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阿波羅中止導航系統（英語：AGS或英語：Abort Guidance Section）是一個提供中止功能的備用的計算機系統。它 The Apollo Abort Guidance System (AGS, also known as Abort Guidance Section) was a backup computer system providing an abort capability in the event of failure of the Lunar Module's primary guidance system (Apollo PGNCS) during descent, ascent or rendezvous. As an abort system, it did not support guidance for a lunar landing.

中止導航系統由TRW研發，與阿波羅PGNCS系統各自獨立開發。

這是第一個使用捷聯慣性測量單元而非萬向陀螺儀慣性測量單元的導航系統 。雖然並沒有萬向陀螺儀慣性測量單元測量準確，它依託於與光學望遠鏡和交會雷達的配合仍然提供了令人滿意的準確度。AGS在尺度上也更小更輕。

簡介

中止導航系統包含以下幾個部分:：

• 中止電子組件：計算機
• 中止傳感器組件：一個小型捷聯慣性測量單元
• 數據輸入和顯示組件：一個類似於DSKY的接口

該系統使用的計算機是MARCO 4418。該計算機尺寸為12.7cm*20.3cm*60.33cm，重14.93千克，功率90瓦。因為內存支持串行通信，它比AGC Because the memory had a serial access it was slower than AGC, although some operations on AEA were performed as fast or faster than on AGC.

該計算機有以下特徵：

• 它的內存為4096個字節。低2048個字節為隨機存取存儲器，高2048個字節為只讀存儲器。

• It had 4096 words of memory. Lower 2048 words were erasable memory (RAM), higher 2048 words served as fixed memory (ROM). The fixed and erasable memory were constructed similarly so the ratio between fixed and erasable memory was variable.

• It was an 18-bit machine, with 17 magnitude bits and a sign bit. The addresses were 13 bits long; MSB indicated index addressing.

• Data words were two's complement and in fixed-point form.

Registers

The AEA has the following registers:

• A: Accumulator (18 bit)
• M: Memory Register (18 bit), holds data that are being transferred between the central computer and memory
• Q: Multiplier-Quotient Register (18 bit), stores the last significant half of result after multiplication and division. It can be also used as extension of Accumulator
• Index Register (3 bit): used for index addressing

Other less important registers are:

• Address Register (12 bit): holds the memory address requested by central computer
• Operation Code Register (5 bit): holds 5-bit instruction code during its execution
• Program Counter (12 bit)
• Cycle Counter (5 bit): controls shift instructions
• Timers (2 registers): produce the control timing signals
• Input Registers: 13 registers

Software

First design ideas of the Abort Guidance System did not include the use of the computer but rather a sequencer without any navigation capability. This would be adequate to put the Lunar Module to the lunar orbit where the crew would wait for rescue by the Apollo CSM. Later design included a digital computer to provide some autonomy.

The AGS software was written in LEMAP assembly language that uses 27 instructions described above and a set of pseudo-operations used by the assembler.

The main computation cycle was 2 seconds long. This 2-second cycle was divided into 100 segments; each of these segments had a duration of 20 ms. These segments were used for computations that needed to be recalculated every 20 ms (like IMU signal processing, update of PGNCS downlink data, direction cosines update, etc.).

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.)

The software for AGS was reviewed many times to find program errors and to reduce the size of the software. There are some known versions of the software that were used for unmanned and manned tests.

User interface

The AGS User interface unit was named DEDA (Data Entry and Display Assembly). Its function was entry and readout of data from the AGS. Some of the system's functionality was built into DEDA unlike the DSKY used by AGC.

DEDA had the following elements:

• Numeral keys 0 - 9

• + and - sign key

• CLR key: clears the entry display and clears the OPR ERR light

• ENTER key: for data/address entry

• READOUT key: reads the data from the specified address and displays the refreshed data every half second

• HOLD key: stops the continuous outputting of data

• OPR ERR light: indicates Operator's error

• displays are used to enter and read the data

Use of AGS

There are few actual descriptions of the use of the AGS, as a landing abort was never needed during the Apollo missions. There were, however, three cases in which the AGS was used.

Its first (and intended) use was during the Apollo 10 mission, following the separation of the Lunar Module descent stage, prior to the ascent stage burn. An incorrect switch setting led to the AGS causing extensive gyrations of the ascent stage.

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.

The AGS played an important role in the safe return of Apollo 13 after an oxygen tank explosion left the Service Module crippled and forced the astronauts to use the Lunar Module as a "lifeboat." Supplies of electrical power and water on the LM were limited and the Primary Guidance and Navigation System used too much water for cooling. As a result, after a major LM descent engine burn 2 hours past its closest approach to the moon to shorten the trip home, the AGS was used for most of the return, including two midcourse corrections.^{pp. III-17,32,35,40}