Apollo Abort Guidance System

[1] The AGS was designed by TRW independently of the development of the Apollo Guidance Computer and PGNCS.

It was the first navigation system to use a strapdown Inertial Measurement Unit rather than a gimbaled gyrostabilized IMU (as used by PGNCS).

[2] Although not as accurate as the gimbaled IMU, it provided satisfactory accuracy with the help of the optical telescope and rendezvous radar.

The Abort Guidance System included the following components:[3] The computer used was MARCO 4418 (MARCO stands for Man Rated Computer) whose dimensions were 5 by 8 by 23.75 inches (12.7 by 20.3 by 60.33 centimeters); it weighed 32.7 pounds (14.83 kg) and required 90 watts of power.

CLZ (Clear, Add and Zero): Similar to CLA instruction; the contents of memory are set to zero.

First design ideas of the Abort Guidance System did not include the use of the computer but rather a sequencer without any navigation capability.

There was also a set of computations that had to be performed every 40 ms (engine commands, external signal sampling, attitude control, etc.).

Other computations were performed every 2 seconds and these equations were divided into smaller groups so they could be recalculated during the remaining (i.e. unused) time of 20 ms segments (e.g. radar data processing, calculation of orbital parameters, computation of rendezvous sequence, calibration of IMU sensors, etc.

[6] The AGS User interface unit was named DEDA (Data Entry and Display Assembly).

DEDA had the following elements: There are few actual descriptions of the use of the AGS, as a landing abort was never needed during the Apollo missions.

Its first use was for testing of the Lunar Module descent stage in Earth orbital flight during the Apollo 9 mission.

[7] It was used again in the Apollo 10 mission, following separation of the Lunar Module descent stage prior to the APS burn.

The next use of the AGS was during the lunar ascent phase of the Apollo 11 mission, when the LM crew performed a sequence of rendezvous maneuvers that resulted in gimbal lock; the AGS was subsequently used to acquire attitude control.

As a result, after a major LM descent engine burn two hours past its closest approach to the Moon to shorten the trip home, the AGS was used for most of the return, including two mid-course corrections.[10]pp.