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###  SeaBeam 2000 Sidescan Data Processing  ###
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Description by S.M.Smith, SIO/GDC, January 1995

The SeaBeam 2000 multibeam echosounder installed on R/V Melville
collects acoustic intensity (sidescan) data in addition to the 
120 beams of bathymetric data.

The logged sidescan data contains both acoustic intensity vs. angle
which has 16bit resolution and a subset of 4bit (aka grayscale) data,
1000 pixels per ping, which has been geometrically corrected for
realtime output to the EPC graphic recorder. As of August 1994, SIO/STS
has developed programs for extracting the grayscale data, merging them
with navigation, applying a cross-track median filter and generating
navigated mosaics in Mercator projection.

NEAR REALTIME DISPLAYS (on mlvl61)
----------------------------------
Sidescan data are displayed on one half of a strip record made on
an EPC Graphic recorder (a profile of the SB2000 vertical beam is
recorded on the other half). Paper speed of the recorder is adjusted
to ship speed to make the record true scale in both along and across
track directions. Scale (meters/pixel) and min/max dB values to be
covered by the 4 bit (0-15) range are entered via the SeaBeam Control
window on the u/w watch terminal. Note that the scale and min/max dB
values also control the logged 4bit data, so it is necessary to monitor
the width and intensity of the image which will change with depth and
reflectivity of the seafloor.

The 4bit sidescan data are also displayed by program 'xchart' in a
scrolling window on the u/w watch terminal.

LOGGED DATA FILE FORMATS, NAMES AND TAPE STRUCTURES:
----------------------------------------------------
Logged sidescan data totals about 250 Mbytes per day, broken up into
hour-long files of about 8 Mbytes each, and given names tagged with
date (yr,mon,dy)and end time (hr,min) of the file, e.g:
	sb2000_ss.940327-0100

Each day at sea, these daily files are dumped to tape by the Shipboard
Computer Group (SCG) tech. One copy is made on an Exabyte (8mm-5.6Gb mode)
tape and a duplicate on a DAT (4mm) tape using the Unix 'tar' tape archive
utility.  Multiple days of data are put on each tape, with each day being
a single tar archive file, separated by EOF marks. (Therefore, to access
data after the first day on tape, you must use the 'mt' command to skip
to the proper file before issuing the tar extraction command.)

The logged tapes are returned to the SIO Geological Data Center (GDC)
for indexing and archiving. As a safety precaution, the DAT tapes are
normally kept onboard Melville until after the next leg, unless secure
alternative transportation is available back to Scripps.  At GDC, the
log tapes are assigned names in the form of:
	GLOR07SS03d     [cruiseid,SS,tape number,d(DAT) or e(Exabyte)]

Tar (tv) tape lists are made of the Exabyte copy and these lists are
stored online for quick reference.

Format documentation: the logged sidescan data format consists of a
short header containing flags and byte count added by SIO, followed by the
data structure provided by SBI.  Documentation for these are in files:
	sb2000_ss.5     [to list: troff -t sb2000_ss.5 | lpr ]
	ss_format.h     [to list: cat ss_format.h ]

[Note to Programmers: due to occasional glitches in transferring data
between the SeaBeam and SIO computers, data from two pings get combined
into one record with the second ping often being incomplete.
Therefore, the number of bytes contained in the first field of the SIO
header should be used to determine the start of the next good data
record, not the total length of the SS record contained in the 2nd
field of the SBI data record structure.]

Grayscale (4bit) Data File Formats, Names and Programs:
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SSraw File:
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A single file containing grayscale sidescan data for each day is
extracted from the hour-long logged data files.  In a naming convention
parallelling that for the SB2000 bathymetry data, these daily files
are named:
	SSraw.<date>   where date is in the form of yr_mon_dy (e.g. 94may03)

The date assigned is the date the file ends. If more than one file ends
on a date, subsequent files are named may03.1, may03.2, etc.  (Prior to 1994,
the cut off of daily logging occurred just after GMT midnight, resulting
in files containing data from the date prior to that given in the file name.)

The program which extracts the SSraw file from the logged data is:
	ss2k_convert

DOtar.conv.ssraw is a script which will tar in one day's tar archive file
of logged data from tape and extract grayscale into a single SSraw file.

RUNdotar.conv.ssraw is an example of a script that will process multiple
days from a tar tape.

As of mid-1994, the SSraw file extraction is being incorporated into the
automatic daily processing script executed by the SCG techs at sea.

SSraw file format: The SSraw files are in the SIO Swath format, a data
structure that accomodates files containing multibeam data from a number
of different sources (SB2000 bathymetry or sidescan; 16-beam SeaBeam;
Hydrosweep; and ECHOS (Russian) systems).  This data structure is 
described in:
	swath_bathy.5	[to list: troff -t swath_bathy.5 | lpr ]


SSmrg and SSmed Files:
---------------------
The SSraw file is merged with navigation to create a file named
	SSmrg.<date>
using program:
	swath_merge

The SSmrg files is then passed through a 3-point, across-ping median
filter to create a file named:
	Ssmed.<date>
using program:
	ss2k_median

RUNss_merge.median is a script which executes the above two programs
and also runs script RUNswath_check.SSmed that checks the SSmed file
for begin/end times, time gaps, lat/long bounds, etc.

The SSmed file is currently regarded as the final product to be
archived by GDC.

SSmrg and SSmed file formats: Identical to SSraw.

#end