Dual-transponder Precision Navigation System for Synthetic ApertureSonar

E N Pilbrow, M P Hayes, and P T Gough

Acoustics Research Group:Dept. Electrical and Computer Engineering,

University of Canterbury,Te Whare Wananga o Waitaha,

Private Bag 4800, Christchurch, NEW ZEALAND.

Email: e.pilbrow,m.hayes,p.gough@elec.canterbury.ac.nz

Abstract: The technical details of a dual-transponder, long-baseline positioning system to mea-sure the sway of a free towed Synthetic Aperture Sonar (SAS) are presented. The swayis measured with respect to freely deployed, battery powered, transponders which sitstationary on the seabed connected via cables to floating buoys housing high-accuracyGPS timing receivers. A T/R switch allows a single hydrophone on each transponderto alternately receive and transmit linear FM chirp signals. The time of flight of thesignals is determined by matched-filtering using a DSP and transmitted to the towboatfor storage in real time using RF modems. The sway information is completely in-dependent for each sonar ping and allows the deblurring of the SAS images by postprocessing. A Matlab simulation predicts a worst case sway accuracy of±1.5 cm.

Keywords: Baseline, Positioning, Synthetic, Transponder, Navigation

1. INTRODUCTION

Synthetic Aperture Sonar (SAS) is a technique forhigh resolution sea floor imaging that provides con-stant resolution, independent of range. However, im-age quality is severely degraded by unknown motionsof the sonar [1] with magnitudes greater than 1/10 thof the imaging wavelength [2].

The Acoustics Research Group at The University ofCanterbury has designed and built a free-towed SAS,KiwiSAS III, shown in Fig. 1. It has been operationalfor many years and the flat-nosed front-towed designhas proven to be stable, however, its use is limitedto fairly calm weather conditions. Even in “perfect”sea conditions, the residual towfish motion causes theprocessed images to be blurred. If this motion couldbe measured, the blurring could be removed by post-processing the image. Such a positioning system hasbeen designed and simulated [3] and is currently underconstruction. It will operate in conjunction with theKiwiSAS IV towfish (also currently under construc-tion) to measure its sway. The technical details of itsdesign and simulation results of its expected perfor-mance are presented in this paper.

Figure 1: The KiwiSAS III towfish. The rectangu-lar transmitter is mounted within the body at the frontwhile the square receiver keel hangs down below thefins at the rear. The two imaging frequency bands arecentered on 30 kHz and 100 kHz and the ping rate is14 pings per second.

2. SWAY - THE CRUCIAL PARAMETER

The KiwiSAS towfish has six degrees of freedom asshown in Fig. 2. Any deviations in the towfish positionfrom the mean “flight” path1 are known as sway, surge,and heave for the x, y, and z directions respectively.

Whilst any unknown deviations cause degradation ofthe image, not all of the six degrees of freedom haveto be measured accurately to improve the image qual-

1 The mean flight path is a straight level path traveled at constantspeed. Its direction is determined by the orientation of the posi-tioning system.

Figure 2: The six degrees of freedom of the KiwiSAStowfish. Variations in the across-track motion (sway)cause the most severe blurring of the SAS images andwill be measured by the proposed positioning system.

ity. This is due to both the towing arrangement of theKiwiSAS and the nature of SAS imaging characteris-tics. Ideally, a positioning system measures all the un-knowns. However, much improvement can be gainedby designing a simple system that initially removes thelargest deviations and can be expanded to remove theremaining errors in the future.

The KiwiSAS III has an Inertial Navigation System(INS) installed to measure angular towfish motion[4]. This provides data which is especially useful forbathymetric work but the main cause of SAS imageblur, sway motion [5], cannot be measured accuratelyusing this system. Even very small sway deviationsfrom a straight tow path cause severe blurring of theSAS image. To correct for this effect, accurate swaymeasurements are required for every transmitted ping.

Sway is defined as the difference between the towfishacross-track position and the across-track componentof the mean flight path. Knowing the sway allows theimage data collected at each ping to be time shiftedto bring the blurred SAS image back into alignment,i.e., as if the towfish had traveled along the mean flightpath. The goal is to achieve an accuracy (sway error)of ±1 cm over the entire tow path.

3. THE KiwiSAS IV POSITIONING SYSTEM

The design is based around the concept of a long base-line positioning system (LBL). A LBL works by mea-suring the time of flight of sonar signals from the tow-fish to a number of separate transponders. Knowingthe speed of sound underwater allows the time of flightdata to be converted directly to distances and the posi-tion of the towfish to be calculated with respect to thepositions of the transponders.

In the KiwiSAS design, two transponders will be de-ployed to sit on the seabed, connected to floating sur-face units by cables as shown in Fig. 3. Antennas onthe surface units allow real time communication withthe towboat where all the measurements can be safelystored.

Figure 3: A conceptual drawing of the two surfaceand seabed units connected by a cable which formthe dual-transponder LBL. The dashed line intersect-ing the transponders indicates the across-track direc-tion in which sway is measured.

The geometry of the system means that the directionin which sway can be measured is along a line joiningthe two transponders so, ideally, the towfish must betowed perpendicular to this line while collecting im-ages. This perpendicular towing arrangement is illus-trated in Fig. 4. The transponders alternately transmit

Figure 4: A birds eye view of a perpendicular, dual-transponder, LBL. The pair of dashed lines and pair ofsolid lines indicate the time of flight between the tow-fish transmitter and the numbered seabed transpondersfor two successive towfish pings.

a linear Frequency Modulated (FM) chirp from 60 kHzto 80 kHz and receive the sonar’s dual linear FM chirpsof 20 kHz to 40 kHz and 90 kHz to 110 kHz. An initialcalibration phase of transmitting and receiving chirpsbetween the transponders is required after deploymentto establish the distance between them (i.e., the base-

line) which will typically be around 50 m.

4. TRANSPONDER HARDWARE DETAILS

4.1 Seabed Compartment

The seabed electronics are controlled by a MicrochipPIC16F876 microcontroller responding to serial com-mands from the surface unit. It gathers data from allthe sensors, controls the gains of the amplifiers andmonitors the batteries status. This information is sentback to the surface unit as NMEA0183-compliant se-rial strings.

4.1.1 Motion Sensors.The seabed compartmentmust sit motionless on the sea floor. Any movementwill change the baseline and necessitate recalibration.This is monitored by an Assemtech MS24 motion sen-sor.

4.1.2 Pressure Sensors.Depth is measured by aSensym ICT 19C050PA4K absolute pressure sensor.Depth information is required because the two seabedunits may come to rest on mounds or hollows and thecoordinate system which they define must be trans-formed to be level with the sea surface.

4.1.3 Hydrophone. The hydrophone used to al-ternately transmit and receive is an Ametek StrazaSB45C-95. It is designed to operate from 60 kHz to110 kHz and has a 15 dB variation in response overthis band. A Transmit/Receive (T/R) switch is usedto allow simultaneous connection to a preamp and apower amplifier. This is shown in Fig. 5. When thepower amp is transmitting, the preamp input is pro-tected by the Vishay Siliconix 2N4117A JFETs1 con-ducting through the 100 kΩ resistors limiting the volt-age at the inputs to 0.7 V above and below the supplyrails. When the preamp is receiving, none of the hy-drophone signal is lost into the poweramp output stagebecause the signal levels are well below the 0.7 V re-quired for the back to back diodes to conduct. The1 MΩ resistors provide the necessary DC path for theinput of the preamplifier.

The hydrophone maximum power rating of 7.5 WRMS means a maximum signal amplitude of around220 V is permissible for the 60 kHz to 80 kHz fre-quency band. PSPICE simulations of the hydrophone2

using this level of input results in a power consump-tion of 7.1 W RMS with 1.5 W RMS radiated into thesea.

1 In this configuration, the JFETs act as pn junction diodes with ex-tremely low reverse leakage currents. Their response is slow but thatdoes not matter here.

2 At 60 kHz, it can be represented by a 330Ω resistor and a 0.94 nFcapacitor in series.

When transmitting, the hydrophone will appear as abright point on the SAS mid-band images, roughlyequal in intensity to a large retro reflector (see Fig. 6) ifboth are located 40 m from the towfish. At larger dis-tances, the hydrophone will appear as a brighter pointthan a retro. This is because its intensity drops by afactor of 4 for a doubling in distance whereas a retroreflector’s intensity drops by a factor of 16 due to thetwo-way propagation path.

Figure 6: A retro reflector is commonly used as a SASimage target. It reflects incident sound waves back inthe direction they came from so appears as a brightpoint in the image.

4.1.4 Preamp. The preamp is based around a BurrBrown INA111AU instrumentation amplifier produc-ing a single ended output with a gain of 40 dB. Thisis amplified further by a Texas Instruments THS4140line driver with an adjustable gain controlled by a Dal-las DS1808 digital potentiometer. This allows the mi-crocontroller to adjust the gain on the fly (using theI2C communication protocol) depending on how faraway the towfish is from the transponders.

4.1.5 Poweramp. The poweramp is based on aSGS-Thompson TDA2030 amplifier and a 1:20 step-up transformer. This provides up to 220 V amplitudeas a floating output to drive the hydrophone. The trans-mit waveform is generated in the sea surface compart-ment (Sec. 4.2) as a differential signal and sent downthe connecting cable. When it arrives, it is convertedinto a single ended signal using a Burr Brown INA133line receiver and the gain is adjustable by a DS1808digital potentiometer.

4.1.6 Temperature Sensors.The speed of sound inwater is a crucial parameter and in shallow water it

Figure 5: T/R switch used to alternately connect the hydrophone to the preamp and poweramp. This design wasarrived at by experiment due to the lack of information available on T/R switches.

varies with temperature. Dallas DS18B20 digital tem-perature sensors are used to measure the water temper-ature at the towfish (mid depth), transponders (seabeddepth) and surface plus inside each unit to check forelectrical overheating problems. They have an accu-racy of±0.5C and use a ‘one wire’ bus for commu-nication with the microcontroller. A one wire bus usesa single wire for bidirectional communication (andpower supply if desired) along with a ground wire.Any number of devices can be connected because eachis individually addressable via a unique 64 bit built-in code. The microcontroller runs through a complexsearch routine [6] to identify these codes when the cir-cuit is first switched on.

4.1.7 Power Supply. Each of the two transpondersare powered from a pair of 12 V Sealed Lead Acid(SLA) batteries in series. This +24 V supply is useddirectly by the poweramp. For the lower poweredanalog and digital circuits, the supply is converted to+12 V and -12 V using Texas Instruments PT5100 andPT78NR200 switching regulators respectively. Linearregulators are used for further reduction as required.

The batteries power both units and are located inthe seabed unit to help weigh it down. The recom-mended [7] charging voltage for a constant voltageSLA charging system is 13.5 V. Charging is controlledby two STMicroelectronics L200 voltage regulatorsand a Nais TX2-5V relay, automatically switching thebatteries in parallel when a voltage is detected at thecharging terminals. This allows each battery to beindependently charged and monitored. The chargingsupply can be connected to the surface unit so that theseabed unit does not have to be opened up to chargethe batteries.

4.1.8 Cable. The cable connecting the two unitsis a General Cable 24880016 comprising six twistedpairs with individual shields plus an overall shield andone additional unshielded wire. This is the same typeas used for the KiwiSAS tow cable for many yearsand has proven to be capable of withstanding repeatedflexing and abrasion in an underwater marine environ-ment. Each wire is rated to carry 3.2 A so can be usedfor either signals or power supplies.

4.2 Sea Surface Compartment

4.2.1 Microcontroller. A Microchip PIC16F877microcontroller controls the entire operation of thesurface unit. It receives commands and transmits datato the towboat through an RF modem while keepingtrack of the timing pulses from a GPS and control-ling the operation of a DSP. It also issues commandsto the seabed unit and receives sensor data in return. AMaxim MAX3100 chip provides two extra serial portsand connects to the microcontroller using an SPI bus.

4.2.2 Power Supply. The surface power supply ismuch simpler than the seabed power supply, consist-ing of linear regulators to drop the +24 V battery sup-ply down as required.

4.2.3 High Stability Clock. A C-MAC IQTCXO-251 10 MHz oscillator is used as the clock source forthe microcontroller, DSP and ADC. It has a temper-ature stability of 2.5 ppm and an ageing stability of1 ppm per year. This low drift ensures that the sam-pling of the received signals and the generation of thetransmit signals are highly accurate in time.

4.2.4 ADC. The output of the preamp in the seabedunit is transmitted up the cable as a differential sig-nal to be digitized using an Analog Devices AD772316 bit sigma-delta ADC. It uses the high-stabilityclock signal to ensure the sampling is as accurate aspossible. The samples are sent to the DSP via a 16 bitparallel data bus.

4.2.5 DAC. The DAC is used to convert the FMchirp generated by the DSP, a linear sweep from60 kHz to 80 kHz, to an analog signal. A Burr BrownDAC902 DAC using a 16 bit parallel data bus con-nected to the DSP performs this function. The dif-ferential analog current output is converted to a differ-ential voltage output by a Texas Instruments THS4140line driver and transmitted down the cable to the pow-eramp in the seabed unit.

4.2.6 DSP. A Texas Instruments TMS320VC33DSP running at 50 MHz is used to alternately match-filter the digitized preamp signals from the ADCand generate the transmit waveform for the DAC.A matched filter provides high immunity to signalstrength variations and background noise. The re-ceived SAS signal is correlated with a copy of theoriginal signal and the peak of the correlation enve-lope identifies the time of arrival at that transponder.

4.2.7 GPS. The system requires an absolute timingreference in both transponders and in the towfish sothe time of flight of sonar signals between them canbe calculated. The Motorola Oncore UT+ GPS is usedhere with a Motorola Timing 2000 antenna in the sur-face unit of each transponder. The UT+ is designedspecifically for high precision timing applications andoutputs a 100 Hz square wave with each rising edgeaccurate to±130 ns. This accuracy is achievable un-der dynamic motion provided the antenna has an unob-structed view skywards. A serial string is transmittedto the microcontroller, once per second, containing theabsolute GPS time corresponding to the current risingedge. The 100 Hz waveform is monitored by the mi-crocontroller, using its capture port, with an accuracyof ±50 ns and used to update the high stability clockwhich, although has high stability, will drift by a smallamount between each update.

4.2.8 RF Modem. The towboat and transponderscommunicate using Bluechip MOD433 RF modemsfeaturing built-in error correction. They operate on theunlicensed 433 MHz band with a transmission powerof 10 mW equating to a range of 600 m in the open.For every towfish ping, the transponders transmit backthe time of arrival of that ping at 9600 baud plus allthe sensor data. This real-time transfer of data fromthe transponders eliminates the need for large on-board memories and the possibility of losing data ifa transponder malfunctions.

The data format will be based on the NMEA0183 stan-dard used for interconnecting marine electronic equip-ment. Some modifications will be required becausethe standard is designed for one-way communicationonly.

A proposed feature, not yet implemented, is the abil-ity to download the DSP program code through theRF modem. This would be controlled by the micro-controller which would receive the code from the RFmodem and transmit it serially into the DSP in boot-loader mode. This provides an easy way to update theDSP code without opening the case.

5. SIMULATION RESULTS

A Matlab simulation of a perpendicular, dual-transponder, LBL was performed. Fig. 7 shows thelayout used. The simulation revealed a worst casesway error of±1.5 cm for a 50 m transponder sep-aration with 20 m across-track motion range, a 30 macross-track point of closest approach, and±50 m ofalong-track range. Although this does not meet the tar-get of±1 cm, it is a worst case figure, so much of thetime the error is likely to be less than±1.5 cm.

Figure 7: The layout (birds eye view, not to scale) ofthe KiwiSAS dual-transponder LBL simulation. Theorigin is located at transponder one.

6. CONCLUSIONS

The technical design details of the transponders for theKiwiSAS IV LBL were presented. Whilst most of thedesign follows from ‘standard circuits’, the T/R switchdesign is original and preliminary experiments haveshown it to work well. Additional challenges remaine.g., bootloading the DSP through the RF modem butshould also be surmountable.

The sway data measured by this system will be com-pletely independent for each transmitted sonar pingand should allow significant improvements in the Ki-wiSAS image quality. Although the simulations sug-gest the±1 cm goal may not be obtainable, the swaydata will still provide an excellent starting point forSAS autofocus algorithms to further sharpen the im-ages. Future expansion is possible by deploying addi-tional transponders. This additional data would enableadditional parameters to be measured and further im-prove the accuracy.

7. REFERENCES

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[2] D. W. Hawkins.Synthetic Aperture Imaging Algo-rithms: with application to wide bandwidth sonar.PhD thesis, Department of Electrical and Com-puter Engineering, University of Canterbury, Oc-tober 1996.

[3] E N Pilbrow, P T Gough, and M P Hayes. Longbaseline precision navigation system for syntheticaperture sonar. InProceedings of The 11th Aus-tralasian Remote Sensing and PhotogrammetryAssociation Conference, Brisbane, Australia, 3-5November 2002.

[4] E N Pilbrow, P T Gough, and M P Hayes. In-ertial navigation system for a synthetic aperturesonar towfish. InProceedings of Electronics NewZealand Conference, Dunedin, New Zealand, 14-15 November 2002.

[5] P T Gough, M P Hayes, H J Callow, and S A For-tune. Autofocussing procedures for high-qualityacoustic images generated by a synthetic aperturesonar. InProceedings of International Congresson Acoustics, Rome, Italy, September 2001.

[6] Dallas Semiconductor.iButton book of standards.

[7] Jaycar Electronics.Using and charging SLA bat-teries, 2001.