Get the hell out of there: the AGS

As ever, there was a backup system for the PCNS, although in this case the philosophy was a little unusual because, in the event of failure, it was not meant to replace the PCNS in order to allow the mission to continue. As its name indicated, the abort guidance system (AGS pronounced biggs’) was intended to be used only in ease of an abort.

Given the limited capabilities of the lunar module, it was considered to be almost impossible to accomplish a landing without a working guidance system. The highly optimised nature of the approach trajectory and the complexities of keeping the spacecraft balanced on top of the descent engine’s thrust meant that there was very little room for error. A fully functioning backup system would have been excessively heavy. Yet there were reasonable concerns about the possibility of the PGNS failing while two men were descending to the rocky surface of a hostile world. Its systems were complex, exotic and very new. It was decided, therefore, that if the PGNS did fail, the descent to the surface should simply be aborted, the descent stage should be jettisoned, and the ascent stage should fire its engine to return to orbit. To achieve this, designers added a separate pared-down guidance system, the abort electronics assembly, which included a computer at its heart, the AGS, which was even simpler than that in the PGNS. Instead of having its own heavy IMU, the AGS received its attitude reference from a set of body-mounted gyros and accelerometers. ‘These were intrinsically less accurate and more prone to

drift than a full IMU, but they would only be required for a short period while the abort took place.

Throughout a normal descent, the lunar module pilot closely monitored the AGS to ensure that its knowledge of velocity and position kept track of the PGNS. At regular intervals, he fed it updates from the more accurate system and then watched how the two compared. Then, if the crew lost the PGNS, the AGS was ready to take over and automatically guide them to a safe orbit, from which the CSM could rescue them.

The LMP worked with the AGS using an interface that was basic even compared to the DSKY, the data entry and display assembly (DEDA). It had one single 5-digit display and a simplified keyboard. Since it had little in the way of a user-friendly interface, he had to get down to its machine level to use it. For example, to access its memory he had to supply the address where a value was stored in a manner that will be familiar to people who used the very early microprocessor machines of the 1970s. In order to achieve high functionality, he had to understand it well and be slick at interrogating it. It could keep an LMP very busy. By Apollo 17, Jack Schmitt and the engineers he worked with on simulations had done so much with the AGS that they believed they could have used it to continue to a landing had the PGNS failed.