1 WEBB G143/MAPLD 2005 The LM Abort Guidance Section Julian Webb University of the West of England, Bristol, UK [email protected]

1WEBB G143/MAPLD 2005

The LM Abort Guidance Section

Julian WebbUniversity of the West of England, Bristol, UK

[email protected]


Introduction

The Lunar Module (LM) Abort Guidance Section (AGS) was developed (primarily 1964 - 1968) by TRW to provided a backup guidance system in case of failure of the PGNS (including the LGC, the LM version of the AGC)

This presentation covers the function, organisation, operation and experience of the AGS

As will be seen the name Abort Guidance Section does not really reflect the role of the system - the AGS was in fact a backup guidance and navigation system


AGS mission function AGS provides:

LM trajectory and CSM orbital position calculations

routine follow-up monitoring of PGNS operation throughout descent, landing and ascent phases of a lunar-landing mission

act as a backup to PGNS in abort situations leading to ascent, orbit and rendezvous with the CSM


AGS components

AGS comprises three major assemblies: Abort Sensor Assembly (ASA)

inertial platform

Abort Electronics Assembly (AEA)general purpose computer

Data Entry and Display Assembly (DEDA)astronaut I/O interface PGNS

Attitude commands (CES)Engine commands

DisplaysTelemetry


AGS components

Attitude control is achieved by outputting error angles to the CES, which then orients vehicle attitude, using the RCS, so as to null the errors

AEA can start and stop the ascent and descent engines

AEA can display attitude information on the FDAI (8-ball) displays

A telemetry stream is provided to mission control AGS can be initialised by capturing the PGNS

downlink telemetry stream


ASA

The ASA comprises a set of three strapdown gyros and three accelerometers

These components are physically mounted close to the PGNS IMU in the AOT housing at the front of the ascent stage

Thus both inertial systems and telescope (used for staralignment) are held in rigid alignment with each other


Strapdown Gyro Systems

The ASA gyros were not mounted in a set of gimbals like the IMU

Rather, each gyro was pivoted in a casing fixed to the LM structure


Strapdown Gyro Systems

Strapdown gyro systems cannot enter a gimbal lock situation (unlike the 3-gimbal Apollo IMU) An advantage in possible abort situations

They are also physically small: ASA(AGS) : 530 in3, 21lb IMU(PGNCS): 1023 in3, 42lb

However, the accuracy of strapdown systems is more difficult to predict than gimballed gyros, as a tradeoff is required between the time taken for calculation and accuracy

Accuracy of around 1 deg/hr was typical


DEDA In earliest design for AGS no astronaut interface was

provided (mission variables loaded via GSE) The astronaut interface to AGS is via the Data Entry

and Display Assembly (DEDA)

Besides simple input and outputfunctions, DEDA also checks the input keystrokes and lights operator error light if the sequence is improper, thus removing any need for input checking in the AEA


DEDAPermitted input sequences are (d=decimal

digit, o=octal digit): Clr o o o ReadOut (contents of memory location ooo

displayed) Clr o o o ± d d d d d Entr (ddddd written to location ooo)

The three octal digits define the desired memory location

Only locations 0268-7048 are user-accessible Illegal, sequences result in Opr Err light

being illuminated - cleared by Clr buttonHold button prevents display updating until

ReadOut pressed


DEDANote that in contrast to most LGC routines

(except self-test), all AGS routines are initiated by altering a memory location to some value (rather than specifying a verb/noun combination)

Results of routines are displayed by reading specified memory locations


AEA 27 instructions (10-70s) Memory

18-bit, 2’s complement, fixed-point (no parity) 4096 words (2048 volatile,

2048 hardwired) 5s cycle time

No interrupt system AEA polls for input from DEDA and PGNCS

No timer as such all routine program sections take < 20ms (or are split into

<20ms chunks) (see slide 14) DLY instruction pauses processing until an every-20ms signal

received if 20ms signal occurs at other time, CWEA warning issued

(program has probably entered a loop)

23.75 inches


Software DesignThe AEA executes one computational

cycle every 2 secondsEach cycle comprises 100 20ms segments

the DLY instruction times the start of each segment

Each 20ms segment comprises two partsi) functions performed every 20msii) alternately, either functions performed every

40ms… or part of an every-two-seconds function


Software Design20ms functions:

Gyro, accelerometer data processing Attitude direction cosine updating PGNCS downlink data input routine, Telemetry output,

PGNCS/AGS or body axis align computations

40ms functions include: Main engine thrust selection and control Output AGS attitude error signals Computation and output to the instrument panel of FDAI

angles DEDA and external discrete sampling (CES, GSE)

2s functions include: Decision logic for AGS guidance LM navigation Various manoeuvre and orbital calculations


AEA Code

ADD 2PIB3

SICOE TMI *-1 # SET PLUS

STQ SREX

STO TS1

SUB 2PIB3 # SET BETWEEN 0-2PI

TMI *+2

STO TS1

CLA PID2 # PI/2

SUB TS1

STO TS0 # PI/2-ALPHA

TMI SICO1 # -- IS GREATER THAN 90

AXT 1,1

Some sample code (start of sine/cosine routine)...


Development IssuesInitially a digital differential analyser (with

no user interface) was the favoured solution

Studies then indicated a shift to a full general-purpose digital computer of 500 x 18-bit word memory capacity was necessary to accommodate require mission functionality

After several intermediate designs, 4096 words (and DEDA) were required to meet expanded mission requirements


Development Issues Great care was taken to minimise power

consumption (AEA required 75W maximum) memory split into 2048 hardwired words and 2048 word

erasable scratch pad (however, ratio between hardwired and scratchpad memory was (potentially) flexible)

erasable memory technology used destructive read, so immediate rewrite required after each read access

hardwired memory obviated need for rewrite for hardwired program memory accesses

special instructions provided to reduce power consumption by not rewriting memory after read

Scratchpad memory more 0-bits than 1-bits scratchpad memory held in inverted form to reduce inhibit

driver power consumption


Development Issues

An apparently short-lived plan (1966) was to offer the AEA as a commercial computer (MARCO [MAn-Rated-COmputer] 4418)

TRW believed it had ‘developed a digital computer whose current capabilities and future potential transcend its original design objectives’

The 4K memory of the AEA could be extended to 8K (the implementation details of this are unknown)

The author of this presentation is not aware of any sales of the MARCO 4418 (except in AEA guise) and welcomes further information on this


Development Issues

Budget was a major issue Testing was carried out by NASA in a modified

“milk-wagon like” van (MISER - Mobile Inertial Sensor Evaluation Rogatory), housing an AGS plus test equipment

This was driven round the Houston streets to test the operation of hardware and software


In-flight performanceThe AGS was popular with crews - e.g.

“AGS seemed to work extremely well” (Armstrong, Apollo 11)

“[AGS] performed admirably and agreed with the PGNS…” (Mitchell, A14)

but some problems (excluding procedural) encountered: ‘Clr’ key required two depressions (A9) Inoperative DEDA segment (A11) Broken DEDA electroluminescent display (A14) AGS failed just prior to rendezvous (A14)


Using the AGS - demo

(Demonstration of AEA simulator)


AEA v LGCWhich is ‘better’?Analysis of sine/cosine routines

AEA17 magnitude bits accuracycalculates both sine and cosine of angle at one timememory usage: 41 words = 738 bitstiming (worst case): 1173s

LGC28 magnitude bits accuracy (double-word)calculates either sine or cosinememory usage: 52 words = 780 bits (dedicated memory

only)timing (worst case): 3872s (sine), 4083s (cosine)


AEA v LGC Clearly both use almost the same memory

capacity Both use same polynomial approximation

technique (AEA: 3 terms, LGC: 4 terms) Adjusting for the greater accuracy of the LGC, in

terms of speed of execution the AEA is approximately twice as fast as the LGC ...

… and the AEA calculates both sine and cosine in one subroutine call

However, the LGC has the advantage of having an easily extendable memory addressing structure - vital as demands on the LGC grew


AEA v AGC AEA benefits from:

simple instruction set simple programming language simple memory structure user input error checking handled in DEDA

LGC benefits from: easily expanded memory DSKY interface sophisticated timing mechanisms multi-level interrupt structure interpreted program instruction set to extend basic

functionality


AEA v AGC The AEA suffers from:

polling for inputs 20ms ‘slots’ and time wasted in the DLY instruction pause inefficient user interface (e.g. many inputs require user to

pad with zeros - can almost double number of key strokes and hence chances for input error)

limited error reporting (only via CWEA, or by blanking DEDA displays)

The LGC suffers from: two complex programming languages one’s-complement arithmetic very complex memory structure relatively slow


Conclusion

The AGS provided a lightweight, low-power backup to the PGNS

The AEA was a fast, straightforward processor, but with limited possibilities for expansion

The simple DEDA user interface was popular with crews, though inefficient in terms of the number of keystrokes required

Though never used in anger, AGS proved that it could successfully guide the LM back to the locale of the CSM


Acknowledgements

(Major sources color-coded in references) Mary Nelson, Wichita State University

from James E Tomayko Collection Box 33, ff 33 Davis Peticolas and John Pultorak via Ron Buckey

(www.ibiblio.org/apollo/yaAGS.html)

http://www.ibiblio.org/apollo/yaAGS.html#AGS_Documentation








Acronyms ACA – Attitude Controller Assembly AEA – Abort Electronics Assembly AGC – Apollo Guidance Computer (cf LGC) AGS – Abort Guidance Section – backup to PGNS to allow rendezvous CES - Control Electronics Section DEDA - AEA keyboard and display DSKY – DiSplay and KeYboard (AGC) FDAI - Flight Director/Attitude Indicator (8-ball display) IMU – Inertial Measurement Unit (part of PGNS) ISS – Inertial SubSection LGC – Lunar module Guidance Computer LM - Lunar Module PGNCS – Primary Guidance, Navigation and Control Section PGNS – Primary Guidance and Navigation Section RCS – Reaction Control System (on LM, 16 jets arranged in two

systems)


References Lunar Module / Abort Guidance System (LM/AGS) Design Survey,

NASA/ERC Design Criteria Program, Guidance and Control (06414-6008-T000), TRW Systems Group, 1968

Apollo Operations Handbook, Lunar Module, LM6 and Subsequent Vol1, Grumman Aerospace Corporation, 1968

LM AGS Programmed Equations Document, Flight Program 6, TRW Systems Group, April 1969

LM/AGS Flight Equations, Narrative Description, TRW Systems Group, 25 January 1967

Various TRW Press Releases and product leaflets Beraru, J; The TRW Systems MARCO 4418 - A Man Rated

Computer, TRW Systems, ND (probably 1966) Bettwy, T.S. & Baker, K.L; Flight Program 8, TRW Systems Inc., 18

December 1970 Stiverson, H.L.; Abort Electronic Assembly, Programming

Reference, TRW Systems Group, April 1966 Wie, B.; Space Vehicle Dynamics and Control, AIAA Education

Series, AIAA, Reston VA, 1998

Documents

1 WEBB G143/MAPLD 2005 The LM Abort Guidance Section Julian Webb University of the West of England, Bristol, UK [email protected]