diff --git a/src/grb2index/CMakeLists.txt b/src/grb2index/CMakeLists.txt
index 39603b94..c62f6c42 100644
--- a/src/grb2index/CMakeLists.txt
+++ b/src/grb2index/CMakeLists.txt
@@ -12,12 +12,9 @@ elseif(CMAKE_Fortran_COMPILER_ID MATCHES "^(GNU)$")
   set(CMAKE_Fortran_FLAGS_RELEASE "-O3 ${CMAKE_Fortran_FLAGS}")
 endif()
 
-# This is the fortran source code.
-set(fortran_src grb2index.F90)
-
 # Build the executable.
 set(exe_name grb2index)
-add_executable(${exe_name} ${fortran_src})
+add_executable(${exe_name} grb2index.F90)
 target_link_libraries(${exe_name} PRIVATE g2::g2_4 w3emc::w3emc_4 bacio::${bacio_name})
 
 # Install the utility.
diff --git a/src/grb2index/docs/user_guide.md b/src/grb2index/docs/user_guide.md
index 44dbd0b2..85c61377 100644
--- a/src/grb2index/docs/user_guide.md
+++ b/src/grb2index/docs/user_guide.md
@@ -21,35 +21,36 @@ the name of the output index file.
 
 # Index File Format
 
-Version 2 of the index file format is used with GRIB2 files, and has
-the following format:
-
 The index file has two header records:
-- 81-byte "Steve Lord" header with 'GB2IX1' in columns 42-47
-- 81-byte header with number of bytes to skip before index
-records, total length in bytes of the index records, number of
-index records, and GRIB file basename written in format
-('IX1FORM:',3i10,2x,a40).
-
-Each following index record corresponds to a GRIB1 message and
-contains the following fields. All integers are in big-endian format
-in the file.
-
-- byte 001 - 004 length of index record
-- byte 005 - 008 bytes to skip in data file before grib message
-- byte 009 - 012 bytes to skip in message before lus (local use) set = 0, if no local section.
-- byte 013 - 016 bytes to skip in message before gds
-- byte 017 - 020 bytes to skip in message before pds
-- byte 021 - 024 bytes to skip in message before drs
-- byte 025 - 028 bytes to skip in message before bms
-- byte 029 - 032 bytes to skip in message before data section
-- byte 033 - 040 bytes total in the message
-- byte 041 - 041 grib version number (currently 2)
-- byte 042 - 042 message discipline
-- byte 043 - 044 field number within grib2 message
-- byte 045 -  ii identification section (ids)
-- byte ii+1-  jj grid definition section (gds)
-- byte jj+1-  kk product definition section (pds)
-- byte kk+1-  ll the data representation section (drs)
-- byte ll+1-ll+6 first 6 bytes of the bit map section (bms)
+1. an 81-byte header with 'GB2IX1' in columns 42-47
+2. an 81-byte header with the index version number, the number of
+bytes to skip before index records, total length in bytes of the
+index records, number of index records, and the GRIB file basename.
+
+Each record in the index table contains the following fields. All
+integers are in big-endian format in the file. The only difference
+between index version 1 and index version 2 is the size of the
+field containing the number of bytes to skip in file before
+message. To accomodate files > 2 GB, this must be a 64-bit int.
+
+Index Version 1 | Index Version 2 | Contents
+----------------|-----------------|---------
+001 - 004 | 001 - 004 | length of index record
+005 - 008 | 005 - 012 | bytes to skip in data file before grib message
+009 - 012 | 013 - 016 | bytes to skip in message before lus (local use) set = 0, if no local section.
+013 - 016 | 017 - 020 | bytes to skip in message before gds
+017 - 020 | 021 - 024 | bytes to skip in message before pds
+021 - 024 | 025 - 028 | bytes to skip in message before drs
+025 - 028 | 029 - 032 | bytes to skip in message before bms
+029 - 032 | 033 - 036 | bytes to skip in message before data section
+033 - 040 | 037 - 044 | bytes total in the message
+041 - 041 | 045 - 045 | grib version number (always 2)
+042 - 042 | 046 - 046 | message discipline
+043 - 044 | 047 - 048 | field number within grib2 message
+045 -  ii | 045 -  ii | identification section (ids)
+ii+1-  jj | ii+1-  jj | grid definition section (gds)
+jj+1-  kk | jj+1-  kk | product definition section (pds)
+kk+1-  ll | kk+1-  ll | the data representation section (drs)
+ll+1-ll+6 | ll+1-ll+6 | first 6 bytes of the bit map section (bms)
+
 
diff --git a/src/grb2index/grb2index.F90 b/src/grb2index/grb2index.F90
index f4da9124..ec6b3c60 100755
--- a/src/grb2index/grb2index.F90
+++ b/src/grb2index/grb2index.F90
@@ -4,7 +4,6 @@
 
 !> This program creates an index file from a GRIB2 file.
 !>
-!>
 !> @return
 !> - 0 successful run
 !> - 1 GRIB message not found
@@ -12,150 +11,153 @@
 !> - 8 error accessing file
 !>
 !> @author Iredell @date 1992-11-22
-PROGRAM GRB2INDEX
-  PARAMETER(MSK1=32000,MSK2=4000)
-  CHARACTER CGB*256,CGI*256
-  CHARACTER(LEN=1),POINTER,DIMENSION(:) :: CBUF
-  CHARACTER CARG*300
-  INTEGER NARG,IARGC
-  INTERFACE
-     SUBROUTINE GETG2IR(LUGB,MSK1,MSK2,MNUM,CBUF,NLEN,NNUM, &
-          NMESS,IRET)
-       INTEGER,INTENT(IN) :: LUGB,MSK1,MSK2,MNUM
-       CHARACTER(LEN=1),POINTER,DIMENSION(:) :: CBUF
-       INTEGER,INTENT(OUT) :: NLEN,NNUM,NMESS,IRET
-     END SUBROUTINE GETG2IR
-  END INTERFACE
+program grb2index
+  implicit none
+  
+  integer :: msk1, msk2
+  parameter(msk1=32000,msk2=4000)
+  character cgb*256,cgi*256
+  character(len=1),pointer,dimension(:) :: cbuf
+  character carg*300
+  integer narg,iargc
+  integer :: numtot, nnum, nlen, ncgi, mnum, lcarg, kw
+  integer :: ios, iret, irgi, iw, ncgb, nmess
+  
+  interface
+     subroutine getg2ir(lugb,msk1,msk2,mnum,cbuf,nlen,nnum, &
+          nmess,iret)
+       integer,intent(in) :: lugb,msk1,msk2,mnum
+       character(len=1),pointer,dimension(:) :: cbuf
+       integer,intent(out) :: nlen,nnum,nmess,iret
+     end subroutine getg2ir
+  end interface
 
-  !  GET ARGUMENTS
-  NARG=IARGC()
-  IF(NARG.NE.2) THEN
-     CALL ERRMSG('grb2index:  Incorrect usage')
-     CALL ERRMSG('Usage: grb2index gribfile indexfile')
-     CALL ERREXIT(2)
-  ENDIF
-  CALL GETARG(1,CGB)
-  NCGB=LEN_TRIM(CGB)
-  CALL BAOPENR(11,CGB(1:NCGB),IOS)
-  !CALL BASETO(1,1)
-  IF(IOS.NE.0) THEN
-     LCARG=LEN('grb2index:  Error accessing file '//CGB(1:NCGB))
-     CARG(1:LCARG)='grb2index:  Error accessing file '//CGB(1:NCGB)
-     CALL ERRMSG(CARG(1:LCARG))
-     CALL ERREXIT(8)
-  ENDIF
-  CALL GETARG(2,CGI)
-  NCGI=LEN_TRIM(CGI)
-  CALL BAOPEN(31,CGI(1:NCGI),IOS)
-  IF(IOS.NE.0) THEN
-     LCARG=LEN('grb2index:  Error accessing file '//CGI(1:NCGI))
-     CARG(1:LCARG)='grb2index:  Error accessing file '//CGI(1:NCGI)
-     CALL ERRMSG(CARG(1:LCARG))
-     CALL ERREXIT(8)
-  ENDIF
+  !  get arguments
+  narg=iargc()
+  if(narg.ne.2) then
+     call errmsg('grb2index:  Incorrect usage')
+     call errmsg('Usage: grb2index gribfile indexfile')
+     call errexit(2)
+  endif
+  call getarg(1,cgb)
+  ncgb=len_trim(cgb)
+  call baopenr(11,cgb(1:ncgb),ios)
+  !call baseto(1,1)
+  if(ios.ne.0) then
+     lcarg=len('grb2index:  Error accessing file '//cgb(1:ncgb))
+     carg(1:lcarg)='grb2index:  Error accessing file '//cgb(1:ncgb)
+     call errmsg(carg(1:lcarg))
+     call errexit(8)
+  endif
+  call getarg(2,cgi)
+  ncgi=len_trim(cgi)
+  call baopen(31,cgi(1:ncgi),ios)
+  if(ios.ne.0) then
+     lcarg=len('grb2index:  Error accessing file '//cgi(1:ncgi))
+     carg(1:lcarg)='grb2index:  Error accessing file '//cgi(1:ncgi)
+     call errmsg(carg(1:lcarg))
+     call errexit(8)
+  endif
 
-  !  WRITE INDEX FILE
-  MNUM=0
-  CALL GETG2IR(11,MSK1,MSK2,MNUM,CBUF,NLEN,NNUM,NMESS,IRGI)
-  IF(IRGI.GT.1.OR.NNUM.EQ.0.OR.NLEN.EQ.0) THEN
-     CALL ERRMSG('grb2index:  No GRIB messages detected in file ' &
-          //CGB(1:NCGB))
-     CALL BACLOSE(11,IRET)
-     CALL BACLOSE(31,IRET)
-     CALL ERREXIT(1)
-  ENDIF
-  NUMTOT=NUMTOT+NNUM
-  MNUM=MNUM+NMESS
-  CALL WRGI1H(31,NLEN,NUMTOT,CGB(1:NCGB))
-  IW=162
-  CALL BAWRITE(31,IW,NLEN,KW,CBUF)
-  IW=IW+NLEN
+  !  write index file
+  mnum=0
+  call getg2ir(11,msk1,msk2,mnum,cbuf,nlen,nnum,nmess,irgi)
+  if(irgi.gt.1.or.nnum.eq.0.or.nlen.eq.0) then
+     call errmsg('grb2index:  No GRIB messages detected in file ' &
+          //cgb(1:ncgb))
+     call baclose(11,iret)
+     call baclose(31,iret)
+     call errexit(1)
+  endif
+  numtot=numtot+nnum
+  mnum=mnum+nmess
+  call wrgi1h(31,nlen,numtot,cgb(1:ncgb))
+  iw=162
+  call bawrite(31,iw,nlen,kw,cbuf)
+  iw=iw+nlen
 
-  !  EXTEND INDEX FILE IF INDEX BUFFER LENGTH TOO LARGE TO HOLD IN MEMORY
-  IF(IRGI.EQ.1) THEN
-     DO WHILE(IRGI.EQ.1.AND.NNUM.GT.0)
-        IF (ASSOCIATED(CBUF)) THEN
-           DEALLOCATE(CBUF)
-           NULLIFY(CBUF)
-        ENDIF
-        CALL GETG2IR(11,MSK1,MSK2,MNUM,CBUF,NLEN,NNUM,NMESS,IRGI)
-        IF(IRGI.LE.1.AND.NNUM.GT.0) THEN
-           NUMTOT=NUMTOT+NNUM
-           MNUM=MNUM+NMESS
-           CALL BAWRITE(31,IW,NLEN,KW,CBUF)
-           IW=IW+NLEN
-        ENDIF
-     ENDDO
-     CALL WRGI1H(31,IW,NUMTOT,CGB(1:NCGB))
-  ENDIF
-  CALL BACLOSE(11,IRET)
-  CALL BACLOSE(31,IRET)
+  !  extend index file if index buffer length too large to hold in memory
+  if(irgi.eq.1) then
+     do while(irgi.eq.1.and.nnum.gt.0)
+        if (associated(cbuf)) then
+           deallocate(cbuf)
+           nullify(cbuf)
+        endif
+        call getg2ir(11,msk1,msk2,mnum,cbuf,nlen,nnum,nmess,irgi)
+        if(irgi.le.1.and.nnum.gt.0) then
+           numtot=numtot+nnum
+           mnum=mnum+nmess
+           call bawrite(31,iw,nlen,kw,cbuf)
+           iw=iw+nlen
+        endif
+     enddo
+     call wrgi1h(31,iw,numtot,cgb(1:ncgb))
+  endif
+  call baclose(11,iret)
+  call baclose(31,iret)
 
-END PROGRAM GRB2INDEX
+end program grb2index
 
 !> Write index headers.
 !>
-!> ### Program History Log
-!> Date | Programmer | Comments
-!> -----|------------|---------
-!> 95-10-31 | Iredell | modularize system calls
-!> 2005-02-25 | Gilbert | et Header bytes  49-54 to blanks.
-!> 2012-08-01 | Vuong | changed hostname to hostnam
-!>
 !> @param[in] lugi integer logical unit of output index file
 !> @param[in] nlen integer total length of index records
 !> @param[in] nnum integer number of index records
 !> @param[in] cgb character name of GRIB file
 !>
 !> @author Iredell @date 93-11-22
-SUBROUTINE WRGI1H(LUGI,NLEN,NNUM,CGB)
-  CHARACTER CGB*(*)
+subroutine wrgi1h(lugi, nlen, nnum, cgb)
+  implicit none
+
+  integer :: lugi, nlen, nnum
+  character cgb*(*)
+  character cd8*8, ct10*10, hostname*15
 #ifdef __GFORTRAN__
-  CHARACTER CD8*8,CT10*10,HOSTNAME*15
-  INTEGER ISTAT
+  integer istat
 #else
-  CHARACTER CD8*8,CT10*10,HOSTNAM*15
+  character hostnam*15
+  integer hostnm
 #endif
-  CHARACTER CHEAD(2)*81
+  character chead(2)*81
+  integer :: kw, ncgb, ncgb1, ncgb2, ncbase
 
-  !  FILL FIRST 81-BYTE HEADER
-  NCGB=LEN(CGB)
-  NCGB1=NCBASE(CGB,NCGB)
-  NCGB2=NCBASE(CGB,NCGB1-2)
-  CALL DATE_AND_TIME(CD8,CT10)
-  CHEAD(1)='!GFHDR!'
-  CHEAD(1)(9:10)=' 1'
-  CHEAD(1)(12:14)='  1'
-  WRITE(CHEAD(1)(16:20),'(I5)') 162
-  CHEAD(1)(22:31)=CD8(1:4)//'-'//CD8(5:6)//'-'//CD8(7:8)
-  CHEAD(1)(33:40)=CT10(1:2)//':'//CT10(3:4)//':'//CT10(5:6)
-  CHEAD(1)(42:47)='GB2IX1'
-  !CHEAD(1)(49:54)=CGB(NCGB2:NCGB1-2)
-  CHEAD(1)(49:54)='      '
+  !  fill first 81-byte header
+  ncgb = len(cgb)
+  ncgb1 = ncbase(cgb,ncgb)
+  ncgb2 = ncbase(cgb,ncgb1-2)
+  call date_and_time(cd8,ct10)
+  chead(1) = '!GFHDR!'
+  chead(1)(9:10) = ' 1'
+  chead(1)(12:14) = '  1'
+  write(chead(1)(16:20),'(i5)') 162
+  chead(1)(22:31) = cd8(1:4) // '-' // cd8(5:6) // '-' // cd8(7:8)
+  chead(1)(33:40) = ct10(1:2) // ':' // ct10(3:4) // ':' // ct10(5:6)
+  chead(1)(42:47) = 'gb2ix1'
+  chead(1)(49:54) = '      '
 #ifdef __GFORTRAN__
-  ISTAT=HOSTNM(HOSTNAME)
-  IF(ISTAT.eq.0) THEN
-     CHEAD(1)(56:70)='0000'
-  ELSE
-     CHEAD(1)(56:70)='0001'
-  ENDIF
+  istat = hostnm(hostname)
+  if(istat.eq.0) then
+     chead(1)(56:70) = '0000'
+  else
+     chead(1)(56:70) = '0001'
+  endif
 #else
-  CHEAD(1)(56:70)=HOSTNAM(HOSTNAME)
+  chead(1)(56:70) = hostnam(hostname)
 #endif
-  CHEAD(1)(72:80)='grb2index'
-  CHEAD(1)(81:81)=CHAR(10)
+  chead(1)(72:80) = 'grb2index'
+  chead(1)(81:81) = char(10)
 
-  !  FILL SECOND 81-BYTE HEADER
-  CHEAD(2)='IX1FORM:'
-  WRITE(CHEAD(2)(9:38),'(3I10)') 162,NLEN,NNUM
-  CHEAD(2)(41:80)=CGB(NCGB1:NCGB)
-  CHEAD(2)(81:81)=CHAR(10)
+  !  fill second 81-byte header
+  chead(2) = 'IX1FORM:'
+  write(chead(2)(9:38),'(3i10)') 162,nlen,nnum
+  chead(2)(41:80) = cgb(ncgb1:ncgb)
+  chead(2)(81:81) = char(10)
 
-  !  WRITE HEADERS AT BEGINNING OF INDEX FILE
-  CALL BAWRITE(LUGI,0,162,KW,CHEAD)
+  !  write headers at beginning of index file
+  call bawrite(lugi,0,162,kw,chead)
 
-  RETURN
-END SUBROUTINE WRGI1H
+  return
+end subroutine wrgi1h
 
 !> Locate basename of a file.
 !>
@@ -169,14 +171,16 @@ END SUBROUTINE WRGI1H
 !> @return The index of the basename within the string.
 !>
 !> @author Iredell @date 93-11-22
-FUNCTION NCBASE(C,N)
-  CHARACTER C*(*)
+integer function ncbase(c,n)
+  implicit none
+  character c*(*)
+  integer :: n
+  integer :: k
 
-  K=N
-  DO WHILE(K.GE.1.AND.C(K:K).NE.'/')
-     K=K-1
-  ENDDO
-  NCBASE=K+1
+  k = n
+  do while (k .ge. 1 .and. c(k:k) .ne. '/')
+     k = k - 1
+  enddo
+  ncbase = k + 1
 
-  RETURN
-END FUNCTION NCBASE
+end function ncbase