# nmea.perl
# Convert generic NMEA output into something more spreadsheet-friendly.
# Understands some Garmin-specific sentences.
#
# Note: we depend on the $GPRMC sentence appearing once in every
# "data burst" from the GPS, preferably as the first sentence.
# If this is not true for a particular GPS, the code that calls
# output() must be moved to a more appropriate place.
#
# Dave Martindale, May 2000
#

$maxtrack = 12;

$infile = "<-";
$outfile = ">-";
open (INFILE, $infile) or die "couldn't open input file $infile\n";
open (OUTFILE, $outfile) or die "couldn't open output file $outfile\n";

print OUTFILE "GPRMC,,,,,,,,";
print OUTFILE "GPGGA,,,,,,,,,,";
print OUTFILE "GPGSA,,,,,,,,,,,,,,,,,";
print OUTFILE "GPGSV,", "," x (4 * $maxtrack);
print OUTFILE "GPVTG,,,,";
print OUTFILE "GPGLL,,,,";
print OUTFILE "PGRME,,,";
print OUTFILE "PGRMZ,,";
print OUTFILE "PGRMM";
print OUTFILE "\n";

print OUTFILE "Time,Valid,Lat,Long,SOG,CMG,Date,MVar,"; # GPRMC
print OUTFILE "Time,Lat,Long,Qual,Sats,HDOP,Alt,GeoH,DGPSAge,DGPSID,"; # GPGGA
print OUTFILE "Auto,Dim,PRN,,,,,,,,,,,,PDOP,HDOP,VDOP,"; # GPGSA
print OUTFILE "View,"; # GPGSV
for ($sat = 1; $sat <= $maxtrack; $sat++) {
print OUTFILE "PRN ", $sat, ",Elev,Azim,S/N,";
}
print OUTFILE "CRS (T),CRS (M),SOG (Kt),SOG (km/h),"; # GPVTG
print OUTFILE "Lat,Long,Time,Valid,"; # GPGLL
print OUTFILE "HPE,VPE,SPE,"; # PGRME
print OUTFILE "Alt,Dim,"; # PGRMZ
print OUTFILE "Datum"; # PGRMM
print OUTFILE "\n";

sub output {
print OUTFILE $time_rmc, ',', $ok_rmc, ',', $lat_rmc, ',',
$long_rmc, ',', $speed, ',', $cmg, ',', $date, ',',
$mvar, ',';
print OUTFILE $time_gga, ',', $lat_gga, ',', $long_gga, ',',
$fixqual, ',', $nsat, ',', $hdop_gga, ',', $alt_gga, ',',
$gheight, ',', $DGPS_age, ',', $DGPS_ID, ',';
print OUTFILE $fixsel, ',', $fixdeg, ',',
join(',', @satsused), ',', $pdop, ',', $hdop, ',',
$vdop, ',';
print OUTFILE $nsiv, ',';
for ($sat = 0; $sat < $maxtrack; $sat++) {
if ($sat < $nsiv) {
print OUTFILE $prn[$sat], ',', $elev[$sat], ',',
$azim[$sat], ',', $signal[$sat], ',';
} else {
print OUTFILE ",,,,";
}
}
print OUTFILE $crs_t, ',', $crs_m, ',', $sog_kt, ',', $sog_km, ',';
print OUTFILE $lat_gll, ',', $long_gll, ',', $time_gll, ',',
$ok_gll, ',';
print OUTFILE $hpe, ',', $vpe, ',', $epe, ',';
print OUTFILE $alt_rmz, ',', $alt_type, ',';
print OUTFILE $datum;
print OUTFILE "\n";
}

sub latitude {
my ($deg, $min) = unpack "a2a*", $_[0];
my $lat = $deg + $min / 60;
$lat = - $lat if $_[1] =~ /[Ss]/;
return $lat;
}

sub longitude {
my ($deg, $min) = unpack "a3a*", $_[0];
my $long = $deg + $min / 60;
$long = - $long if $_[1] =~ /[Ww]/;
return $long;
}

$first = 1;
while ($line = <INFILE>) {
chomp($line);
@field = split /[,*]/, $line;
SWITCH: {

# recommended minimum specific GPS/Transit data
if ($field[0] eq '$GPRMC') {

# We don't know exactly what sentences to expect
# from an arbitrary GPS, but we assume that RMC
# will always be included. Each time we see this
# sentence, print out the accumulated information
# from the previous burst.
output() unless ($first) ;
$first = 0;

# Now process the new RMC record
$time_rmc = join ':', unpack "a2" x 3, $field[1];
$ok_rmc = $field[2];
$lat_rmc = latitude(@field[3..4]);
$long_rmc = longitude(@field[5..6]);
$speed = $field[7];
$cmg = $field[8];
$date = join '/', unpack "a2" x 3, $field[9];
$mvar = $field[10] . $field[11];
# field[12] is checksum
last SWITCH;
}

# GPS fix data
if ($field[0] eq '$GPGGA') {
$time_gga = join ':', unpack "a2" x 3, $field[1];
$lat_gga = latitude(@field[2..3]);
$long_gga = longitude(@field[4..5]);
$fixqual = $field[6];
$nsat = $field[7];
$hdop_gga = $field[8];
$alt_gga = $field[9];
# $field[10] is altitude units (always M)
$gheight = $field[11];
# $field[12] is geoid height units (always M)
$DGPS_age = $field[13];
$DGPS_ID = $field[14];
# field[15] is checksum;
last SWITCH;
}

# GPS DOP and active satellites
if ($field[0] eq '$GPGSA') {
$fixsel = $field[1]; # A for auto selection, M for manual
$fixdeg = $field[2]; # 2 or 3 for 2D or 3D fix
@satsused = @field[3..14];
$pdop = $field[15];
$hdop = $field[16];
$vdop = $field[17];
# field[18] is checksum;
last SWITCH;
}

# satellites in view
if ($field[0] eq '$GPGSV') {
# $field[1] is total number of sentences for full data
# $field[2] is current sentence number
$nsiv = $field[3];
#
# Unpack next 16 fields as PRN, elevation, azimuth, and
# signal quality for next 4 satellites
#
$sat = ($field[2] - 1) * 4;
$lim = $sat + 4;
if ($nsiv < $lim) {
$lim = $nsiv;
}
$f = 4;
while ($sat < $lim) {
$prn[$sat] = $field[$f++];
$elev[$sat] = $field[$f++];
$azim[$sat] = $field[$f++];
$sig = $field[$f++];
$sig = '' if $sig == 0;
$signal[$sat] = $sig;
$sat++;
}
# field[20] is checksum;
last SWITCH;
}

# track/ground speed
if ($field[0] eq '$GPVTG') {
$crs_t = $field[1];
# $field[2] is reference (always T);
$crs_m = $field[3];
# $field[4] is reference (always M);
$sog_kt = $field[5];
# $field[6] is units (always N);
$sog_km = $field[7];
# $field[8] is units (always K);
# $field[9] is checksum;
last SWITCH;
}

# position error info
if ($field[0] eq '$PGRME') {
$hpe = $field[1];
# $field[2] is units (always M)
$vpe = $field[3];
# $field[4] is units (always M)
$epe = $field[5];
# $field[6] is units (always M)
# field[7] is checksum;
last SWITCH;
}

# latitude/longitude
if ($field[0] eq '$GPGLL') {
$lat_gll = latitude(@field[1..2]);
$long_gll = longitude(@field[3..4]);
$time_gll = join ':', unpack "a2" x 3, $field[5];
$ok_gll = $field[6];
# field[7] is checksum;
last SWITCH;
}

# altitude
if ($field[0] eq '$PGRMZ') {
$alt_rmz = $field[1];
# $field[2] is units (always F)
$alt_type = $field[3]; # 2: user alt, 3: 3D fix
# field[4] is checksum;
last SWITCH;
}

# datum
if ($field[0] eq '$PGRMM') {
$datum = $field[1];
# field[2] is checksum;
last SWITCH;
}
}
}