Apollo Abort Guidance System
The Abort Guidance System (AGS ) was the computer-aided control system reserve the Apollo moon landers. It should be the Lunar Module used in a failure of the Primary Guidance, Navigation and Control System ( PGNCS ) during the descent to the moon, during the ascent from the moon and rendezvous with the command module (CM). In contrast to PGNCS it was not meant for landing on the moon.
The AGS was completely different from the Apollo Guidance Computer PGNCS with his ( AGC), and was developed independently of this at TRW.
Description
The Abort Guidance System (AGS ) included the following components:
- The Abort Electronic Assembly ( AEA) - the computer,
- The Abort Sensor Assembly ( ASA) - a simple strapdown inertial measurement unit (Inertial Measurement Unit (IMU ) ) and
- The Data Entry and Display Assembly ( DEDA ) - the user interface.
Abort Electronic Assembly
First draft ideas for the abortion Guidance System (AGS ) did not provide for the use of a computer. Rather, a sequencer without any navigation capability should be used. This would have been enough to bring in the necessary, the lunar module in a lunar orbit, where the crew on the command module would have to wait. Only later designs foresaw the use of a digital computer, in order to give the lunar module here more independence.
Hardware
The computer was used in the AEA 4118 MARCO ( 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. As has been accessed serially at AEA on the store, it was slower than the AGC. Nevertheless, here are some operations were faster than running on the AGC.
The AEA had a memory capacity of 4096 data words. This was the lower 2048 data words, the write - read memory (RAM) and the top 2048 of the data words read-only memory (ROM). The read-write memory and read only memory were the same design, so that the ratio between the two was variable.
In the AEA there was a 18 -bit machine ( 17-bit data and 1 sign bit ( two's complement) ). The data were represented as fixed-point numbers. The memory addresses were 13 bits long. There the indexed addressing is used.
Software
The AGS software was written in assembly language LEMAP.
A calculation cycle had a length of 2 seconds. This 2-second cycle was divided into 100 segments of 20 ms. These segments were used for calculation of operations that had to be calculated every 20 ms (for example, the processing of the IMU data). A series of calculations were carried out every 40 ms ( for example, the engine control or the control of the attitude ). Other calculations were carried out every 2 seconds (e.g., the processing of the radar data, the calculation of the orbital parameters, the calculation related to the rendezvous with the command module, or the calibration of the IMU sensors). However, this could also - together in smaller groups - are calculated in unused 20 -ms segments.
The software of the AGS has been revised several times to find program errors and to reduce the size of the programs.
Instruction set
The instruction set of the AEA comprised 27 commands. The instruction format consisted of a 5-bit instruction code index bit 1 and bit 12 for addressing.
Abort Sensor Assembly
The AGS was the first inertial navigation system in which instead of a gimbaled gyro platform (like the PGNCS ) a so-called gyro strapdown platform was used. The strapdown IMU was not as accurate as a gimbaled IMU. However, the system had a satisfactory accuracy, with the support of the optical telescope and the rendezvous radar. However, the ASA was smaller and lighter than the inertial measurement unit of PGNCS.
Data Entry and Display Assembly
The user interface of the AGS was called Data Entry and Display Assembly ( DEDA ). It was used for input and output of data from the AGS. The implementation of some system functions differently here was to realize the DSKY of the AGC.
The DEDA had the following elements:
Use of the AGS
None of the missions of the Apollo program, there was a crash landing of the lunar module on the moon. However, there were two cases in which the AGS was used. The first time in Apollo 10, as a result of an incorrect switch position, the attitude of the Lunar Module was not stable. The second time in Apollo 11 when it came to so-called gimbal lock in carrying out the maneuver to rendezvous with the command module by the crew of the lunar module when PGNCS. Both times was used for attitude control of the AGS.