Source Code
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These source-code files are part of a reconstructed copy of Skylark 048, the
Block II Command Module (CM) Apollo Guidance Computer (AGC) software for the
Skylab-2, Skylab-3, Skylab-4, and Apollo-Soyuz Test Project missions.
They have been created via disassembly of binary dumps of the original core rope
memory modules actually flown on Skylab-2, part numbers 2010802-541, 2010802-551,
2010802-561, 2010802-571, 2010802-581, and 2010802-591. Access to these modules
was provided by the New Mexico Museum of Space History, who we are much indebted to.
The source code for the Apollo 15, 16, and 17 software, Artemis 072, was used as
a starting point. Heavy use was made of TRW 4900.5-244, Programmed Guidance Equations
for Skylark Command Module Earth Orbital Program, dated 14 February 1972. This document
contains pseudocode of essentially all of Skylark, using original program labels which
have been copied for this source reconstruction.
Since only binary dumps (rather than listings) of Skylark are available as source material,
all comments and labels are approximate.
They have been taken from the Programmed Guidance Equations or other AGC programs where
possible, or, in some places, written from scratch to match what we believe would have
been in the original listing.
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049923,000002: ## Copyright: Public domain.
049924,000003: ## Filename: LATITUDE_LONGITUDE_SUBROUTINES.agc
049925,000004: ## Purpose: A section of Skylark revision 048.
049926,000005: ## It is part of the source code for the Apollo Guidance Computer (AGC)
049927,000006: ## for Skylab-2, Skylab-3, Skylab-4, and ASTP. No original listings of
049928,000007: ## this software are available; instead, this file was created via
049929,000008: ## disassembly of dumps of the core rope modules actually flown on
049930,000009: ## Skylab-2. Access to these modules was provided by the New Mexico
049931,000010: ## Museum of Space History.
049932,000011: ## Assembler: yaYUL
049933,000012: ## Contact: Ron Burkey <info@sandroid.org>.
049934,000013: ## Website: www.ibiblio.org/apollo/index.html
049935,000014: ## Mod history: 2023-09-04 MAS Created from Artemis 072.
049936,000015: ## 2024-03-04 MAS Updated for Skylark 48.
049937,000016:
049938,000017: # SUBROUTINE TO CONVERT RAD VECTOR AT GIVEN TIME TO LAT,LONG AND ALT
049939,000018:
049940,000019: # CALLING SEQUENCE
049941,000020:
049942,000021: # L-1 CALL
049943,000022: # L LAT-LONG
049944,000023:
049945,000024: # SUBROUTINES USED
049946,000025:
049947,000026: # R-TO-RP, ARCTAN, SETGAMMA, SETRE
049948,000027:
049949,000028: # ERASABLE INIT. REQ.
049950,000029:
049951,000030: # AXO, -AYO, AZO, TEPHEM (SET AT LAUNCH TIME)
049952,000031: # ALPHAV = POSITION VECTOR METERS B-29
049953,000032: # MPAC -- TIME (CSECS B-28)
049954,000033: # ERADFLAG =1, TO COMPUTE EARTH RADIUS, =0 FOR FIXED EARTH RADIUS
049955,000034:
049956,000035: # OUTPUT
049957,000036:
049958,000037: # LATITUDE IN LAT (REVS. B-0)
049959,000038: # LONGITUDE IN LONG (REVS. B-0)
049960,000039: # ALTITUDE IN ALT METERS B-29
049961,000040:
049962,000041: 11,2000 SETLOC LATLONG
049963,000042: 11,2000 BANK
049964,000043:
049965,000044: 11,2461 COUNT* $$/LT-LG
049966,000045: 11,2461 E4,1453 EBANK= ALPHAV
049967,000046: 11,2461 40220 LAT-LONG STQ SETPD
049968,000047: 11,2462 02143 INCORPEX
049969,000048: 11,2463 00001 0D
049970,000049: 11,2464 24007 STOVL 6D # SAVE TIME IN 6-7D FOR R-TO-RP
049971,000050: 11,2465 02054 ALPHAV
049972,000051: 11,2466 51406 PUSH ABVAL # 0-5D= R FOR R-TO-RP
049973,000052: 11,2467 36167 STCALL ALPHAM # ABS. VALUE OF R FOR ALT FORMULA BELOW
049974,000053: 11,2470 55224 R-TO-RP # RP VECTOR CONVERTED FROM R B-29
049975,000054: 11,2471 77656 UNIT # UNIT RP B-1
049976,000055: 11,2472 36054 STCALL ALPHAV # U2= 1/2 SINL FOR SETRE SUBR BELOW
049977,000056: 11,2473 22645 SETRE # CALC RE METERS B-29
049978,000057: 11,2474 63545 DLOAD DSQ
049979,000058: 11,2475 02054 ALPHAV
049980,000059: 11,2476 63525 PDDL DSQ
049981,000060: 11,2477 02056 ALPHAV +2
049982,000061: 11,2500 75415 DAD SQRT
049983,000062: 11,2501 76405 DMP SL1R
049984,000063: 11,2502 22600 B2/A2
049985,000064: 11,2503 14021 STODL COSTH # COS(LAT) B-1
049986,000065: 11,2504 02060 ALPHAV +4
049987,000066: 11,2505 34023 STCALL SINTH # SIN(LAT) B-1
049988,000067: 11,2506 22605 ARCTAN
049989,000068: 11,2507 14742 STODL LAT # LAT B0
049990,000069: 11,2510 02054 ALPHAV
049991,000070: 11,2511 14021 STODL COSTH # COS(LONG) B-1
049992,000071: 11,2512 02056 ALPHAV +2
049993,000072: 11,2513 34023 STCALL SINTH # SIN(LONG) B-1
049994,000073: 11,2514 22605 ARCTAN
049995,000074: 11,2515 14744 STODL LONG # LONG. REVS B-0 IN RANGE -1/2 TO 1/2
049996,000075: 11,2516 02167 ALPHAM
049997,000076: 11,2517 77625 DSU # ALT= R-RE METERS B-29
049998,000077: 11,2520 02142 ERADM
049999,000078: 11,2521 34746 STCALL ALT # EXIT WITH ALT METERS B-29
050000,000079: 11,2522 02143 INCORPEX
050001,000080:
050002,000081: # SUBROUTINE TO CONVERT LAT,LONG,ALT AT GIVEN TIME TO RADIUS VECTOR
050003,000082:
050004,000083: # CALLING SEQUENCE
050005,000084:
050006,000085: # L-1 CALL
050007,000086: # L LALOTORV
050008,000087:
050009,000088: # SUBROUTINES USED
050010,000089:
050011,000090: # SETGAMMA, SETRE, RP-TO-R
050012,000091:
050013,000092: # ERASABLE INIT. REQ.
050014,000093:
050015,000094: # AXO, AYO, AZO, TEPHEM SET AT LAUNCH TIME
050016,000095: # LAT -- LATITUDE (REVS B0)
050017,000096: # LONG -- LONGITUDE (REVS B0)
050018,000097: # ALT -- ALTITUDE (METERS) B-29
050019,000098: # MPAC -- TIME (CSECS B-28)
050020,000099: # ERADFLAG =1 TO COMPUTE EARTH RADIUS, =0 FOR FIXED EARTH RADIUS
050021,000100:
050022,000101: # OUTPUT
050023,000102:
050024,000103: # R-VECTOR IN ALPHAV (METERS B-29)
050025,000104:
050026,000105: 11,2523 40220 LALOTORV STQ SETPD # LAT,LONG,ALT TO R VECTOR
050027,000106: 11,2524 02143 INCORPEX
050028,000107: 11,2525 00001 0D
050029,000108: 11,2526 00007 STORE 6D # 6-7D = TIME FOR RP-TO-R
050030,000109: 11,2527 73545 DLOAD SIN # COS(LONG)COS(LAT) IN MPAC
050031,000110: 11,2530 00742 LAT # UNIT RP = SIN(LONG)COS(LAT) 2-3D
050032,000111: 11,2531 65275 DMPR PDDL # PD 2 GAMMA*SIN(LAT) 0-1D
050033,000112: 11,2532 22600 B2/A2
050034,000113: 11,2533 00742 LAT # 0-1D = GAMMA*SIN(LAT) B-2
050035,000114: 11,2534 65346 COS PDDL # PD4 2-3D = COS(LAT) B-1 TEMPORARILY
050036,000115: 11,2535 00744 LONG
050037,000116: 11,2536 57356 SIN DMPR # PD 2
050038,000117: 11,2537 71525 PDDL COS # PD 4 2-3D = SIN(LONG)COS(LAT) B-2
050039,000118: 11,2540 00742 LAT
050040,000119: 11,2541 71525 PDDL COS # PD 6 4-5D = COS(LAT) B-1 TEMPORARILY
050041,000120: 11,2542 00744 LONG
050042,000121: 11,2543 55475 DMPR VDEF # PD4 MPAC = COS(LONG)COS(LAT) B-2
050043,000122: 11,2544 41456 UNIT PUSH # 0-5D = UNIT RP FOR RP-TO-R SUBR.
050044,000123: 11,2545 36054 STCALL ALPHAV # ALPHAV +4= SINL FOR SETRE SUBR.
050045,000124: 11,2546 22645 SETRE # RE METERS B-29
050046,000125: 11,2547 77624 CALLRPRT CALL
050047,000126: 11,2550 55213 RP-TO-R # EXIT WITH UNIT R VECTOR IN MPAC
050048,000127: 11,2551 16054 STODL ALPHAV
050049,000128: 11,2552 02142 ERADM
050050,000129: 11,2553 74215 DAD VXSC # (RE + ALT)(UNIT R) METERS B-30
050051,000130: 11,2554 00746 ALT
050052,000131: 11,2555 02054 ALPHAV
050053,000132: 11,2556 77772 VSL1 # R METERS B-29
050054,000133: 11,2557 36054 STCALL ALPHAV # EXIT WITH R IN METERS B-29
050055,000134: 11,2560 02143 INCORPEX
050056,000135:
050057,000136: # SUBROUTINE TO COMPUTE EARTH RADIUS
050058,000137:
050059,000138: # INPUT
050060,000139:
050061,000140: # 1/2 SIN LAT IN ALPHAV +4
050062,000141:
050063,000142: # OUTPUT
050064,000143:
050065,000144: # EARTH RADIUS IN ERADM AND MPAC (METERS B-29)
050066,000145:
050067,000146: 11,2561 63545 GETERAD DLOAD DSQ
050068,000147: 11,2562 02060 ALPHAV +4 # SIN**2(L)
050069,000148: 11,2563 44352 SL1 BDSU
050070,000149: 11,2564 15724 DP1/2 # COS**2(L)
050071,000150: 11,2565 44275 DMPR BDSU
050072,000151: 11,2566 22602 EE
050073,000152: 11,2567 15724 DP1/2
050074,000153: 11,2570 75465 BDDV SQRT
050075,000154: 11,2571 22576 B2XSC
050076,000155: 11,2572 77622 SR4R
050077,000156: 11,2573 02142 STORE ERADM
050078,000157: 11,2574 77616 RVQ
050079,000158:
050080,000159: # THE FOLLOWING CONSTANTS WERE COMPUTED WITH A=6378166, B=6356784 METERS
050081,000160: # B2XSC = B**2 SCALED B-51
050082,000161: # B2/A2 = B**2/A**2 SCALED B-1
050083,000162: # EE = (1-B**2/A**2) SCALED B-0
050084,000163:
050085,000164: 11,2575 00446 00305 B2XSC 2DEC .0179450689 # B**2 SCALED B-51
050086,000165: 11,2577 06,3723 DP1/2 = XUNIT
050087,000166: 11,2577 17711 05254 B2/A2 2DEC .9933064884 B-1 # GAMMA= B**2/A**2 B-1
050088,000167: 11,2601 00155 25250 EE 2DEC 6.6935116 E-3 # (1-B**2/A**2) B-0
050089,000168: 11,2603 00302 17755 ERAD 2DEC 6373338 B-29 # PAD RADIUS
050090,000169:
050091,000170: # ARCTAN SUBROUTINE
050092,000171:
050093,000172: # CALLING SEQUENCE
050094,000173:
050095,000174: # SIN THETA IN SINTH B-1
050096,000175: # COS THETA IN COSTH B-1
050097,000176: # CALL ARCTAN
050098,000177:
050099,000178: # OUTPUT
050100,000179: # ARCTAN THETA IN MPAC AND THETA B-0 IN RANGE -1/2 TO +1/2
050101,000180:
050102,000181: 11,2605 77600 ARCTAN BOV
050103,000182: 11,2606 22607 CLROVFLW
050104,000183: 11,2607 63545 CLROVFLW DLOAD DSQ
050105,000184: 11,2610 00023 SINTH
050106,000185: 11,2611 63525 PDDL DSQ
050107,000186: 11,2612 00021 COSTH
050108,000187: 11,2613 77615 DAD
050109,000188: 11,2614 75454 BZE SQRT
050110,000189: 11,2615 22633 ARCTANXX # ATAN=0/0 SET THETA=0
050111,000190: 11,2616 40065 BDDV BOV
050112,000191: 11,2617 00023 SINTH
050113,000192: 11,2620 22640 ATAN=90
050114,000193: 11,2621 67542 SR1 ASIN
050115,000194: 11,2622 00025 STORE THETA
050116,000195: 11,2623 50125 PDDL BMN
050117,000196: 11,2624 00021 COSTH
050118,000197: 11,2625 22627 NEGCOS
050119,000198: 11,2626 43545 DLOAD RVQ
050120,000199: 11,2627 57545 NEGCOS DLOAD DCOMP
050121,000200: 11,2630 43244 BPL DAD
050122,000201: 11,2631 22635 NEGOUT
050123,000202: 11,2632 15724 DP1/2
050124,000203: 11,2633 00025 ARCTANXX STORE THETA
050125,000204: 11,2634 77616 RVQ
050126,000205:
050127,000206: 11,2635 52025 NEGOUT DSU GOTO
050128,000207: 11,2636 15724 DP1/2
050129,000208: 11,2637 22633 ARCTANXX
050130,000209: 11,2640 75345 ATAN=90 DLOAD SIGN
050131,000210: 11,2641 33412 LODP1/4
050132,000211: 11,2642 00023 SINTH
050133,000212: 11,2643 00025 STORE THETA
050134,000213: 11,2644 77616 RVQ
050135,000214:
050136,000215: 11,2645 06,3725 2DZERO = DPZERO
050137,000216:
050138,000217:
050139,000218: # ..... SETRE SUBROUTINE .....
050140,000219: # SUBROUTINE TO SET RE (EARTH RADIUS)
050141,000220:
050142,000221: # RE = RREF FOR FIXED EARTH RADIUS OR COMPUTED RF FOR FISCHER ELLIPSOID
050143,000222:
050144,000223: # CALLING SEQUENCE
050145,000224: # L CALL
050146,000225: # L+1 SETRE
050147,000226:
050148,000227: # SUBROUTINES USED
050149,000228: # GETERAD
050150,000229:
050151,000230: # INPUT
050152,000231: # ERADFLAG = 0 FOR FIXED RE, 1 FOR COMPUTED RE
050153,000232: # ALPHAV +4 = 1/2 SINL IF GETERAD IS CALLED
050154,000233:
050155,000234: # OUTPUT
050156,000235: # ERADM = ERAD OR COMPUTED RF FOR EARTH (METERS B-29)
050157,000236:
050158,000237: 11,2645 71220 SETRE STQ DLOAD
050159,000238: 11,2646 00051 SETREX
050160,000239: 11,2647 22604 ERAD
050161,000240: 11,2650 45014 BOFF CALL # ERADFLAG=0 FOR FIXED RE, 1 FOR COMPUTED
050162,000241: 11,2651 00742 ERADFLAG
050163,000242: 11,2652 22654 SETRXX
050164,000243: 11,2653 22561 GETERAD
050165,000244: 11,2654 36142 SETRXX STCALL ERADM # EXIT WITH RE OR RM METERS B-29
050166,000245: 11,2655 00051 SETREX
050167,000246: 11,2656 0000051 SETREX = S2
End of include-file LATITUDE_LONGITUDE_SUBROUTINES.agc. Parent file is MAIN.agc