Platform realignment: the LM way

The LM possessed a full guidance and navigation system similar to that in the CSM but with different names. It was the primary guidance and navigation system or just PGNS and, as often happened, the people of Apollo quickly transmogrified the pronunciation of this clumsy acronym to ‘pings’. It had its own inertial measurement unit and optical system. Upon power-up, it needed to know three things: what time it was. where it was, and which way was ‘up’. A call from the CMP in the command module allowed the commander to set the mission clock to the right time, and this information was eventually passed to the computer. Other variables were loaded into the computer to prepare it for the proper operation of the spacecraft; the LM mass, the settings for its digital autopilot and trim angles for the engine gimbals. An uplink from mission control direct to the computer’s memory provided a state vector to tell it where it was, how fast it was moving and in what direction. They also uploaded a REFSMMAT which would provide a reference for which way was ‘up’, but only when the guidance platform was aligned in accordance with it.

Although the computer at the core of the LM guidance and navigation system was essentially identical to the one in the command module, the systems connected to it were quite different. This reflected how engineering constraints altered when designing a super-light, rocket-powered Moon-lander instead of an interplanetary

spacecraft that had to withstand atmospheric re-entry and parachute drop onto the surface of Earth.

For the first alignment of its guidance platform, the LM was still docked to the CSM, and procedures reflected this. First, as a starting point, the known orientation of the platform in the CSM provided an approximate alignment. Since there was no computer-to-computer connection between the two spacecraft, gimbal angles were recorded manually by the CMP and radioed through to his colleagues. A few simple calculations had to be applied to these angles to account for the different orientations of the coordinate systems of the two spacecraft, and to take into account the angle indicated in the tunnel that measured any mutual misalignment. The gimbals of the LM’s fMU were then commanded to drive the platform to this orientation. While not sufficiently accurate for precise manoeuvring, this procedure gave the platform a reasonably good idea of which way was ‘up’.

The next step was to carry out a fine alignment that used Program 52, as in the CM. However, whereas the CM sported a sophisticated motor-driven sextant and telescope, the LM had a much simpler periscope arrangement called the alignment optical telescope (AOT), which was mounted at the top of the cabin between the two crewmembers. This was a remarkably ingenious device, whose elegance was in the simplicity of its design. Its main component was a unity-power telescope with a 60- degree field of view that could be manually rotated between six fixed positions: forward, forward right, aft right, aft, aft left and forward left. It incorporated two methods of using the stars to determine the orientation of the platform. One was for in-flight use when the LM was free to rotate; the other was for use on the surface, or for when it was attached to the CSM. Despite its simplicity, the AOT allowed the commander to align the LM’s platform just as accurately as the CMP could align the platform in the command module.

Sighting the stars was done against an illuminated graticule on which were inscribed a series of patterns. A pair of cross-hairs was used when the LM was in free flight, and a pair of radial lines and spirals came into play for surface or docked alignments. In both cases, the computer was told which of the six detents the AOT was in, and which star was to be marked. In each case, it was then a two-step process.

To mark on a star during free flight, the LM was manoeuvred to make the star move across the X and Y cross-hairs, with marks being taken when it coincided with each line so that the computer could define two intersecting planes whose vertex pointed to the star. A similar pair of marks on a second star gave the two vectors the computer required to calculate the platform’s orientation.

The second method was normally used on the lunar surface, but it could also be brought into play when the LM was docked to the CSM. It was considered undesirable to try to manoeuvre the entire stack from the lightweight end so this method did not require that the LM be manoeuvred. It was also a simple two-step process once the computer knew which star was being viewed at which detent. First, the graticule was rotated until the star lay between the two radial lines. Pressing the ‘Mark X’ button yielded the shaft angle. The graticule was rotated again until the star lay between the two spirals. Pressing ‘Mark Y’ gave the reticle angle. The

computer could then convert this information into a vector to the star. This process was repeated using a second star. When completed, the computer could determine the platform’s actual orientation which allowed it to be accurately aligned to the required REFSMMAT, in this case, the landing site REFSMMAT.