the synthetic aperture sonar revolution -...

DYNAMICS TECHNOLOGY, INC. DTI/rec-1-10/09/2000

Drs. Ralph ChathamEnson ChangMatt NelsonDavid Marx

Angela PutneyKieffer Warman

Ken ChickSteve Borchardt

21311 Hawthorne Blvd., Suite 300 Torrance, CA 90503-5610 l 1555 Wilson Blvd., Suite 320, Arlington, VA 22209-2405 - (703) 841-0990

The Synthetic Aperture Sonar Revolution

A zim u th (meters, arbitrary zero point)

R ang e (m )

365 370 375 380 385 390 395 400

350

355

360

365

370

SAS Image: 10 cm resolution

350 m

This presentation discusses recent results of applying synthetic aperturetechniques to sonar. We show a number synthetic aperture sonar (SAS) imagesprocessed by Dynamics Technology, Inc. (DTI) from data collected by avariety of different hardware suites. The scientists listed on this page have allcontributed to the work.


Ralph Chatham and Dynamics Technology, Inc.

♦ DTI is a 24 year old small business that does contract research on hardtechnical problems, primarily for U.S. government customers

—40 employees, ~ 2/3 with Ph.D. level education—Synthetic Aperture Sonar (SAS) is one of 4 major business areas

◊ Currently working on 9 government & industry SAS research & development contracts

♦ Ralph Chatham—1970-90: U.S. Navy submarine officer (diesel submarines when possible)—1984-88: DARPA program manager

◊ Submarine laser communication, extraction of dissolved O2 from seawater for UUVuse, submarine self defense

—1989-90: Director, Survivability Navy Theater Nuclear Warfare Office—1990- 94: Chief Scientist & Corporate Technical Director, Global Associates—1994- Vice president, Dynamics Technology, Inc.

◊ SAS business area leader.

—Along the way Chatham has also been:◊ One of 120 Astronaut candidate finalists (but he was told in 1980 you are too susceptible to

motion sickness; you may go back to sea. )

◊ Chairman, 1999-2000 Defense Science Board Task Force on Training and Education

◊ Tom Clancy s novel The Hunt for Red October was dedicated to him

◊ An occasional public teller of folktales and sea stories...

DTI is an American small business specializing in applying physics to signalprocessing and measurement problems of government and industry. One of ourfour business areas is the application of synthetic aperture techniques toacoustic data. We have been at this since 1989 and have a number of currentcontracts for SAS-related research and development. We have five full-time-equivalent Ph.D. level scientists and engineers working in the area.

For the most part we work with major hardware manufacturer partners andprocess data collected by systems they build. We help specify systemparameters, monitor the development, participate in the data collection testsand then SAS process collected data. We are also directing the building of asmall SAS system as prime contractor under a U.S. government contract.

Ralph Chatham is the business area leader in DTI for SAS. The chart listssome, possibly relevant, notes on his background.


Synthetic Aperture Sonar

♦ Coherent addition of multiple pings gives:High resolution independent of range & frequency

—Applicable on scales from meters to tens of kilometers—Long range SAS is new

◊ 1989: DTI recognized that advances in SAR might correct ocean & motion induced phase errors

◊ 1992: Acoustic medium stability experiment

◊ 1995: ~200 m range with 5 cm resolution at 50 kHz (DARPA)

◊ 1997: 1 km range with ~10 cm resolution at 50 kHz (DARPA)

980

1001

Sla

nt

Ran

ge

(m)

1 kmRange

10 m resolution single ping sonar10 cm resolution 50 kHz SAS system

DARPA/Raytheon 3.2 m 50 kHz array

Synthetic aperture processing gives high resolution independent of range bycreating a long virtual aperture, generated by the coherent addition of multiplepings. The radar world developed synthetic aperture radar (SAR) almost 40years ago but synthetic aperture sonar (SAS) did not work then because theslow speed of sound allowed uncompensated motion to generate much largerphase errors between pings.

Moreover, the ocean is a much less transparent medium that also generatesphase errors. Exploiting the full coherence that exists in underwaterpropagation had to wait until new techniques for data-driven focusingalgorithms were developed in SAR world.

SAS at long ranges (greater than 20 or so meters) has only been done since1995, but there are now a number of existence proofs at many scales that it ispossible to exploit the coherence in the ocean, and that there is more coherencethan had previously been believed.

The figure shows fixed aperture (side looking sonar, SLS) vs. SAS images at 1km. DARPA’s Raytheon-built 50 KHz sonar illuminated a sunken PB4-Y2airplane in Lake Washington. Coherent processing by DTI focused the SASimage on the left. The physical aperture was 3.2 meters and was towed at 2.3knots. The ~11cm resolution achieved was limited by the number of datachannels available in the tow-body (16) and by the low system SNR at thisrange. Had 16 more channels been available, the resolution would have been ~5.5cm. A longer physical receive array could have allowed proportionallyfaster tow speeds. The synthetic aperture array length was about 300m or10,000 wavelengths.

We will return to this image later.


SAS is real♦ DTI has focused SAS images from wide variety of sonar systems

◊ 2.5 cm out to 50 m (180 kHz 0.55 m CSS array)

◊ 7.5 cm out to 50 m (20 kHz 0.55m CSS array)

◊ 6cm out to 100m (240 kHz 2 m Boeing/Sonatech array)

◊ 5 cm out to ~300m (50 kHz 1.6 m DARPA array)

◊ 10 cm out to 1,000m (50 kHz 3.2m DARPA array)

◊ . and six othe r systems

♦ SAS enables order of magnitude performance increases—Constant high cross-range resolution to x10 range of current MCM systems

◊ Area coverage rate ~same as for current HF side-looking sonars

◊ SAS increases pixel rate by >10X for conventional arrays (2-3m long)

♦ BUT—Real-time long range SAS not yet demonstrated—Robustness under wider range of conditions needs to be shown—The biggest benefit of SAS will come at long ranges

◊ This implies; lower frequencies, lower grazing angles, new propagation paths

—Caveat: if ray-paths don t illuminate the target, SAS processing won t help

SAS is real, it works in a number of conditions, but there is not yet enoughdata taken in in large quantities in many environments to give customersconfidence that SAS performance will be worth the investment to change theircurrent approaches.


SAS Performance Limits: resolution is independent of range

Sw

ath

(tw

ice

rang

e)

Azimuth Resolution1000 m 100 m 10 m 1 m 10 cm 1 cm

100 m

1 km

10 km

100 km

GLORIA

EG&G 500

EG&G 100

Klein 50

Sea Mark I

Sea Mark II

SAS Limit(Wavelength dependent

attenuation)~SLSTechnology

Limit

Klein MBFS

DARPA SAS(data)

CSS SAS(data)

Boeing-SAS data

Swath width (twice range) is plotted here against azimuth resolution. Real-aperture systemsare plotted in black; SAS data is plotted in color.

Resolution with real-aperture systems is bounded by practical array lengths and thewavelength-dependent exponential attenuation of sound in seawater. SAS breaks that barrierby allowing high resolution with longer wavelengths. The longer wavelengths penetratefarther but resolution is maintained (or increased) by by summing many pings coherentlythus generating a long synthetic aperture.

The ultimate SAS limit also comes from the frequency-dependent attenuation of sea water.All practical sonars (real or synthetic aperture) run up against a wall at some range; verylittle additional range can be achieved even for enormous increases in system power or gainand that limiting range gets shorter as the wavelength gets shorter. One consequence is thatif you want 2.5cm resolution you probably won’t get ranges much over a kilometer evenwith SAS.


10 m

180 m

Front view

♦ SAS gives high cross-range resolution independent of range andfrequency, bringing the SAR revolution to sonar.

♦ Coherent combination of many pings allows synthesis of larger aperture

Processing techniques, algorithms & optimizations are key— 30 years of SAR research have provided the appropriate tools

◊ DARPA/DTI’s autofocus algorithms routinely process out 40,000¡ o f phase error

• both from uncompensated vehicle motion (over 3m) & from medium fluctuations— Real-time computing requirements can be satisfied by common workstations

◊ And within the reach of the highest end personal computers on the market— Extensive hardware R&D is not required (but new combinations of hardware will be)

Synthetic Aperture Sonar (SAS): why it works

DARPA program data,1996. Raytheon 50 KHz

Sonar, 10 (-36dB) &20cm spheres at 180mrange. DTI processing

DTI-Raytheon SASSLS (side looking sonar)

One look: resolution degrades with range Many looks, coherently integrated: resolutionis constant with range

Synthetic aperture sonar works by creating a virtual aperture out ofmultiple pings. The coherently-summed extra ‘looks’ at more distant targetscompensate for the linear degradation of resolution with range that is inherentin real-aperture sonar. The example, from the DARPA program, shows DTI’sSAS-processed images of data collected by Raytheon’s 50 KHz sonar in 1996.The targets were hollow 10-cm (-36dB) & 20-cm spheres at 180 m range.They are hanging in the water column well above the bottom.

The key to success in long range SAS is to correct for phase errors arisingfrom uncompensated motion and from medium instabilities. DTI’s data-drivenfocusing algorithms routinely removed 40,000° of otherwise uncompensatedphase error over the whole length of up to 10,000 wavelength syntheticapertures. We estimate processing loads for real-time SAS (0 to 1000m range,10 cm resolution) to be the order of 800 MFLOP using frequency-domainalgorithms.


36 m

Overview of DTI s SAS Processor

Create synthetic aperture usingmultiple returns

Generate image with frequency-domain all-range focusing algorithmused in seismic and SAR domains, modified for SAS

Apply data-driven auto-focus algorithms to image

Display and record

Receive element level data.

Apply initial motion compensation.Motion corrections can come

from inertial sensors or from the data itself.

SAS image with no motionor medium compensation

All-range focused image withinitial motion compensation

Image after autofocus

R = 419 m

Range

R = 452 m

50 kHz 3.2m aperture SAS in Dabob Bay. 10 cm resolution.DARPA/Rayteon/DTI program. Water depth <30 m

Image without SAS:Broadside Beam SLS

This chart sketches the process we use to generate SAS images. Element-leveltime-series data is recorded. Multiple pings are stitched together to form asynthetic aperture. If the platform motion is measured directly, as with aninertial measurement unit, that information is used to decrease the residualphase errors due to motion. If it is not available, as was the case for all theimages shown in this brief, then information in the sonar data itself is used tomake initial phase corrections for unmeasured motion.

An image is then generated by an all-range focusing algorithm, originallydeveloped by the seismic geology and synthetic aperture radar communities,now modified by DTI for the sonar application. Data-driven auto focusingalgorithms are then applied to complete the process.

The object used to illustrate parts of this process was in Dabob Bay. We don’tknow what it is. Note that there is a faint shadow behind of the unidentifiedbow-shaped outcropping. The object is in water of about 30m depth at a rangefrom the sonar of 420m. We show a larger view of this data later.


Imaging Improvements Through SAS Processing

Initial SAS image SAS image after DTI autofocus processing

Side Looking Sonar (SLS) performance

6.2 m

5 m

Look Direction

187 m

198 m

Sla

nt

Ran

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Hollow air-filled steel spheres (10 and 20cm diameter) above PVC frame and two corner-reflectors.Splitting along range line due to existence of both bottom-bounce and direct path returnsHollow air-filled steel spheres (10 and 20cm diameter) above PVC frame and two corner-reflectors.Splitting along range line due to existence of both bottom-bounce and direct path returns

A set of test targets was laid on the bottom in Lake Washington and imagedwith the DARPA/Raytheon system. These are the same spheres we showed ina previous chart, but now held a meter above a soft mud bottom on water-filled PVC posts. We show the the data at various stages of processing.

The test spheres focus well in cross range, but are split in the range direction.This, we believe, is due to the existence of multiple paths for the sound toreach the spheres and return. One path is direct. Another includes a bottombounce. The different paths are different lengths and therefore focus atdifferent ranges in the images above.

The brightest returns (at the furthest ranges) are two corner reflectors. Theyare not split because they can only support reflections that return by the samepath as the arrival. The returns from the corner reflectors are much moreintense than those from the small spheres (10cm and 20cm), and some of theresidual cross-range side-lobe structure can be seen in them.

Here is a object and its shadow as measured in one segment of a SAS run withthe 50kHz DARPA/Raytheon hardware. More data was collected in the rangedirection (~100 to 500m), but due to processing limitations in our 1996-classPentium-based computers, we only processed a 40m wide strip at a time.


Azimuth (m)

)

384 386 388 390 392 394 396 398 400 402

235

240

Range (m245

250

Unknown Object & Shadow at 230m: SAS vs SLS

Resolution: 11 cmsynthetic aperture * SLS image as shown is actually over-sampled showing

pixels about 1.5 m wide. Data only justifies 2.2m pixels.

382 m

Azimuth (m)

Range (m)

384 386 388 390 392 394 396 398 400

235

240

245

250

SLS

255 m

215 m SAS

SAS

DARPA2.2 m* real aperture (SLS)


Dabob Bay Shallow Water 50kHz SAS Data

DARPA

Looking up-slope (top tobottom): water depth atsonar was 30m,shallower at longerranges.

Sand ripples can beseen in the large box inthe SAS image.

An unknown objectabout 5 m wide is shownat the same scale withboth SLS and SASresolution. Notice thefaint shadow in the SASimage.

SAS image

R = 419 m

Range

R = 452 m

133 m

SASR = 117 m

R = 477 m

SLSRange

Image without SAS:Broadside Beam SLS

Several sets of data were taken by the 50 kHz DARPA sonar in Dabob Bay.Here are SAS and SLS images of an area from 117 m to 477 m range. Theazimuthal extent of the image is approximately 133 m. The bottom depth atthe sonar was approximately 30 m. The depth at the longest range shown isnot known, but, since the sonar was pointing in-shore, the depth at the longestrange was less than 30 m. Therefore, the images shown represent ranges up to10 times the water depth.

The large box surrounds a field of sand ripples detectable in the SAS image,but not apparent in the SLS view. The small box outlines the data used togenerate a blown-up image seen in a previous slide that describes the DTI SASprocessor. A clear shadow of an unidentified outcrop is visible in that imagedespite complications of shallow water propagation.


DTI-Processed SAS Image at 1 kilometer

♦ Sunken Aircraft Imaged DespiteBottom-Bounce Ray-Path

♦ PB4Y at 1 km is illuminated only byreflected ray-paths—at the extreme edge of the vertical

beamwidth and at the extreme edge ofthe system design range

980 m

1001 m

55

45

35

25

15

5

1430

1450

1470

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SoundVelocity (m/s)

Range (km)

Dep

th (

m)

Ray angles: -5o to +5o

55

45

35

25

15

5

0 0.2 0.4 0.6 0.8 1 1.2

Raytheon 3.2 m 50 kHz array

DARPA

We return to the 1000m range, 50 kHz, image of the aircraft in LakeWashington, noting this time that it was illuminated by the edge of the 2°vertical beam after a bottom bounce. The return energy also suffered a bottombounce. There was, nevertheless, sufficient coherence in the signal to focus tonear-theoretical resolution. Features of the image include: shadows of theaircraft structure on the wings and shadows of the single vertical rudder on thestarboard horizontal stabilizer, and some of the rib structure (in the forwardfuselage).

This was an accidental discovery; given the predicted ray-paths, we expectedthere to be nothing to see beyond 500m, but when bright returns were noted inthe data at 1 km, we applied SAS processing with the result you see here.


We have a better mousetrap. Whe re is the path to our door?

♦ What is different between SAS and current bottom imaging sonars?—SAS has much better resolution at much longer ranges—SAS has an array-length/range/speed l imit—Any sonar requires longer wavelengths (~3cm vice 3mm) at longer ranges

◊ Long wavelength sonars see more highlights; surfaces are no longer rough

♦ Detection performance not yet adequately proven

—New wavelength regime—New geometries (very low grazing angle)—New propagation regime

♦ Customer base is very cautious

—They want a stock number for SAS systems before they buy—Robustly engineered, thoroughly tested, second-generation SAS hardware

is not here yet.

Currentsonars

SAS

SAS still a very new technology. SAS with data-driven focusing is evennewer. The technology has yet not moved into commercial or military systems.It will eventually reach them, but there are impediments to their introduction.This chart outlines a few of them. We will elaborate on some of these issues inthe rest of this document.


Implications of longer wavelengths

Azimuth (meters, arbitrary zero point)

Range (m)

365 370 375 380 385 390 395 400

350

355

360

365

370

SAS Image: 10 cm resolution

50 Khz

Imaging a sunken airplane in Lake Washington at 350m

350m from sonar 15m fromsonar

High frequency(~450kHz)image of thesame sunkenairplane -unknown sonar

DARPA

DYNAMICS TECHNOLOGY, INC.

350

355

360

)Range (m

365

370

Azimuth (m)430 435 440 445 450 455 460 465 470 475

SLS Image:300 cm resolution

50kHz

We show three images of the sunken aircraft in Lake Washington. The upperleft is the SAS image. On the lower left is the image made from the sameDARPA/Raytheon array used as if it were a conventional side-looking sonar.To the right is the same aircraft imaged by an unknown a high frequency sonarsystem. Note the different character of the images and that the resolutiondegrades visibly in the conventional sonar image as the range increases.

At first we were puzzled by the extraordinary difference between the very highfrequency image taken at 15m from the aircraft and our SAS images collectedat 100, 350 and 1000m, particularly the lack of shadows and the ghostlycharacter of the aircraft. (The 350m range image is shown.) We explain thereasons for the differences in the following charts.


SAS Image of PB4Y-2 in Lake Washington (April 1997)

40 m

348 m

374 m

Range

Ribs? Spacing

≈ 60 cm (23")

Is the 50KHz SAS seeing through the skin intothe water-filled interior of the aircraft?

Navy patrol aircraft basedupon B-24 design,modified to have a singletail and a pair of AAblisters on aft quarter offuselage. Wrecked nearSeattle in 1956. Attemptsat salvage tore off portinboard engine anddamaged starboardinboard engine. Resting in50m of water. Starboardwing tip is 4.9m feetabove the bottom.

This data taken withDARPA-Hughes 3m arrayand processed as asynthetic aperture imageby DTI with 11 cm cross-range resolution, and 3 cmin-range resolution.Vertical angle was about2¡ from grazing.

Looking at this image it doesn’t look much like a World War II bomber. Forexample, the wing aspect ratio is too narrow. There are also stripes across thefuselage. Our DARPA sponsor, Dr. Theo Kooij speculated that these stripeswere the structural ribs of the aircraft and challenged us to prove or disprovethis.


PB4-Y Wing and Wing Spars

The Naval Air Museum, Pensacola, Florida kindly furnished us copies of theplans of this class of aircraft (Navy PB4Y-2). Here you see the wing structure.Note that the wing spars are I-beams. They pass straight through the fuselage.


SAS Measured Spacing of PB4Y-2 Tail Ribbing

22 22 24 25 21 22 2423 25

Range vs. amplitude cuts along aft fuselage

One pixel = 3 cm (0.85 pixels/inch)

I (Chatham) initiated a blind test with our (DTI) researchers in Torrance,California. Holding the plans secret from them, I asked them to makeamplitude vs range cuts longitudinally along the fuselage and from these todetermine the rib spacing. I gave them only one piece of information: thespacing was not uniform.

Here are the cuts and the estimates of the spacings for the after fuselage. Eachcolor represents a different azimuthal cut through the image.


Spacing of PB4Y-2 Tail Ribbing

Plan dimensions

SAS measurement22 22 24 25 21 22 2423 25

22 22 22 24 20 22 2323 2319

17

Here are the plan dimensions superimposed on the amplitude vs. fuselagedistance that we measured from the SAS sonar data. The results of our blindtest matched the plan dimensions within 2%.

It is clear now that Raytheon’s 50 kHz sonar penetrated the skin of the water-filled sunken PB4-Y2. If you look back at the SAS image a few pages above,you can see that the ball-turret is clearly imaged at the nose. The stripes visiblewithin the fuselage are the aircraft’s ribs. The skin of the wings is not seen, butthe wing spars, which pass straight through the fuselage, are clearly imaged,The inboard port engine is missing, having been broken off during a failedsalvage attempt.


Theoretical Justification

♦ There is theoretical justification (calculated after the fact) for thehypothesis that we are seeing ribs and wing spars.

—Calculated acoustic transmission through an infinite 0.1" thick aluminumplate immersed in water shows at most 2dB loss at 50 KHz*

10

20

30

Transmission loss (dB)

Incidence angle (degrees from perpendicular)

50 KHz

180 KHz

450 KHz

*After Junger & Feit, Sound, Structure, and Their Interaction, MIT Press 1986 p 347 and others.

Model not to be trusted at grazing incidence since hydrodynamic no-slip condition is ignored

After the fact we calculated that a thin metal skin with water on both sides wasvirtually transparent to the 50kHz (3cm wavelength) sound used by our SAS.At 450kHz, however, the skin was opaque. This explains both the skeletalappearance of the SAS image and the lack of shadows that are clearly visiblein the high frequency (but short range) image.

SAS imaged the sunken aircraft at an order of magnitude greater range thanconventional side-looking high frequency sonars and did so with very highresolution at all ranges. However, what the lower frequency SAS system saw adifferent view of the object than did conventional systems at their muchshorter ranges but higher frequencies.


Implications of multipath propagation


0 0.05 0.1 0.15 0.2 0.25 0.3

0

1

2

3

4

5

6

7

8

9

10

Shadow From Direct Path Signal (180 m range)

♦ A narrow bundle of rays from -3.5o to -4o propagates to the targetdirectly

—Extended shadow is formed by these rays

Dep

th (

m)

Range (km)

Target

Range

160 m

200 m

-25 dB 0 dB

20 rays (-3.5o to 4o)

5.8 m

simulation

Simple TrapezoidalPlate Target

.48 m

.98 m

.47 m

At the longer ranges that SAS will enable, shadows may no longer be a usefultool for identification of objects. We modeled the shadow that might resultfrom a vertical trapezoidal plate if a very narrow vertical range of acousticrays were directed at it. In the next chart we show what a more realisticvertical aperture would illuminate.


0 0.05 0.1 0.15 0.2 0.25 0.3

0

1

2

3

4

5

6

7

8

9

10

Shadow From All Paths (180 m Target)

♦ Shadow is shortened by surfacereflected paths

♦ Target multipath also spreads intoshadow

Dep

th (

m)

Range (km)

20 rays (-25o to 0o)

Range

160 m

200 m

Target only

Shadow only

-25 dB

0 dB

-35 dB

0 dB

5.8 m

Simulated data - breaking apart the shadow andtarget contributions for 25¡ vertical beamwidth.

The shadow disappears and the object image spreads in range into where theshadow would have been when a taller (25°) vertical aperture is modeled.

A number of approaches can be taken to minimize this effect, for example,tailoring of the vertical beam in intensity. Moreover, at longer ranges thephenomenon of mode-stripping (multiple bounces attenuating all but a fewmore-or-less direct paths) may also help. Nevertheless, the character of imagesat longer ranges will be different than they are at the shorter ranges ofconventional high resolution sonars. If SAS is to be used to its fullest, it mustwork in this longer range regime. Data must be collected at these ranges andthe resulting system performance must be characterized if we are to haveconfidence in SAS capabilities. This will take a second-generation set of SAShardware. It is not clear when this will happen.

DYNAMIC S TECHNOLOGY, INC.

The SAS Range/Resolution Revolution

DARPA/Raytheon/DTI SAS

200 m 400 m 800 m600 m 1000 m

5 cm

10 cm

High resolution side-looking sonars stop ~ here

♦ SAS technology will change underwater imaging as SAR did for radar

♦ But there are obstacles—There is insufficient data in the new long range regimes

◊ Longer wavelengths

◊ Multipath propagation

◊ Low grazing angles—Real-time processing not yet done at long ranges—Potential users can not afford much research and development—Well-engineered second-generation SAS systems do not yet exist

There is a revolution coming to a sonar near you. Synthetic apertureprocessing revolutionized the way that radar imaging is done. The same thingwill happen to sonar. There are now clear existence proofs that SAS works andthat it can exploit untapped coherence in underwater sound. High resolutionsonar imaging at long ranges and high coverage rates is on its way. It is not,however, here quite yet.

the synthetic aperture sonar revolution -...

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