The LM Abort Guidance Section

1
The LM Abort Guidance Section

• Julian Webb
• University of the West of England, Bristol, UK
• julian2.webb_at_uwe.ac.uk

2
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

3
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

4
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 Displays T
elemetry
5
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

6
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 star
• alignment) are held in rigid
• alignment with each other

7
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

8
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

9
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 output
• functions, 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

10
DEDA

• Permitted input sequences are (ddecimal digit,
  ooctal 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 button
• Hold button prevents display updating until
  ReadOut pressed

11
DEDA

• Note 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

12
AEA

• 27 instructions (10-70?s)
• Memory
• 18-bit, 2s complement,
• fixed-point (no parity)
• 4096 words (2048 volatile,
• 2048 hardwired)
• 5?s cycle time
• No interrupt system
• AEA polls for input from DEDA and PGNCS
• No timer as such
• all routine program sections take lt 20ms (or are
  split into lt20ms 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)

13
(No Transcript)
14
Software Design

• The AEA executes one computational cycle every 2
  seconds
• Each cycle comprises 100 20ms segments
• the DLY instruction times the start of each
  segment
• Each 20ms segment comprises two parts
• i) functions performed every 20ms
• ii) alternately, either functions performed every
  40ms
• or part of an every-two-seconds function

15
Software Design

• 20ms 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

16
AEA Code

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

• 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

17
Development Issues

• Initially 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

18
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

19
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

20
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

21
In-flight performance

• The 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)

22
Using the AGS - demo

• (Demonstration of AEA simulator)

23
AEA v LGC

• Which is better?
• Analysis of sine/cosine routines
• AEA
• 17 magnitude bits accuracy
• calculates both sine and cosine of angle at one
  time
• memory usage 41 words 738 bits
• timing (worst case) 1173?s
• LGC
• 28 magnitude bits accuracy (double-word)
• calculates either sine or cosine
• memory usage 52 words 780 bits (dedicated
  memory only)
• timing (worst case) 3872?s (sine), 4083?s
  (cosine)

24
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

25
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

26
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
• ones-complement arithmetic
• very complex memory structure
• relatively slow

27
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

28
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)

29
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)

30
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