minutes].’’

“Okay, Houston," replied Conrad. He w’as waiting for two indicator lights on the DSKY to go out, which would mean that the landing radar was producing valid measurements of their altitude and horizontal velocity. “I have an altitude light out; and I have a velocity light out.”

“Roger.’’

Conrad then looked at the DSKY’s display for a number. "I’m showing minus 918. Minus 1.000. Looks good. How’’s it look to you, Houston?"

The number, called ‘dcita-H’ was telling him that their height, as measured by the landing radar was 1.000 feet or 300 metres lower than the computer’s estimation based on its knowledge of their orbit.

“Roger; it looks good. Recommend you incorporate it,” said Carr as the flight controllers passed on their wisdom.

“No sooner said than done. Let me know witen it converges. I’m going back to my normal displays.” With this declaration that the radar’s height measurement was reasonable, Conrad commanded the computer to accept the radar data, compare it to its current estimation of their height and rate of descent, then attempt to lly a compromise between the two. Having done so. il then revised their trajectory to high gate and repeated the cycle until its estimation of their height converged with the data coming from the radar. This gradually folded the new’ data into the flight path without causing a sudden transient. The delta-H figure on Apollo 12 was small – radar and computer were almost in agreement. Had the radar shown them to be 10.000 feet or 3.000 metres higher than the computer believed them to be. an abort would have been called for because they would have run out of fuel before reaching high gate.