Although the IMU was finely engineered, the platform inevitably drifted very slowly out of perfect alignment with the REFSMMAT, and had to be realigned at regular intervals. Naturally, the CMP turned to the spacecraft's optical system to take sightings of the stars. Because this task was carried out in conjunction with the computer using Program 52, it was simply known as doing a P52, another ubiquitous term in Apollo jargon. If the crew needed to perform any kind of propulsive burn, which always required accurate aiming of the spacecraft's engines, then a P52 was performed beforehand as a matter of standard procedure. It was also carried out if some time had elapsed since the previous P52 because many other operations, such as the aiming of cameras and cislunar navigation, depended on a properly aligned platform. At any rate, engineers were keen to monitor the rate of drift of the platform as an indicator of the IMU's overall health.
With two stars, a P52 could determine the orientation of the entire universe around the spacecraft. With the spacecraft held in a steady attitude, the sextant's movable line
of sight was pointed towards a specified star. Once the star was accurately aligned in the eyepiece graticule, a button was pressed to tell the computer to note the star's apparent position with respect to the slightly misaligned platform. The sextant was then aimed at a second star, where another mark was taken. The computer now knew where the stars appeared to be and, from its internal knowledge of where they really were, could calculate the amount by which the platform had drifted since its previous realignment. These angles were usually very small, and were expressed to an accuracy of thousandths of a degree. Their values were displayed on the DSKY to be passed on to the controllers in Houston, and indicated how the gimbals needed to be rotated, or torqued, to bring the platform back into accurate alignment.
As in every aspect of their work, the command module pilots were competitive about aligning the platform well, and the P52 procedure included a measurement of their sighting accuracy so as to let them gauge their performance. From its internal tables, the computer knew what the angle between two stars should be. Once the CMP had made the two star sightings, it also knew what the measured angle between them was and could display the difference between these two angles to hundredths of a degree. If the pilot had made a perfect P52, the zero difference would be displayed on the DSKY as a row of five noughts, 00000, which the crew gleefully referred to as 'all balls'. A zero figure was commonplace, as was 'four balls one' - an error of only one hundredth of a degree. Only occasionally was an error of 0.02 degrees extant. Of course, a well-aligned platform was as important for an accurate determination of the state vector as it was for the crews' bragging rights.
The concept behind the P52 became familiar to amateur astronomers of the generation after Apollo as powerful computers became small and cheap enough to build into backyard telescopes. By aligning these inexpensive instruments on two stars in succession, their computers learned the orientation of the universe around them and could quickly and easily aim themselves at any desired celestial object in a manner greatly reminiscent of the Apollo G&N system.
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