#======================================================================

#                    F I N D _ S E A B E D . P L 

#                    doc: Sun May 23 20:26:11 2010

#                    (c) 2010 A.M. Thurnherr

#======================================================================

# HISTORY:

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

#	Dec 25, 2010: - adapted to changes in [LADCP_w]

#	Oct 11, 2011: - moved defaults to [defaults.pl]

#				  - increased z_offset from 10km to 15km

#	Oct 19, 2011: - added $SS_max_allowed_range

#				  - renamed $SS_min_allowed_hab to *_range

#	Jan 26, 2016: - added %PARAMs

#	May 18, 2016: - removed assumption of 1500m/s soundspeed setting

#	May 19, 2016: - updated to ADCP_tools V1.6

# 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) To be reasonably accurate, DEPTH must be from the CTD at this stage.

#======================================================================

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

#======================================================================

my($z_offset) = 15000;		# shift z to ensure +ve array indices

sub find_seabed($$$)

{

	my($d,$be,$beamCoords) = @_;

	my($i,$dd,$sd,$nd);

	my(@guesses);

				   'SS_search_window_halfwidth',$SS_search_window_halfwidth,

	return undef unless ($be-$SS_search_window_halfwidth >= 0 &&

						 $be+$SS_search_window_halfwidth <= $#{$d->{ENSEMBLE}});

	for ($i=$be-$SS_search_window_halfwidth; $i<=$be+$SS_search_window_halfwidth; $i++) {

		next unless (defined($d->{ENSEMBLE}[$i]->{CTD_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) = $beamCoords ? velBeamToEarth($d,$i,@{$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} *= $CTD{SVEL}[$d->{ENSEMBLE}[$i]->{CTD_SCAN}]/$d->{ENSEMBLE}[$i]->{SPEED_OF_SOUND};

		next unless ($d->{ENSEMBLE}[$i]->{DEPTH_BT} >= $SS_min_allowed_range &&

					 $d->{ENSEMBLE}[$i]->{DEPTH_BT} <= $SS_max_allowed_range);

		$d->{ENSEMBLE}[$i]->{DEPTH_BT} *= -1

			if ($d->{ENSEMBLE}[$i]->{XDUCER_FACING_UP});

		$d->{ENSEMBLE}[$i]->{DEPTH_BT} += $d->{ENSEMBLE}[$i]->{CTD_DEPTH};

		if ($d->{ENSEMBLE}[$i]->{DEPTH_BT} > $d->{ENSEMBLE}[$be]->{CTD_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-$SS_search_window_halfwidth; $i<=$be+$SS_search_window_halfwidth; $i++) {

		next unless defined($d->{ENSEMBLE}[$i]->{DEPTH_BT});

		if (abs($d->{ENSEMBLE}[$i]->{DEPTH_BT}-$mode) <= $SS_max_allowed_depth_range) {

			$dd += $d->{ENSEMBLE}[$i]->{DEPTH_BT};

			$nd++;

		} else {

			undef($d->{ENSEMBLE}[$i]->{DEPTH_BT});

		}

	}

	return undef unless ($nd >= 2);

	$dd /= $nd;

	for ($i=$be-$SS_search_window_halfwidth; $i<=$be+$SS_search_window_halfwidth; $i++) {

		next unless defined($d->{ENSEMBLE}[$i]->{DEPTH_BT});

		$sd += ($d->{ENSEMBLE}[$i]->{DEPTH_BT}-$dd)**2;

	}

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

}