Abort Guidance System

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The Abort Guidance System ( AGS ) was the computer-aided reserve guidance system of the Apollo lunar lander . It should be used in the event of a failure of the Lunar Module's Primary Guidance, Navigation and Control System ( PGNCS ) during the descent to the moon, during the ascent from the moon and when rendezvous with the command module (CM) . Unlike the PGNCS, it was not intended to land on the moon.

The AGS was completely different from the PGNCS with its Apollo Guidance Computer ( AGC ) and was developed independently from it at TRW .

description

The Abort Guidance System (AGS) comprised the following components:

  • the Abort Electronic Assembly ( AEA ) - the computer,
  • the Abort Sensor Assembly ( ASA ) - a simple strapdown inertial measurement unit ( IMU ) and
  • the Data Entry and Display Assembly ( DEDA ) - the user interface.

Abort Electronic Assembly

Initial design ideas for the Abort Guidance System (AGS) did not include the use of a computer. Rather, a sequencer without any navigation capability should be used here. This would have been enough to put the lunar module into orbit, if necessary, where its crew would have had to wait for the command module. Only later drafts provided for the use of a digital computer to give the lunar module more independence.

hardware

The computer used by the AEA was called MARCO 4118 (MARCO stands for "Man rated Computer"). This had a size of 60.3 cm x 20.3 cm x 12.7 cm, weighed 14.83 kg and had a power consumption of 90 W. Since the memory was accessed serially with the AEA, it was slower than the AGC. Even so, some operations were performed faster here than on the AGC.

The AEA had a storage capacity of 4096 data words . The lower 2048 data words were the read / write memory (RAM) and the upper 2048 data words were the read-only memory (ROM) . The read / write memory and the read-only memory were constructed in the same way, so that the relationship between the two was variable.

The AEA was an 18- bit machine (17 bits of data and 1 sign bit (two's complement) ). The data were presented as fixed point numbers . The memory addresses were 13 bits long. It was the indexed addressing uses.

software

The AGS software was written in the LEMAP assembly language.

A calculation cycle was 2 seconds long. This 2-second cycle was divided into 100 segments of 20 ms each. These segments were used to compute operations that had to be computed every 20 ms (for example processing the IMU data). A number of calculations were performed every 40 ms (for example engine control or flight attitude control). Other calculations were carried out every 2 seconds (for example processing the radar data, calculating the orbit parameters, calculating the rendezvous with the command module or calibrating the IMU sensors). However, these could also be calculated - summarized in smaller groups - in unused 20 ms segments.

The software of the AGS has been revised several times to bugs to find and reduce the size of the programs.

Command scope

The command scope of the AEA comprised 27 commands. The command format consisted of a 5-bit command code, 1 index bit and 12 bits for addressing.

Abort sensor assembly

The AGS was the first inertial navigation system to use a so-called strapdown gyro platform instead of a gimbal-mounted gyro platform (as in the PGNCS) . The strapdown IMU was not as accurate as a gimbal IMU. However, with the assistance of the optical telescope and the rendezvous radar, the system had satisfactory accuracy. However, the ASA was smaller and lighter than the inertial measurement unit of the PGNCS .

Data Entry and Display Assembly

The user interface of the AGS was called Data Entry and Display Assembly (DEDA). It was used to input and output data from the AGS. The implementation of some system functions was different here from the implementation in the AGC's DSKY .

The DEDA had the following elements:

  • 10 numeric keys (0-9),
  • Plus and minus buttons,
  • several function keys and
  • an ad.

Use of the AGS

In none of the missions of the Apollo program was the landing of the lunar module on the moon aborted. However, there were two cases where the AGS was used. The first time at Apollo 10 , when the attitude of the lunar module was not stable due to an incorrect switch position. The second time at Apollo 11 , when the crew of the lunar module at the PGNCS came to a so-called gimbal lock while performing the maneuvers for the rendezvous with the command module . The AGS was used both times to control the attitude.

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