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The LM Abort Guidance Section

The LM Abort Guidance Section. Julian Webb University of the West of England, Bristol, UK julian2.webb@uwe.ac.uk. Introduction.

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The LM Abort Guidance Section

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  1. The LM Abort Guidance Section Julian Webb University of the West of England, Bristol, UK julian2.webb@uwe.ac.uk G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 Telemetry G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 G143/MAPLD 2005

  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 G143/MAPLD 2005

  10. DEDA • Permitted 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 button • Hold button prevents display updating until ReadOut pressed G143/MAPLD 2005

  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 G143/MAPLD 2005

  12. 23.75 inches AEA • 27 instructions (10-70s) • Memory • 18-bit, 2’s 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 < 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) G143/MAPLD 2005

  13. 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 G143/MAPLD 2005

  14. 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 G143/MAPLD 2005

  15. 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 G143/MAPLD 2005

  16. 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 G143/MAPLD 2005

  17. 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 G143/MAPLD 2005

  18. 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 G143/MAPLD 2005

  19. 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 G143/MAPLD 2005

  20. 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) G143/MAPLD 2005

  21. Using the AGS - demo • (Demonstration of AEA simulator) G143/MAPLD 2005

  22. 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) G143/MAPLD 2005

  23. 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 G143/MAPLD 2005

  24. 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 G143/MAPLD 2005

  25. 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 G143/MAPLD 2005

  26. 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 G143/MAPLD 2005

  27. 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) G143/MAPLD 2005

  28. 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) G143/MAPLD 2005

  29. 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 G143/MAPLD 2005

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