#======================================================================
#                    M E R G E C T D + L A D C P . S E A B E D 
#                    doc: Sun May 23 20:26:11 2010
#                    dlm: Mon May 31 13:15:07 2010
#                    (c) 2010 A.M. Thurnherr
#                    uE-Info: 56 0 NIL 0 0 72 0 2 4 NIL ofnI
#======================================================================

# HISTORY:
#	May 23, 2010: - adapted from [perl-tools/RDI_Utils.pl]

# NOTES:
#	1) BT range is corrected for sound speed at the transducer. This is not
#	   accurate, but unlikely to be very wrong, at least for deep casts, 
#	   because the vertical sound speed variability near the seabed tends
#	   to be small. The seabed depth is only used for sidelobe editing,
#	   which is done with a generous safety margin (from the UH shear
#	   method implementation).
#	2) Acquisition sound speed of 1500m/s assumed.
#	3) To be reasonably accurate, DEPTH must be from the CTD at this stage.

#======================================================================
# (seabed depth, stddev) = find_seabed(dta ptr, btm ensno, coord flag)
#======================================================================

my($search_width) = 200;	# # of ensembles around bottom to search
my($mode_width) = 10;		# max range of bottom around mode
my($min_dist) = 20;			# min dist to seabed for good data
my($z_offset) = 10000;		# shift z to ensure +ve array indices

sub find_seabed($$$)
{
	my($d,$be,$beamCoords) = @_;
	my($i,$dd,$sd,$nd);
	my(@guesses);

	return undef unless ($be-$search_width >= 0 &&
						 $be+$search_width <= $#{$d->{ENSEMBLE}});
	for ($i=$be-$search_width; $i<=$be+$search_width; $i++) {
		next unless (defined($d->{ENSEMBLE}[$i]->{DEPTH}) &&
					 defined($d->{ENSEMBLE}[$i]->{BT_RANGE}[0]) &&
					 defined($d->{ENSEMBLE}[$i]->{BT_RANGE}[1]) &&
					 defined($d->{ENSEMBLE}[$i]->{BT_RANGE}[2]) &&
					 defined($d->{ENSEMBLE}[$i]->{BT_RANGE}[3]));
		my(@BT) = $beamCoords
				? velInstrumentToEarth($d,$i,
					velBeamToInstrument($d,
						@{$d->{ENSEMBLE}[$i]->{BT_VELOCITY}}))
				: velApplyHdgBias($d,$i,@{$d->{ENSEMBLE}[$i]->{BT_VELOCITY}});
		next unless (abs($BT[3]) < 0.05);
		$d->{ENSEMBLE}[$i]->{DEPTH_BT} =
			 $d->{ENSEMBLE}[$i]->{BT_RANGE}[0]/4 +
			 $d->{ENSEMBLE}[$i]->{BT_RANGE}[1]/4 +
 			 $d->{ENSEMBLE}[$i]->{BT_RANGE}[2]/4 +
			 $d->{ENSEMBLE}[$i]->{BT_RANGE}[3]/4;
		$d->{ENSEMBLE}[$i]->{DEPTH_BT} *= cos(rad($d->{BEAM_ANGLE}));
		$d->{ENSEMBLE}[$i]->{DEPTH_BT} *= $d->{ENSEMBLE}[$i]->{SOUND_SPEED}/1500;
		next unless ($d->{ENSEMBLE}[$i]->{DEPTH_BT} >= $min_dist);
		$d->{ENSEMBLE}[$i]->{DEPTH_BT} *= -1
			if ($d->{ENSEMBLE}[$i]->{XDUCER_FACING_UP});
		$d->{ENSEMBLE}[$i]->{DEPTH_BT} += $d->{ENSEMBLE}[$i]->{DEPTH};
		if ($d->{ENSEMBLE}[$i]->{DEPTH_BT} > $d->{ENSEMBLE}[$be]->{DEPTH}) {
			$guesses[int($d->{ENSEMBLE}[$i]->{DEPTH_BT})+$z_offset]++;
			$nd++;
		} else {
			undef($d->{ENSEMBLE}[$i]->{DEPTH_BT});
		}
	}
	return undef unless ($nd>5);

	my($mode,$nmax);
	for ($i=0; $i<=$#guesses; $i++) {			# find mode
		$nmax=$guesses[$i],$mode=$i-$z_offset
			if ($guesses[$i] > $nmax);
	}

	$nd = 0;
	for ($i=$be-$search_width; $i<=$be+$search_width; $i++) {
		next unless defined($d->{ENSEMBLE}[$i]->{DEPTH_BT});
		if (abs($d->{ENSEMBLE}[$i]->{DEPTH_BT}-$mode) <= $mode_width) {
			$dd += $d->{ENSEMBLE}[$i]->{DEPTH_BT};
			$nd++;
		} else {
			undef($d->{ENSEMBLE}[$i]->{DEPTH_BT});
		}
	}
	return undef unless ($nd >= 2);

	$dd /= $nd;
	for ($i=$be-$search_width; $i<=$be+$search_width; $i++) {
		next unless defined($d->{ENSEMBLE}[$i]->{DEPTH_BT});
		$sd += ($d->{ENSEMBLE}[$i]->{DEPTH_BT}-$dd)**2;
	}

	return ($dd, sqrt($sd/($nd-1)));
}