Pre-launch Review
LATITUDE LONGITUDE SUBROUTINES
Scanned pages: 1236-1242
# Copyright: Public domain.# Filename: LATITUDE_LONGITUDE_SUBROUTINES.agc# Purpose: Part of the source code for Colossus 2A, AKA Comanche 055.# It is part of the source code for the Command Module's (CM)# Apollo Guidance Computer (AGC), for Apollo 11.# Assembler: yaYUL# Contact: Ron Burkey <info@sandroid.org>.# Website: www.ibiblio.org/apollo.# Pages: 1236-1242# Mod history: 2009-05-14 RSB Adapted from the Colossus249/ file of the# same name, using Comanche055 page images.## 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 055 of AGC program Comanche by NASA# 2021113-051. 10:28 APR. 1, 1969## This AGC program shall also be referred to as# Colossus 2A
# Page 1236# 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,SETGAMMA,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 13/LT-LG
EBANK= ALPHAVLAT-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 CALLRTRPCALLRTRP 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# Page 1237 DAD SQRT 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 1238# 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 # PD4 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 MPACCALLRPRT CALL RP-TO-R # EXIT WITH UNIT R VECTOR IN MPAC STODL ALPHAV ERADM# Page 1239 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 = XUNITB2/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
ERAD 2DEC 6373338 B-29 # PAD RADIUS
# Page 1240# 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 CLROVFLWCLROVFLW 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 RVQNEGCOS DLOAD DCOMP BPL DAD NEGOUT DP1/2ARCTANXX STORE THETA RVQ
NEGOUT DSU GOTO DP1/2 ARCTANXXATAN=90 DLOAD SIGN LODP1/4 SINTH STORE THETA RVQ
2DZERO = DPZERO
# Page 1241# ***** 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 GAMMASETGMEX STORE GAMRP RVQGAMRP = 8D
# Page 1242# ***** 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 RF 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 GETERADSETRXX STCALL ERADM # EXIT WITH RE OR RM METERS B-29 SETREXTSTRLSRM BON VLOAD # ERADFLAG=0,SET R0=RLS ERADFLAG # =1 R0=RM SETRXX RLS ABVAL SR2R # SCALE FROM B-27 TO B-29 GOTO SETRXXSETREX = S2504RM 2DEC 1738090 B-29 # METERS B-29 (MOON RADIUS)