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Printed from: www.oicinc.com on
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http://www.oicinc.com/drinkingfromthefirehose.html |
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Sea Technology
Article: Drinking from the Fire-Hose |
By: Tom Reed, president of Oceanic Imaging
Consultants, Inc.,
INTRODUCTION
The acquisition of seafloor mapping data has matured remarkably
in the past decade. This article reviews some of these advances,
with particular attention to how some new technologies have effectively
dealt with "data-overload" made possible by sensors with
greater resolution and bandwidth.
Seafloor mapping - while admittedly potentially a complex, detail-intensive
task involving numerous electronic sensors, telemetry systems and
logging/display devices - can be, and often is best thought of,
as making pictures of the bottom of the sea. Pictures, as we all
know, contain a lot of information. A typical 35mm color slide,
at 9000 scan lines per inch and approximately a 4x3 aspect ratio,
contains over 2.5 Giga-bits of information. Clearly, mapping the
seafloor can be highly data-intensive, resulting in data overload
on both operators and logging devices. "The Matrix" notwithstanding,
people do not deal gracefully with scrolling alpha-numeric byte-streams.
Scrolling pictorial representations (waterfalls, graphic recorders,
etc.) are the usual solution, but often with compression in both
physical resolution, and data dynamic range. With recording mimicking
display, loss of resolution, or reduction in survey coverage and
pace, was considered inevitable. These mandatory losses are now
a thing of the past. Sustainable, 24-7-365 reliable logging and
real-time processing of seabed mapping data at rates in excess of
10 mega-bytes per second is available in commercial, off the shelf
PC-based packages today.
HISTORICAL PERSPECTIVE
At a first glance, seabed mapping systems can be categorized into
two families: acoustic systems, such as echo-sounders, sidescans,
multi-beams and swath-interferrometric devices, and non-acoustic
devices, such as traditional cameras and videos, electronic still
cameras (ESC's) and laser linescan systems (LLS's). We recognize
that profiling systems such as single and multi-channel seismics,
magnetometers and gravimeters are equally important mapping tools,
but for the moment restrict our analysis to systems providing imaging
in the traditional planimetric format.
A typical two-channel analog sidescan system actually provides
a large amount of information. Fortunately, roll-paper records are
quite efficient at gathering this, albeit somewhat difficult to
carry around in significant quantities. When taken to the digital
domain, a quantitative notion of the data available to us becomes
apparent. Assume a reasonable digitization rate of 24 KHz, yielding
a constant slant-range resolution of 3 centimeters. Assume 16-bits
sample resolution to encompass the likely range of raw backscatter
variations in amplitude. This implies a data rate from a simple
two-channel sidescan of approximately 100 Kbytes per second, or
360 megabytes per hour. This will fill a Zip-disk in 15 minutes,
and a 1 GB Optical platter in under 3 hours. To people planning
a 30 day cruise, this begins to look expensive. In steps downsampling.
Perception is everything, they say. A high-quality roll-paper
recorder can display 4096 samples across a scan, and you don't need
to hire a programmer to show your boss the data. For the typical
sidescan mentioned above, this would allow you to show full resolution
data for any swath of 50 meters range or less. At any range greater
than that, you would have to throw away some of the data. At 100
meter range, you would have to throw away half the data, for example.
Furthermore, the human eye can only distinguish between 16 and 32
shades of grey. This translates to retaining at best one third of
the original dynamic range. If you take this same data to an XGA
computer monitor, you are limited to only 1024 samples across a
line, and a depth resolution of 8-bits. Let's assume you may have
2 screens, so now, in theory, you would need to log only 1024 samples
per side, 8-bits per sample to make maximum use of your digital
display system. If you operate at 100 meters range or greater, and
only log what you display, you have irretrievably discarded over
75% of your data. No amount of post-acquisition signal processing
magic can bring this lost data back. Furthermore, any operator-induced
changes to gains, contrast stretches, etc. are likely to be permanent.
Logging raw data at full resolution and full dynamic range obviates
these problems. It just requires media space, and through-put bandwidth.
NEW DEVELOPMENTS
In 1997, the United States Naval Oceanographic Office (NAVOCEANO)
embarked upon a fleet modernization program for its T-AGS 51, T-AGS
60 and T-AGS 45 class oceanographic survey vessels and accompanying
Hydrographic Survey Launches (HSL's). Mission requirements included
high-resolution sea floor mapping and object location/identification
in littoral areas of the world, typically under adverse environmental
conditions. Specific requirements included simultaneous acquisition,
display and logging of dual-frequency sidescan sonar data, single
and multibeam bathymetry, and beam amplitude and backscatter. Per
the specification, the maximum anticipated data rate would be up
to 1 Gigabyte per hour, which the sonar data acquisition workstation
would be required to log to both tape and hard-drive. Anecdotally,
this prompted the requirement for at best a 9 GB hard drive, as
it was mentioned that "...no government worker was ever required
to work more than an 8 hour day...".
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Figure 1. NAVOCEANO Hydrographic survey launch (34' LOA).
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Oceanic Imaging Consultants (OIC) was selected to provide
GeoDAS as the Sonar Data Acquisition and Processing workstation for
the T-AGS ships and HSL's, interfacing to the Datasonics SIS1502 Dual-frequency
sidescan, as well as the Simrad EM3000 for multibeam bathymetry and
backscatter.
The rack-mount GeoDAS workstation, as shown below, was delivered with
dual 1280x1024 monitors, dual 18-GB hard-drives, and dual 20 GB Exabyte
Mammoth tape drives. A quad-port Ethernet card allows simultaneous
receipt of both the broadcast Simrad multibeam bathymetry and TSS-PosMV
navigation and motion sensor data, as well as re-broadcast of processed
OIC records of merged sidescan and bathymetry to client workstations,
either on the launch, or at a remote location, via Ethernet Radio
Link. The latter technique allows observers back on the host T-AGS
ship or land to view raw or processed data from the HSLs as the data
are being acquired, offering both immediacy of analysis, and easy
remote trouble-shooting of any data quality issues which might arise. |
Figure 2. GeoDAS equipment rack aboard NAVOCEANO ship, with
dual 1280x1024 monitors, rack-mount keyboard, CPU, Sonar MUX and printer,
plus integrated winch controls, USBL tracking unit & deck cameras.
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The sidescan on the HSL's is a Datasonics SIS1501/2
dual-frequency system. The 1501 model indicates CW technology, while
the 1502 indicates that the sonar can operate in both CW and Chirp
mode. GeoDAS interfaces to and controls the SIS1501/2 through the
through the Datasonics multiplexor and proprietary.dll, which allows
full control of sonar range, wet-gains, pulse type, length and power,
as also provide both sensor telemetry and a continuous 192 Kbytes
per second data feed. GeoDAS logs all this data raw, without any filtering
or downsampling, and then applies user-specified processing to allow
realtime data display in a variety of views (profile, waterfall, mosaic).
The multibeam on the HSL's is the Simrad EM3000s, a 300 KHz sounder
which provides 127 beams over a swath of upto 200 meters or four times
water-depth, in waters from 0.5 to 150 meters deep. The system outputs
both raw and corrected bathymetrye datagrams over a 10 Mb/sec UDP
link. It can also broadcast both beam amplitude (average backscatter
per beam) and the raw time-series backscatter data at full resolution
(3 cm). GeoDAS catches all these datagrams and logs them in parallel
with the SIS1501 sidescan data, and offers the user options for both
processing and display selection. By default, the console will present
a waterfall showing platform and sidescan attitude, course, etc.,
along with synchronized color-contoured bathymetry, and either low
or high-frequency sidescan. The user may modify views and processing
on the fly without interrupting data logging, to optimize onboard
analysis such as target detection and survey status. |
Figure 3. GeoDAS user interface, showing color-contour bathymetry
in parallel with sidescan (100 or 500 KHz). |
The combined data streams from the sidescan and multibeam
comes to just under 1 GB per hour, which GeoDAS logs continually to
dual Exabyte 8900 tape drives, and optionally prints to a rack-mounted
roll-paper recorder. The operators may also elect to create realtime
mosaics of both the sidescan and bathymetry data, in a variety of
projections, at user-definable scale and orientation. The geo-coded
mosaics can be export to an on-board GIS package, and merged with
existing data for quick-look mission products. |
Figure 4. "Quick-look" mission product created from
HSL bathymetry and sidescan, mosaicked at 1 centimeter resolution
and merged with available air-photo data for the surrounding region.
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DOUBLE OR NOTHING...
In the fall of 2000, OIC began work on an interface to the Raytheon
LS-4096 Laser Linescan system. The LS-4096 provides continuous 14-bit
imagery from a scanning laser mounted on an underwater vehicle.
Resolution is configurable from 512 to 4096 samples per scan line,
at scan rates up to 4000 RPM. At maximum resolution, the LS-4096
delivers 2.2 Mbytes per second, at approximately 267 scan lines
per second. This data rate doubled that which had been required
of GeoDAS in the past, and exceeds conventional sonar imaging requirements
by at least a factor of 100, if not more. The original top-side
processor allowed saving of user-selected "snapshots",
but any continuous data logging could only be accommodated on video
tape, which could not take advantage of the full imaging resolution
potential of this system. Any geo-coding of data required manual
"cut-and-paste"operations post acquisition.
GeoDAS-LLS provides a complete control, processing and display
interface to the laser linescan system, treating the data stream
as a single-channel sidescan (albeit, a VERY FAST sidescan). This
includes full resolution raw data logging, as well as realtime and
post-acquisition processing, targeting and geo-coding. The laser
data can even be merged with co-registered sidescan data, to provide
an ultra-high resolution "gap-filler" directly beneath
the track of the sonar. Co-registration of sidescan and laser imagery
provides significant improvements for search and recovery, bottom
characterization and mine-hunting operations, with the laser data
offering the potential of on-the-fly target identification, augmenting
the sonar's extended abilities for target detection. |
Figure 5. GeoDAS-LLS, showing data from the Raytheon LS-4096.
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NOT JUST ANOTHER PRETTY PICTURE...
This spring, OIC completed it's modifications to GeoDAS to support
Electronic Still Camera imaging. The completed interface retains the
"look-and-feel" of the orginal GeoDAS user interface, while
allowing continuous acquistion, processing and display of single or
dual camera imagery, at data rates up to 10 MB per second. The interface
supports multiple modes of display, including scrolling, binocular
and single image view, to accommodate images up to 1280 by 1280 pixels
on a side, at 16-bits dynamic range. A dual camera configuration at
one Hertz frame rate in continuous operation mode will fill a 60 GB
8-mm tape in just over 2 hours. Dual tape-drives allow automatic roll-over,
for uninterrupted logging (provided you brought A LOT of tapes...).
Retaining the basic GeoDAS format while accommodating data records
easily 1000 times more than your average sidescan record proved challenging,
but the result provides the same processing, targeting, logging and
QA/QC interface as available for scan-line based systems, minimizing
operator re-training, and maximizing product reliability. |
Figure 6. GeoDAS-ESC, in the dual-camera configuration.
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DATA INTEGRATION
While the above examples demonstrate available solutions for the
mechanics of multi-sensor, high-rate data acquisition, there remains
the issue of operator overload, and interpretation. Simply put,
someone has to look at all this. If we can now simultaneously acquire
swath acoustic data, providing a half-kilometer swath of acoustic
imagery and bathymetry, and optical data, providing sub-centimeter
scale imagery over a patch which might barely cover the nadir footprint
of the sonar, how are we to co-register and compare the two modalities?
ROVer's Eye, a real-time terrain visualization package developed
by OIC under DARPA funding, provides one option. ROVer integrates
a hi-speed 3-D rendering package with GeoDAS's realtime-processing
and target analysis package, to provide an interactive immersive
experience, wherein underwater vehicle operators can work not only
with existing models, but see new data from on-board sensors evolve
into the current model in realtime. ROVer accommodates simultaneous
inputs from sidescan, bathymetry and navigation systems, while accessing
a database of existing data, targets and as-built structure models.
Operators see new data evolve in a model before them, just as headlights
reveal the road ahead to night-time drivers. "Road-signs"
in ROVer reveal not gratuitous advertising (nor Burma Shave ditties)
but full resolution images of proximal targets, which "pop-up"
as the vehicle passes by. The combination of synoptic swath data
with detail-rich target imagery in a fully geo-coded environment
provides a whole new level of data interpretation experience. |
Figure 7. ROVer's Eye view of the bottom of Honolulu Harbor, with
TargetView mode on, providing automatic notification of proximity
of previously marked targets. |
SUMMARY
Seafloor imaging is a data-intensive process. Down-sampling strategies
compromise both image resolution and quantitative information potential.
GeoDAS, an off-the-shelf solution for acoustic and non-acoustic seabed
data acquisition and processing, can provide a uniform interface to
sonar, laser and camera-based imaging systems, handling raw data rates
up to 10 MB per second with no down-sampling losses, while retaining
realtime interactivity. Examples are provided from working installations. |
Dr. Thomas B. Reed IV is founder and president of Oceanic
Imaging Consultants, Inc. of Honolulu, HI. He received his undergraduate
education from Harvard University in 1982, where he majored in Economic
Geology, and completed his graduate work in Marine Geology and Geophysics
at Hawaii Institute of Geophysics, in 1987. |
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