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LATITUDE_LONGITUDE_SUBROUTINES.agc
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LATITUDE_LONGITUDE_SUBROUTINES.agc
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# Copyright: Public domain.
# Filename: LATITUDE_LONGITUDE_SUBROUTINES.agc
# Purpose: Part of the source code for Luminary 1A build 099.
# It is part of the source code for the Lunar Module's (LM)
# Apollo Guidance Computer (AGC), for Apollo 11.
#
# Assembler: yaYUL
# Contact: Jim Lawton <jim DOT lawton AT gmail DOT com>
# Website: www.ibiblio.org/apollo.
# Pages: 1133-1139
# Mod history: 2009-05-28 JL Updated from page images.
# 2011-01-06 JL Fixed interpretive indentation.
#
# This source code has been transcribed or otherwise adapted from digitized
# images of a hardcopy from the MIT Museum. The digitization was performed
# by Paul Fjeld, and arranged for by Deborah Douglas of the Museum. Many
# thanks to both. The images (with suitable reduction in storage size and
# consequent reduction in image quality as well) are available online at
# www.ibiblio.org/apollo. If for some reason you find that the images are
# illegible, contact me at info@sandroid.org about getting access to the
# (much) higher-quality images which Paul actually created.
#
# Notations on the hardcopy document read, in part:
#
# Assemble revision 001 of AGC program LMY99 by NASA 2021112-061
# 16:27 JULY 14, 1969
# Page 1133
# SUBROUTINE TO CONVERT RAD VECTOR AT GIVEN TIME TO LAT,LONG AND ALT
#
# CALLING SEQUENCE
# L-1 CALL
# L LAT-LONG
#
# SUBROUTINES USED
# R-TO-RP, ARCTAN, SFTGAMMA, SETRE
#
# ERASABLE INIT. REQ.
# AXO, -AYO, AZO, TEPHEM (SET AT LAUNCH TIME)
# ALPHAV = POSITION VECTOR METERS B-29
# MPAC -- TIME (CSECS B-28)
# ERADFLAG =1, TO COMPUTE EARTH RADIUS, =0 FOR FIXED EARTH RADIUS
# LUNAFLAG=0 FOR EARTH, 1 FOR MOON
#
# OUTPUT
# LATITUDE IN LAT (REVS. B-0)
# LONGITUDE IN LONG (REVS. B-0)
# ALTITUDE IN ALT METERS B-29
BANK 30
SETLOC LATLONG
BANK
COUNT* $$/LT-LG
EBANK= ALPHAV
LAT-LONG STQ SETPD
INCORPEX
0D
STOVL 6D # SAVE TIME IN 6-7D FOR R-TO-RP
ALPHAV
PUSH ABVAL # 0-5D= R FOR R-TO-RP
STODL ALPHAM # ABS. VALUE OF R FOR ALT FORMULA BELOW
ZEROVEC # SET MPAC=0 FOR EARTH, NON-ZERO FOR MOON
BOFF COS # USE COS(0) TO GET NON-ZERO IN MPAC
LUNAFLAG # 0=EARTH, 1=MOON
CALLRTRP
CALLRTRP CALL
R-TO-RP # RP VECTOR CONVERTED FROM R B-29
UNIT # UNIT RP B-1
STCALL ALPHAV # U2= 1/2 SINL FOR SETRE SUBR BELOW
SETGAMMA # SET GAMMA=B2/A2 FOR EARTH, =1 FOR MOON
CALL # SCALED B-1
SETRE # CALC RE METERS B-29
DLOAD DSQ
ALPHAV
PDDL DSQ
ALPHAV +2
DAD SQRT
# Page 1134
DMP SL1R
GAMRP
STODL COSTH # COS(LAT) B-1
ALPHAV +4
STCALL SINTH # SIN(LAT) B-1
ARCTAN
STODL LAT # LAT B0
ALPHAV
STODL COSTH # COS(LONG) B-1
ALPHAV +2
STCALL SINTH # SIN(LONG) B-1
ARCTAN
STODL LONG # LONG. REVS B-0 IN RANGE -1/2 TO 1/2
ALPHAM
DSU # ALT= R-RE METERS B-29
ERADM
STCALL ALT # EXIT WITH ALT METERS B-29
INCORPEX
# Page 1135
# SUBROUTINE TO CONVERT LAT,LONG.ALT AT GIVEN TIME TO RADIUS VECTOR
#
# CALLING SEQUENCE
# L-1 CALL
# L LALOTORV
#
# SUBROUTINES USED
# SETGAMMA, SETRE, RP-TO-R
#
# ERASABLE INIT. REQ.
# AXO, AYO, AZO, TEPHEM SET AT LAUNCH TIME
# LAT -- LATITUDE (REVS B0)
# LONG -- LONGITUDE (REVS B0)
# ALT -- ALTITUDE (METERS) B-29
# MPAC -- TIME (CSECS B-28)
# ERADFLAG =1 TO COMPUTE EARTH RADIUS, =0 FOR FIXED EARTH RADIUS
# LUNAFLAG=0 FOR EARTH, 1 FOR MOON
#
# OUTPUT
# R-VECTOR IN ALPHAV (METERS B-29)
LALOTORV STQ SETPD # LAT,LONG,ALT TO R VECTOR
INCORPEX
0D
STCALL 6D # 6-7D= TIME FOR RP-TO-R
SETGAMMA # GAMMA=B2/A2 FOR EARTH, 1 FOR MOON B-1
DLOAD SIN # COS(LONG)COS(LAT) IN MPAC
LAT # UNIT RP = SIN(LONG)COS(LAT) 2-3D
DMPR PDDL # PD 2 GAMMA*SIN(LAT) 0-1D
GAMRP
LAT # 0-1D = GAMMA*SIN(LAT) B-2
COS PDDL # PD4 2-3D = COS(LAT) B-1 TEMPORARILY
LONG
SIN DMPR # PD 2
PDDL COS # PD 4 2-3D = SIN(LONG)COS(LAT) B-2
LAT
PDDL COS # PD 6 4-5D = COS(LAT) B-1 TEMPORARILY
LONG
DMPR VDEF # PD 4 MPAC = COS(LONG)COS(LAT) B-2
UNIT PUSH # 0-5D= UNIT RP FOR RP-TO-R SUBR.
STCALL ALPHAV # ALPHAV +4= SINL FOR SETRE SUBR.
SETRE # RE METERS B-29
DLOAD BOFF # SET MPAC=0 FOR EARTH, NON-ZERO FOR MOON
ZEROVEC
LUNAFLAG
CALLRPRT
COS # USE COS(0) TO GET NON-ZERO IN MPAC
CALLRPRT CALL
RP-TO-R # EXIT WITH UNIT R VECTOR IN MPAC
STODL ALPHAV
ERADM
# Page 1136
DAD VXSC # (RE + ALT)(UNIT R) METERS B-30
ALT
ALPHAV
VSL1 # R METERS B-29
STCALL ALPHAV # EXIT WITH R IN METERS B-29
INCORPEX
# SUBROUTINE TO COMPUTE EARTH RADIUS
#
# INPUT
# 1/2 SIN LAT IN ALPHAV +4
#
# OUTPUT
# EARTH RADIUS IN ERADM AND MPAC (METERS B-29)
GETERAD DLOAD DSQ
ALPHAV +4 # SIN**2(L)
SL1 BDSU
DP1/2 # COS**2(L)
DMPR BDSU
EE
DP1/2
BDDV SQRT
B2XSC
SR4R
STORE ERADM
RVQ
# THE FOLLOWING CONSTANTS WERE COMPUTED WITH A=6378166, B=6356784 METERS
# B2XSC = B**2 SCALED B-51
# B2/A2 = B**2/A**2 SCALED B-1
# EE = (1-B**2/A**2) SCALED B-0
B2XSC 2DEC .0179450689 # B**2 SCALED B-51
DP1/2 = XUNIT
B2/A2 2DEC .9933064884 B-1 # GAMMA= B**2/A**2 B-1
EE 2DEC 6.6935116 E-3 # (1-B**2/A**2) B-0
# Page 1137
# ARCTAN SUBROUTINE
#
# CALLING SEQUENCE
# SIN THETA IN SINTH B-1
# COS THETA IN COSTH B-1
# CALL ARCTAN
#
# OUTPUT
# ARCTAN THETA IN MPAC AND THETA B-0 IN RANGE -1/2 TO +1/2
ARCTAN BOV
CLROVFLW
CLROVFLW DLOAD DSQ
SINTH
PDDL DSQ
COSTH
DAD
BZE SQRT
ARCTANXX # ATAN=0/0 SET THETA=0
BDDV BOV
SINTH
ATAN=90
SR1 ASIN
STORE THETA
PDDL BMN
COSTH
NEGCOS
DLOAD RVQ
NEGCOS DLOAD DCOMP
BPL DAD
NEGOUT
DP1/2
ARCTANXX STORE THETA
RVQ
NEGOUT DSU GOTO
DP1/2
ARCTANXX
ATAN=90 DLOAD SIGN
LODP1/4
SINTH
STORE THETA
RVQ
2DZERO = DPZERO
# Page 1138
# ***** SETGAMMA SUBROUTINE *****
# SUBROUTINE TO SET GAMMA FOR THE LAT-LONG AND LALOTORV SUBROUTINES
#
# GAMMA = B**2/A**2 FOR EARTH (B-1)
# GAMMA = 1 FOR MOON (B-1)
#
# CALLING SEQUENCE
# L CALL
# L+1 SETGAMMA
#
# INPUT
# LUNAFLAG=0 FOR EARTH, =1 FOR MOON
#
# OUTPUT
# GAMMA IN GAMRP (B-1)
SETGAMMA DLOAD BOFF # BRANCH FOR EARTH
B2/A2 # EARTH GAMMA
LUNAFLAG
SETGMEX
SLOAD
1B1 # MOON GAMMA
SETGMEX STORE GAMRP
RVQ
GAMRP = 8D
# Page 1139
# ***** SETRE SUBROUTINE *****
# SUBROUTINE TO SET RE (EARTH OR MOON RADIUS)
#
# RE = RM FOR MOON
# RE = RREF FOR FIXED EARTH RADIUS OR COMPUTED RF FOR FISCHER ELLIPSOID
#
# CALLING SEQUENCE
# L CALL
# L+1 SETRE
#
# SUBROUTINES USED
# GETERAD
#
# INPUT
# ERADFLAG = 0 FOR FIXED RE, 1 FOR COMPUTED RE
# ALPHAV +4 = 1/2 SINL IF GETERAD IS CALLED
# LUNAFLAG = 0 FOR EARTH, =1 FOR MOON
#
# OUTPUT
# ERADM = 504RM FOR MOON (METERS B-29)
# ERADM = ERAD OR COMPUTED RE FOR EARTH (METERS B-29)
SETRE STQ DLOAD
SETREX
504RM
BON DLOAD # BRANCH FOR MOON
LUNAFLAG
TSTRLSRM
ERAD
BOFF CALL # ERADFLAG=0 FOR FIXED RE, 1 FOR COMPUTED
ERADFLAG
SETRXX
GETERAD
SETRXX STCALL ERADM # EXIT WITH RE OR RM METERS B-29
SETREX
TSTRLSRM BON VLOAD # ERADFLAG=0, SET R0=RLS
ERADFLAG # =1 R0=RM
SETRXX
RLS
ABVAL SR2R # SCALE FROM B-27 TO B-29
GOTO
SETRXX
SETREX = S2