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Virtual AGC — AGS — LVDC — Gemini

The Apollo Guidance Computer
A Kinder, Gentler Introduction

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Ron's Introduction to the AGC

The Apollo spacecraft used for lunar missions in the late 1960's and early 1970's was really two different spacecraft, the Command Module (CM) and the Lunar Module (LM).  The CM was used to get the three astronauts to the moon, and back again.  The LM was used to land two of the astronauts on the moon while the third astronaut remained in the CM, in orbit around the moon.


Apollo 15 CSM (Command and Service Modules)

Apollo 16 LM

Each of the spacecraft needed to be able to navigate through space, with or without the assistance of the astronauts, and therefore needed to have a "guidance system".  The guidance system was developed by MIT's Instrumentation Lab, now an independent company known as the Charles Stark Draper Laboratory.

An important part of the guidance system was the Apollo Guidance Computer—or just "AGC" for short.  On any given Apollo mission, there were two AGCs, one for the Command Module, and one for the Lunar Module.  The two AGCs were identical and interchangeable, but they ran different software because the tasks the spacecraft had to perform were different.  Moreover, the software run by the AGC evolved over time, so that the AGC software used in later missions like Apollo 17 differed somewhat from that of earlier missions like Apollo 8.

Considered just as a computer, the AGC was severely underpowered by any more-modern standards.  The basic characteristics of the AGC were:
Real DSKY.
It is occasionally quipped—with perhaps greater wit than insight—that the AGC was more like a calculator than a computer.  But to say this is to grossly underestimate the AGC's sophistication. For example, the AGC was multi-tasking, so that it could seemingly run multiple programs simultaneously.

Another important part of the guidance system was the Display/Keyboard unit—or just "DSKY" for short.  The AGC by itself was simply a box with electrical connections, without any built-in way for the astronaut to access it.  The DSKY provided the astronaut with an interface by which to access the AGC.

The Lunar Module had a single DSKY, positioned between the two astronauts where it could be operated by either of them.  The Command Module actually had two DSKYs.  One of the CM's DSKYs was only the main control panel, while the other was positioned near the optical equipment used to mark the positions of stars or other landmarks.

The DSKY as a substantial piece of equipment in its own right:
Perhaps the most important part of the guidance system was the Inertial Measurement Unit—or just "IMU" for short.  The IMU continuously kept track of the acceleration and rotation of the spacecraft, and reported this information back to the AGC.  By mathematically processing this data, the AGC could know on a moment-by-moment basis the orientation and position of the spacecraft.

Fabrizio's Alternate Introduction to the AGC

Fabrizio's poster, in Italian(Fabrizio Bernardini has contributed this alternate introduction to the AGC, from a poster presentation he made, and then was kind enough to give use both the poster, in Italian, and an English translation.  The words and photos are his, and I'll simply present them as-is.  Don't be surprised if there's some duplication of material.  Thanks, Fabrizio!)

Introduction

The first contract assigned within the scope of the Apollo program, a little after the decision by president Kennedy to reach the Moon before the decade was out, has been that for the computerized guidance and navigation system. Requirements for the system were clear and simple, but apparently unreachable at the time: build a small autonomous system, able to steer a crew from the Earth to the Moon and bring it back safely to the Earth.

A giant leap for technology

When in 1962 MIT's Instrumentation Laboratory began, under the direction of the legendary Dr. Charles Stark Draper (a pioneer of inertial navigation systems), the development of a computer to bring people from the Earth to the Moon, computers where in their childhood. They were giant electronic-tubes devices, big as rooms or even building floors, controlled by means of of perforated tapes or cards, with insiggnificat memory capacity compared to the personal computers that would be born at the end of the 1970's.

With an engineering courage that has few precedents, designers (historical characters like Hal Laning Jr, Eldon Hall, Ramon Alonso, Albert Hopkins and many others) elected to use for the first time in history a newly born electronic component: the integrated circuit. Still far away from the microprocessor or microcontrollers of the 1980's, the only functions available were simple logic ones. Using thousands of these logic gates, it was possible to implement what would be called today the central processing unit of the computer and all support circuitry. For memory, since it was still impractical to build it with integrated circuits, the ferrite core technology (where small rings were used to store single bits) was adopted. being reasonable easy to miniaturize. Using the same technology, the program memory was made of tens of thousands of ferrite cores, hand woven with thin copper wire to represent the 1's and 0's of the computer software. These wire were to be woven by specialized workers at least one month before the launch of a mission.

These critical decisions, taken in the era of the "dinosaur computers", were found to be winners in the following years, just seven, that saw men land on the Moon. The onboard computer developed for the Apollo program (and installed into both the Command Module and the Lunar Module, but with different software for each vehicle) was an outstanding technological success and established rules for the development of future computers both for aerospace and other applications. It also opened the way to new applications of integrated circuits (it has been the first big user on an industrial scale) and being at the time the smallest computer in the world, helped push computers into other sectors of technology.

AGC, CMC, LGC?

Internal modules of AGCThe computer and the navigation system developed for the Apollo program were used both on board the Command (and Servce) Module, or CSM, and on board the Lunar Module, or LM. To distinguish them, the one in the Command Module was said CMC (Command Module Computer), that in the Lunar Module was said LGC (Lunar-module Guidance Computer).

The software was different for the two computers, and each "edition" had an independent development path, with shared components. It was in common the management of the various programs, as well as the management of the inertial platform and that of the interface toward the crew (called DSKY, for Display and Keyboard).

Some integrated circuits used in the AGCThe CMC software was different for the presence of the Saturn V rocket monitoring and control program (never used in its controlling functions as the Saturn computer always performed flawlessly). It also contained functions to steer during the trans-lunar trajectory maneuver, during orbital flight and of course for the re-entry into the Earth's atmosphere.

The LGC software was different for the descent and landing programs, and for the ascent program and the subsequent lunar orbit insertion.

Another part in common was devoted to the rendez-vous maneuvres, as both vehicles could perform as the active part in the complex process of rejoining each other in orbit.

Obviously, the software of both vehicles was different in the management of propulsion devices, and the navigation sensor which, while in the CSM were a sextant and a telescope, in the LM they were a single optical device with a special behavior (due to the reduced size of the LM and the need to keep weight within precise limits).

Display and keyboard

DSKYThe most visible component of the whole AGC was the panel used by the crew to interact with the computer. In an era in which "speak" to a computer using a video terminal was still a futuristic advancement, the unit called Display and Keyboard, or DSKY, represented a drastic achievement in the relationship between man and machine. For the first time the famous seven-digits numericla displays were introduced and for the first time it was possible to tell the computer, while it was working, what to do, being also possible to interact with it to change parameters, or to monitor numerical values of different kind (which by itself was a real novelty in an aerospace workd still dominated by gauges).

System limitations imposed, of course, a number of restriictions and ingenuous solutions even if using the language developed for the purpose of user interaction with the computer required patience and training to ensure efficency and confidence.

DSKY in CM control panelThe DSKY's language was based in two-digits numeric codes, used to represent Verbs (that is actions) and Nouns (that is the object of an action). In the figure it is shown part of a checklist (from the Apollo A17 Flight Crew G&N Dictionary) that summarized all the Verb/Noun codes for the Lunar Module computer. Verb and Noun values had a prominent display on the DSKY as well as the active program (or major mode, related usually to a flight phase), which was also assigned a two-digits numerical value and displayed on the top right corner of the unit.

Apollo 17 verb/noun listThe Verb and Noun keys were used to set the Verb/Noun combination of interest, and telling the computer to accept it by pressing the Enter key (e.g,, a typical sequence could have been: Verb 16 Noun 65 Enter). The Clear key was used to correct user typing errors, while the Reset key was used to cancel error warnings from the computer. The Pro key (where Pro stands for Proceed) was sued to confirm critical operations (like the execution of a maneuver or the starting and stopping of the computer). The Key Release key was ued to release the control of the DSKY to other routines that required its use, that is when the corresponding indicator light was flashing to grab the attention of the operator. The latter behavior was another novelty as many programs could run inside the AGC at the same time.

Why "guidance"?

The real task assigned to MIT's Instrumentation Laboratory (now Draper Laboratoroes) was the design and implementation of a guidance system for Apollo, that is a system able to keep a notion of the state of the vehicle in every instant and provide guidance directions for the execution of specific flight phases (for instance, landing on the Moon). The whole AGC is therefore a wider system, not limited only to the computer, that provided sensors, actuators, interfaces for the crew and the communications system (to exchange data with the control center).

Guidance, Navigation, and Control system.For the navigation sensors, designers chose an inertial navigation system based on a "three-axis inertial platform" able to maintain an given orientation (used as reference) independently by the motions of the vehicle. On the "platform", three accelerometers were mounted to measure accelerations and thus update the vehicle state during propulsive phases. In the free-fall phases, when the vehicle moved along orbits dictated by Earth/Moon gravities, the state was updated mathematically by applying Newton's and Kepler's equation. Verification of the state, to correct it in case of possibile deviations from the intended path, were done by means of a sextant mounted with the "platform". By observing stars and the angle between stars and the horizon (or a landmark) crew were able to navigate in the Earth/Moon system like sailors did for centuries before the advent of of modern radio navigation systems. The mathematics behind the whole guidance and navigation system was worked out by prof. Richard Battin, at the time one of the most prominent experts in the sector, who also went to initially lead the software development task for AGC.

In the course of the project, the AGC was added other main functions, like that to act as go-between for the manual control of the vehicle. By all means that was the first "fly-by-wire" guidance system in history, and the first "digital autopilot". This and other additional utilities for secondary tasks demonstrated the versatility of computerized system with respect to more classical approaches.

AGC Software

As in all computerized systems, the physical part of the computer (circuits, logic gates and memory) is useless without a program to manage its operation. Software became soon one of the principal aspects of each Apollo mission and hundreds of programmers (at a time when the word "software" was still unknown but to a few insiders) worked to produce programs of very high quality, and with lots of ingenuity, having to overcome not only the intricacies of the space guidance and navigation equations, but also the limitations imposed by the inflexible hardware architecture. Even in this case, methods and technologies were developed for this specific purposes to soon become part of the required knowledge in the aerospace world.

Software development was not even worthy a mention in the original contract, as it was considered expected, at the time (but often it is still the same today), that programming the computer was a side task in the whole project. In truth, at the peak of the Apollo program, in 1969, software workers were approximately as much as the hardware workers. But being the "software culture" not widely diffused, everybody looked with suspicion at this intangiblebut fundamental, "thing".

Two were the programming languages available: one was the proper central processing unit language, or assembly language, and the other was a higher-level language, that was to be interpreted at execution time within the computer by a program called, exactly, Interpreter. This second language enabled to save memory speeding up the programming of complex mathematic operations, at the expense of a slower execution speed.

During the course of software development, in addition to performance limitations, programmers had to fight against memory limitations. Uncountable tricks were used by programmers to save memory words, often to the detriment of code readability. In addition, to worsen the situation, it was required to adopt particular techniques to protect critical functions from a sudden computer restart. By keeping tab on the state of programs under execution it was possible to ensure that in case of serious problems, the computer were able to restart and resume programs from the place they were at the moment of the trouble, without causing issued with critical functions, like the propulsive phases.

The experience acquired with the Apollo program, that continued after the lunar missions with the Skylab flights and the Apollo-Soyuz mission, was not lost. The perfomance of the software and the reliability of the hardware made the AGC the logical choice for the first computer controlled flight, or "fly-by-wire", of an airplane (a modified F-8 fighter). The experience of programmers was subsequently transferred to the Space Shuttle program, where an evolution of the AGC software, and the methods used to implement it, contributed to the making of a space vehicle entirely controlled by the on-board computer.

Myths and legends

Is it true that the Apollo computer had less power than a digital pocket calculator? No it is not true. The performances of the Apollo Guidance Computer were comparable to that of the central processing unit of computers llike the Apple II, the Commoder 64 and the ZX Spectrum, the personal computer that opened the way for the digital revolution at the end ot the 1970's. The big difference was that the AGC was highly reliable and had many more input/outpu ports to be able to control, and receive data from, sensors and devices within the Command Module and the Lunar Module. In addition, it was able to execute programs in real-time, that is which operated continuously performing more tasks at the same time and guaranteeing that high priority tasks were performed first when required. Its architecture is not much different from that of modern microcontrollers used today for the most disparate tasks, with the difference that the latter are at least 10 to 20 times faster than the AGC and enormously smallet (being integrated in a single chip). For this reason programming and using the AGC is not very far from th world in which control systems and embedded systems designers and hobbysts work today.

Is it true that during the landing of Apollo 11, Neil Armstrong had to take control from the computer because of the alarms it generated? Stupid media remarks often cite "software errors" that generated alarms during Apollo 11 descent to the Moon (the most critical phase of the mission). In truth, the robustness of AGC software, in terms of good desing and reliability, saved the mission when other issues, external to the computer, were causing an overload in it its operation (and was thus correctly alerting the crew of the condition). The LGC kept working correctly, but toward the end of the descent, Armstrong took manual control (as everybody else who followed him on this task) because the landing point targeted by the computer was too dangerous for a safe landing.

Was AGC the first space computer? No, AGC wasn't the first space computer or the first computer who flew on a crewed vehicle. Gemini spacecrafts had also an onboard computer, made with transitors, and able to assist the crew (but not totally control the vehicle) in the propulsive phases, during 'rendez-vous' and during re-entry. But that computer was not an essential part of the mission, while AGC was born to privide assistance in completing the mission without ground support.In addition to that, other digital computers were developed for the Titan rockets, and of course for the Saturn, but they were limited to a single task and were much bulkier and heavier. The Lunar Module also had a second onboard computer, the AGC, Abort Guidance System, much simpler and able to ensure the climb back to orbit and a rendez-vous with the CSM in case of troubles with the primary guidance and navigation system.

Did the AGC have an operating system? In a certain sense it had, in the sense today commonly considered for computers for control applications. The AGC operating system was made of an executive and a manager of queue for waiting programs. The executive provided, in turn and basing on priority, a chance to each program to execute its own instructions and also managed possible interruptions generated by random external events. This way more than a single program could be active at the same time, even if the allocation of "machine time" was not based by a rigid scheduling (preferred technique at the time). but was was asynchronous and linked to the correct design of each function. Also, it was available a separate process to verify the well-being of the computer, one for the management of the DSKY and one for managing external devices. And another part of this "operating" was devoted to the higher level language used for mathematical operations that did not require a real-time response.

Introduction to Virtual AGC

The Virtual AGC project provides a virtual machine which simulates the AGC, the DSKY, and some other portions of the guidance system.  In other words, if the virtual machine—which we call yaAGC—is given the same software which was originally run by the real AGCs, and is fed the same input signals encountered by the real AGCs during Apollo missions, then it will responds in the same way as the real AGCs did.  The Virtual AGC software is free of charge, can be obtained for Windows, Mac OS X, Linux, or as open source software source code so that it can be studied or modified.



The video clip above (courtesy of Dean Koska and YouTube) illustrates Virtual AGC running on a Palm Centro—which isn't supported directly from our download page.  But it's one of the great advantages of open-source software that you can take it an adapt it to your wishes without being at the mercy of the creators of the software.

After you install the Virtual AGC software you can run the simulation by clicking its desktop icon :



While there are all manner of runtime options you can select in this screen, such as which Apollo mission you which to fly, it's simplest as a newcomer to just click the Novice button and then to click Run.  In the screenshot below, the DSKY is displaying the time since startup (00000 hours, 00003 minutes, 00270 hundreds of a second), though that's not something it will do automatically when you start it up.



In this simplest of configurations, you are simulating the AGC of the Lunar Module of Apollo 13, and you are provided with a simulated AGC and simulated DSKY.  You can enter commands at the DSKY keypad, and the AGC will report the results of its actions on the DSKY's display.

How to Operate the AGC from the DSKY

The first thing you need to know when operating the AGC is that it contains a collection of programs, each identified by a two-digit number.  The most-basic program is program 00, usually known as "pooh" (as in "Winnie the Pooh") or P00.  In fact, in the screenshot above, the AGC is running P00, as you can see by looking at the "PROG" area in the upper right-hand corner of the DSKY.  The available programs differ somewhat from mission to mission, and the picture below (taken from reference cards supplied to the astronauts) is a sampling of some of the programs available for the Apollo 15 Command Module.



From the astronaut's perspective, all operations of the computer are controlled by a quirky system of "verbs" and "nouns".  Verbs and nouns aren't words, as you might suppose, but are instead two-digit numbers.  What makes them "verbs" and "nouns" is that they are somewhat analogous to verbs and nouns in natural languages like English, in which the verb describes the action to be taken and the noun describes the data to which the action is applied.  So most commands are applied by hitting a key sequence like the following on the DSKY:

VERB digit digit NOUN digit digit ENTR
or sometimes
VERB digit digit ENTR

For example, a command like V06N36E (VERB 0 6 NOUN 3 6 ENTR) would mean to perform action 06 on data 36.  The exact meanings of the verbs-numbers and noun-numbers differ from mission to mission—i.e., from AGC software version to version.  The pictures below show a sampling of the available verbs and nouns from the astronaut's reference cards for the Apollo 15 Command Module, from which you can see that verb 06 means to perform a decimal display and noun 36 means "time of AGC clock", so V06N36E should display the current time from the AGC's clock, if that happens to be a legal action for the current program.



There are also some exceptions, such as getting the AGC to run the P00 program.  You do that with a key sequence like V37E00E, in which NOUN is replaced by ENTR for some reason.

How did this strange verb/noun system get put into place?  One of the original AGC developers, Ramón Alonso, provides an explanation.  Apparently, nobody had yet arrived at any kind of software requirements for the AGC's user interface when the desire arose within the Instrumentation Laboratory to set up a demo guidance-computer unit with which to impress visitors to the lab.  Of course, this demo would have to do something, if it was going to be at all impressive, and to do something it would need some software. In short order, some of the coders threw together a demo program, inventing and using the verb/noun user-interface concept, but without any idea that the verb/noun concept would somehow survive into the flight software.  As time passed, and more and more people became familiar with the demo, nobody got around to inventing an improvement for the user interface, so the coders simply built it into the flight software without any specific requirements to do so.  Many objections to the verb/noun system were received by the developers, such as "it's not scientific", "it's not dignified", or even "astronauts won't understand it".  Even though the coders of the demo hadn't seriously intended the verb/noun interface to be used in any permanent way, it became a kind of devilish game to counter these objections with arguments as to why the interface was really a good one.  In the end, the coders won and the verbs and nouns stayed.  Nevertheless, the following Shakespearian quote (from Henry IV) is embedded within the AGC source code:  "It will be proved to thy face that thou hast men about thee that usually talk of a noun and a verb, and such abominable words as no Christian ear can endure to hear."

The main Virtual AGC web-page gives you various step-by-step procedures you can perform to see the DSKY actually do something.

It's the Software, Stupid!

You may notice that on the Virtual AGC main display screen there are a number of different Apollo missions listed, but that not all of them are selectable.  That's because Virtual AGC strictly simulates the original AGC computer hardware, and that requires having the original software that ran on the AGC.  But we don't have copies of all of the different software versions used for the different missions!  Collecting this software is one of the principal goals of the Virtual AGC project, so if you happen to know a collector, a museum, or any of the original AGC developers who have printouts of AGC software, let us know!  (We don't want the printouts themselves, just copies of the information in them, and we're willing to travel there to do the copying work ourselves.)

Here's a brief rundown on the known AGC software:

Mission
AGC Software by  Name and Version
Command Module
Lunar Module
Apollo 1
Sunspot build TBD
-
Apollo 4
Solarium build 55
-
Apollo 5
-
Sunburst build TBD
Apollo 6
Solarium build TBD -
Apollo 7
Sundisk build TBD
-
Apollo 8
Colossus build 237?
-
Apollo 9
Colossus build 249? Sundance build 306
Apollo 10
Comanche build 45?
Luminary build 69
Apollo 11
Comanche build 55
Luminary build 99
Apollo 12
Comanche build TBD
Luminary build 116
Apollo 13
Comanche build 67
Luminary build 131
Apollo 14
Comanche build 72
Luminary build 178
Apollo 15
Artemis build 72
Luminary build 210
Apollo 16
Artemis build 72 Luminary build 210
Apollo 17
Artemis build 72 Luminary build 210
Skylab 2
Skylab 3
Skylab 4
Skylark build 48?
-
Apollo-Soyuz
Skylark build TBD
-


These programs, though having different names, are not really independent programs.  There is a complex family heritage, and a lot of overlap between them.  So if you understand one of the programs, you will understand 99% of what's in any of the others.

The AGC software is mostly written in assembly language.  "Assembly language" is a very simple language in which each instruction—usually, each line of software source code—represents a single primitive CPU operation.  Since each type of CPU provides different primitive operations, the assembly language for each type of CPU differs from that of every other type of CPU.  The AGC assembly language, being based on the custom AGC CPU, thus differs in detail from every other type of assembly language but shares certain customary elements.  The specific assembly language of the AGC was referred to as "basic"—not to be confused with the famous later computer language BASIC which is many people's first introduction to simple computer programming.  Here's a short excerpt from the Apollo 11 Lunar Module software to give the flavor of it:

TWIDDLE           INHINT                                         
TS L # SAVE DELAY TIME IN L
CA POSMAX
ADS Q # CREATING OVERFLOW AND Q-1 IN Q
CA BBANK
EXTEND
ROR SUPERBNK
XCH L

WAITLIST INHINT
XCH Q # SAVE DELTA
TS WAITEXIT # WAITEXIT.
EXTEND
INDEX WAITEXIT # IF TWIDDLING, THE TS SKIPS TO HERE
DCA 0 # PICK UP 2CADR OF TASK.
-1 TS WAITADR # BBCON WILL REMAIN IN L
DLY2 CAF WAITBB # ENTRY FROM FIXDELAY AND VARDELAY.
XCH BBANK
TCF WAIT2

The things at the left-hand side (TWIDDLE, WAITLIST, DLY2) are "program labels", and are used to provide names for different blocks of source code.  The next column to the right contains the actual CPU instructions (INHINT, TS, CA, ADS, etc.) while the column to the right of that (L, POSMAX, Q, etc.) specifies the data on which the CPU instructions are supposed to operate.  For example, at the very bottom you'll find "TCF WAIT2"; when the program reaches that point, the instruction TCF causes it to jump to program label WAIT2.  Finally, anything preceded by the '#' symbol is just a comment added for explanatory purposes by the programmer, but not affecting program execution in any way.

You can learn all about AGC basic assembly language, if you so desire, in the Virtual AGC assembly-language manual.  The Virtual AGC software provides an assembler, which is a program that converts assembly-language source code to executable code which can actually be run on the AGC CPU.  So with Virtual AGC, you can actually create and run your own AGC programs.

As it happens, the functionality which the AGC needed to provide was so complex that it wouldn't have fit within the amount of physical memory provided by the AGC if it were written entirely in basic assembly language.  One thing that was done to get around this was to provide also a higher-level computer language referred to as "interpretive", in which each interpretive instruction represents a large number of basic assembly-language instructions.  Interpretive thus has the advantage of cutting down on memory usage, but typically has the disadvantage of running much more slowly, and therefore had to be used with care.  Within any given AGC program like Luminary, basic and interpretive code is intermixed.

Here's a sample of interpretive code:

STATINT1          TC       INTPRET                               
BON RTB
QUITFLAG # KILL INTEGRATION UNTIL NEXT P00.
NOINT
LOADTIME
STORE TDEC1
CALL
INTSTALL
SET CALL
NODOFLAG
SETIFLGS
GOTO
STATEUP
600SECS 2DEC 60000 B-28
ENDINT CLEAR EXIT
STATEFLG
TC PHASCHNG

Some of the things here are the same as in the basic assembly language, such as the program labels and the program comments.  As with basic assembly langauge, the next column to the right of the program labels (BON, STORE, CALL, etc.) contains instructions—though mostly interpretive instructions rather than basic instructions.

In this sample, "TC INTPRET" and "TC PHASCHNG" are basic assembly-language code.  In fact, "TC INTPRET" actually means to begin interpreting the lines that follow as interpretive code, and every block of intepretive code must somehow be preceded by a basic instruction like that.  Near the end, you see the interpretive instruction EXIT, and this means to stop working with interpretive code and to resume working with basic assembly language.

In other respects, though, interpretive is very weird in comparison to basic assembly language.  For example, the column to the right of the instruction column contains mostly names of variables operated on by the instructions ... but can also contain more instructions, because intepretive instructions can be packed two to a word of memory.  Also, many variables don't seem to be preceded by instructions, because a single interpretive instruction may operate on the lines below it as well as on the column to the right of it.  For example, in the two lines of code "CLEAR EXIT / STATEFLG" near the end of the sample, the CLEAR and EXIT instructions are grouped together because they are packed into a single word of memory, but the CLEAR instruction actually operates on the variable STATEFLG.  Oh, those wacky AGC developers!  How they must have chuckled when they dreamed up that scheme.

You can learn all about AGC interpretive language, if you so desire, in the Virtual AGC assembly-language manual.

The Abort Guidance System (AGS)

The AGC was not the only computer in the Lunar Module.  As a backup to the main guidance system, there was a completely separate guidance system called the Abort Guidance System (AGS) developed by TRW Aerospace, now a division of Northop Grumman.  As the name implies, the AGS was intended to be used only in the case of an aborted landing, and its basic function was simply to get the LM into an orbit from which it would be possible for the CM to rendezvous.  Fortunately, this functionality never had to be used in a real mission, but that does not alter the significance of the system.

Naturally, an important part of the AGS was a computer, and this computer was called the Abort Electronics Assembly—or just "AEA" for short.  Since the AGC and AEA were developed independently by unrelated groups, there is no commonality between them in terms either of hardware or of software.  There was essentially no interaction between the AGC and AEA, except that the spacecraft's "state vector"—its current position, velocity, and orientation—could be transferred.

Here are the basic characteristics of the AEA:
Just as the AGC had its DSKY for interfacing to the astronaut, the AEA had its own Data Entry and Display Assembly—or "DEDA" for short.

The astronaut interface was very simple.  You could do one of two things:
In other words, to usefully interact with the AEA, you had to know the numerical values of the memory addresses that had any significant purposes.  But don't be fooled by the simplicity of the interface into believing that the AEA was a mere calculator.  Though its functionality was much less than that of the AGC, it still had to perform very significant guidance functions on demand, and to have very sophisticated software.

Virtual AGC provides the ability to simulate the AEA and DEDA, and you can activate this ability by selecting the "LM Abort Computer (AEA)" option in the Virtual AGC main window.  In the screenshot below, the DEDA is displaying the contents of location 377, which is the number of interations of the units self-test which have been executed (and passed).



As with the AGC software, the AEA software evolved over time and we have only a subset of the software versions known to have been created, so if you know of the existence of others please let us know!  We do have the versions for Apollo 11 ("Flight Program 6") and probably for Apollo 15-17 ("Flight Program 8").

The AEA was programmed in its own assembly language, which was completely different from that of the AGC.  The Virtual AGC software includes an AEA assembler, so you can create and run your own AEA software, if you'd like to do so.

You can learn more about the AEA, including the definition of its assembly language, at the main Virtual AGC web-page for it.

Guiding the Mighty Saturn Rocket

It's natural to think that the AGC might have steered and otherwise controlled the Saturn V rocket that hurled the Command Module and Lunar Module onto its moon-bound trajectory, or the Saturn IB rocket that was used for Earth-orbital missions like Apollo 7, but in fact it isn't true.  The AGC was able to provide limited guidance-system backup for some of the Saturn burns, but the main responsibility for guiding the Saturn rested with yet another onboard computer, the Launch Vehicle Digital Computer (or LVDC for short).  The astronauts were able to monitor various sensor readings from the Saturn's guidance system on their displays, but neither they nor the AGC normally had any control over the rocket.

Unlike the AGC and AGS, the LVDC was not installed in the Command Module or Lunar Module, but was instead installed in the Saturn itself, in a non-propulsive stage called the IU (Instrumentation Unit).  The IU perched above the final propulsive stage, the S-IVB, and therefore was the last part of the Saturn to be discarded by the Command Module.  The picture at right illustrates this for a Saturn IB rocket, but the Saturn V is very similar ... only much bigger!

There's a lot more information about the LVDC on the main Virtual AGC web-page for it, but there's no denying that at present we know far less about the LVDC than about any of the other Apollo flight computers.

To Learn More ...

This web page has provided a brief introduction to the AGC & friends, and to Virtual AGC, but if you'd like to learn more about the AGC without taking the trouble to become a full expert on it, you might want to read the AGC's Wikipedia article

If that's not enough for you, you can learn a lot more by looking at the main Virtual AGC web pages, and of course that's what we think you should do!  In fact, the main Virtual AGC website contains every AGC-related scrap of data, documentation, and software we've been able to find, so it can be somewhat intimidating.  We're sorry about that, but training for an Apollo moon landing isn't easy!



Last modified by Ronald Burkey on 2009-08-17.

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