LATITUDE LONGITUDE SUBROUTINES

Scanned pages: 1133-1139

LATITUDE_LONGITUDE_SUBROUTINES.agc

# 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,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*  $$/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