$DEBUG
	PROGRAM PREPACS
c 06-mar-09 desiderio
c from DAPCS_PREPACS
c (older versions of prepacs did not calculate checksums,
c  and were adapted from code written on board the
c  thompson during a cruise.)
c   
c traps out bad data values which
c result in runtime execution errors:
c ref = 0 causes a divide by 0,
c sig = 0 causes log(0).
c
c command line format:
c    prepacs devfilename binaryfilename outfilename [switch]
c
c prepacs processes acs binary files to give a tab-delimited
c    acs ascii data file with time in msec in the first column
c    and with one row of tab-delimited column headings preceding 
c    the data. it is functionally identical to wetview in its 
c    calculation of acs parameters except that:
c    (1) prepacs processes all the acs packets, and does not
c        skip the first 2 packets as does wetview; 
c    (2) prepacs handles the acs timestamps (which have units
c        of msec) with no loss of significance; wetview handles 
c        time in units of 10s of msec, and seems to truncate the 
c        acs timestamps (and truncates the 3rd packet's timestamp 
c        to use as time = 0).
c    therefore, for a given line of data, the acs time calculated by 
c    prepacs will be about 500 msec later (2 packets' worth) than 
c    that calculated by wetview.
c
c    the optional switch is of the form:
c       +nul    : (or no switch); default;
c                 neither aux nor dark channels written out
c       +aux    : 4 auxiliary acs values written out 
c       +dark   : dark values written out
c       +all    : both auxiliary and dark values written out
c       +wet    : equivalent to +all, but in addition, the appropriate
c                 dev file information is prepended to the output data
c                 file so that the file can be replayed in wetview.
c
c    the 4 auxiliary channels are:
c        (1) internal temperature [degC];
c        (2) seems to always be 0. include for possible future use;
c        (3) depth [meters] (according to wetlabs manuals);
c        (4) external temperature [degC].
c

c dependences: all subroutines are included in this code unit,
c    and are also organized in the order encountered (as listed
c    below) after the main program. some of the subroutines may
c    differ from those of the same name in other programs.
c        prepacs [main]
c            prepacs_command_line_format
c            process_switch
c            read_acs_dev_file
c            find_first_packet
c                check_checksum
c                check_serial_number
c                calc_timestamp
c            write_outfile_header
c            find_packet
c                check_checksum
c                calc_timestamp
c                badpacket_diagnostics
c            parse_packet
c                get_ca_counts
c                calc_scalar_data
c                calc_int_temp_compensation
c            write_out_data
c
c    upper case is used for calls to the above subroutines.
c    calls in lower case are to procedures intrinsic to fortran.
c
c    much of variable transfer between routines is handled by named
c    common block constructs. 
c
c raw acs binary files consist (mostly) of unsigned 2-byte integers 
c    utilizing big-end ordering of the bytes, which is reversed from
c    the way pc computers store integers (little-end ordering). since
c    this version of fortran (fortran 90) does not support unsigned 
c    integers of any type, the fastest and simplest way to convert 
c    the binary data is to swap their bytes and write them into the 
c    least significant bytes of 4-byte integers using an EQUIVALENCE
c    construct:
c
c        integer*1  binaryvalue(2)  ! rawdatum to be converted
c        integer*1  dummy(4)        ! a 4-element array of 1-byte integers
c        integer*4  datum           ! converted value
c        equivalence (dummy,datum)  ! assigns dummy and datum to same memory location 
c        dummy = 0                  ! make sure all bits of dummy (and datum) are 0.
c        dummy(1) = binaryvalue(2)  !   swap bytes into least significant
c        dummy(2) = binaryvalue(1)  !   (first two) bytes of datum
c
c    datum now has the desired value, *without* coding an explicit assignment
c    statement of the type:   datum = ____ .