my %data;
open FILE, '<', 'gravity' or die;
while( <FILE> ) {
    my @fields = split ' ', $_;
    $data{ @fields[ 0, 1 ] } = \@fields;
}
close FILE;

open FILE, '<', 'magnetics' or die;
while( <FILE> ) {
    my @fields = split ' ', $_;

    ## Pad the hash if we didn't see this date/time in the gravity file
    $data{ "@fields[ 0, 1 ]" } //= [ @fields[ 0,1 ], ('n/a') x 3 ];

    push @{ $data{ "@fields[ 0, 1 ]" } }, @fields[ 2 .. $#fields ];
}
close FILE;

open FILE, '<', 'bathymetry' or die;
while( <FILE> ) {
    my @fields = split ' ', $_;

## Pad the hash if we've never seen it before)
## (??? == No of fields added by the magnetics)
    $data{ "@fields[ 0, 1 ]" } //= [ @fields[ 0,1 ], ('n/a') x ( 3 + ??? ) ];

##  We saw it in gravity,  but not magnetics.
    push @{ $data{ "@fields[ 0, 1 ]" } }, ('n/a') x ???
        if @{ $data{ "@fields[ 0, 1 ]" } } < 3 + ???;

    push @{ $data{ "@fields[ 0, 1 ]" } }, @fields[ 2 .. $#fields ];
}
close FILE;

for my $key ( sort keys %data ) {
    my $nFields = @{ $data{ $key } };

## Pad: ??? === total number of fields
    push @{ $data{ $key } }, ('n/a') x ( ??? - $nFields );

    print join "\t", @{ $data{ $key } };
}