Distributed Software System Architecture for Autonomous Launch andRecovery System of Autonomous Underwater Vehicles

Sagar Pai ,Shailabh Suman,Wu Yu Song,Bharath Kalyan and Mandar Chitre

Fig. 1. AutoLARS used in the Experiments

Abstract— In this paper we discuss the software systemarchitecture that is developed for the autonomous launch andrecovery platform (AutoLARS). The architecture comprisesof several components distributed across physical processingunits. The architecture handles the real time data from thehydrophones and subsequent processing of that data to guidethe AutoLARS to intercept the autonomous underwater vehicle(AUV) for subsequent recovery. The architecture is built tobe distributed to facilitate the load balancing of the systemresources. The architecture is modular and makes it easy todeploy based on the system resources

Index Terms— Distributed,Software Architecture,Autonomous, AUV, LARS, AutoLARS

I. INTRODUCTION

Underwater vehicles are increasing their autonomous ca-pabilities more and more in order to carry out more complexand longer missions. The ability to safely launch and recoveran autonomous underwater vehicle (AUV) is critical to anymission involving AUV operations. As AUV gets larger andoperations move to higher sea states, the current practiceis to use larger surface support with high load rated lift-ing equipment [1]. However, when any vessel motions arepresent the operation can become hazardous to personneland equipment, thus limiting AUV operations.More recently,researchers have been working on subsea capture systemsthat have great potential where are AUVs are customized tohome into the subsea capture system [2], [3]. We have beenworking on an elegant alternative called the AutoLARS, thatfacilitates the automatic launch and recovery of AUV [4],[5], [6].

The AutoLARS system fig. 1 is designed to facilitate therecovery of AUV’s which are not equipped to dock into

Fig. 2. AutoLARS intercepts AUV

docking stations. The only requirement for the AUV is tobe able to proceed between two waypoints at a nominalheading and depth. The AutoLARS will triangulate the AUVand proceed to intercept it for subsequent recovery. Since theinterception takes place subsea the AutoLARS can operateat elevated sea states. In this paper we will be describing thedetailed software architecture and its interaction with systemhardware to achieve the goal of AutoLARS. The AutoLARSsystem comprises of the subsea capture and interceptionhoop and a surface unit for power and control. The hoopis connected to the surface unit with a tether capable ofsupplying power and data. The AutoLARS is equipped witha free running commercial off the shelf (COTS) pinger, a setof four hydrophones on the periphery of the hoop, a compassand a depth sensor. The AutoLARS will triangulate towardsthe COTS transponder installed on the AUV see fig. 2.

The paper introduces the distributed software system ar-chitecture used in the design of the AutoLARS system.The requirements for the system architecture is the abilityto perform in real time a myriad of tasks to enable theinterception of the AUV. The system performs a set ofpipelined tasks encompassed in software modules each witha well defined interface. The distributed system architectureused in AutoLARS allows for load balancing the softwaremodules on the hoop and the surface units. The system alsoallows for the playback of captured data from the trialsbetween various submodules for subsequent analysis. Thesystem allows the remote monitoring of the capture processif the system is mounted on the Unmanned Surface Vehicle(USV). The distributed nature of the system allows for theoperator to be in the loop in case on a manual intervention inthe recovery process. The design also caters for a completely

978-0-933957-40-4 ©2013 MTS This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in IEEE Xplore shortly after the conclusion of the conference.

autonomous system with all the processing being done in thehoop. The paper discusses the System architecture in sectionII, the equations for finding the AUV position in section III,the experimental setup and results in section IV.

The development of AutoLARS was started in collabora-tion with NATO Undersea Research Center (NURC), nowCenter for Maritime Research and Experimentation (CMRE)and trials were performed at the NURC facility in LaSpezia. A few challenges in development of AutoLARS wererealized in the trials [5]. In order to address these challengesa scaled version of AutoLARS with same mechanical geom-etry was developed at Acoustics Research Lab (ARL), NUS.The final tracking system and path planning experimentswere performed on the ARL prototype. The scaled prototypedeveloped is for demonstration of capability and it doesnot have a capture mechanism. In the experiments, onlyinterception and homing of AUV were performed.

II. SYSTEM ARCHITECTURE

A. Overview

The fig. 3 shows the different components of the archi-tecture used in the development of the AutoLARS. Thecomponents are defined below

1) Acoustic Acquisition System manages the capture ofthe acoustic data in real time from the set of fourhydrophones mounted on the hoop.

2) Navigation Sensor Interface manages the capture ofdata from the navigation sensors.

3) Control System completes the interception loop, theloop is finally closed by the Path Planning Systemcommanding the control system to direct the Auto-LARS system for interception of the AUV.

4) Signal Processing System handles the correlation andsignal processing of the data. This system also logs theraw data for analysis.

5) AUV Positioning System calculates the AUV positionby triangulation of the transponder signal received bythe four hydrophones on the hoop.

6) Path Planning System executes the trajectory planningof the AutoLARS system with respect to the AUVposition data.

The components of the system architecture follow a pro-ducer consumer paradigm. The consumers register with theproducers. The producers send a notification to the con-sumers on the availability of data. At the system boundariesthe producers run an asynchronous TCP/IP client. The con-sumers run a TCP/IP server to receive the client requests. Theasynchronous nature of the connection allows the producersto fulfil realtime obligations while using the TCP/IP protocol.

B. Acoustic Acquisition System

The hydrophones on the AutoLARS are arranged as shownin fig. 4. The four hydrophones are connected to a 24bit sigma delta Analog to Digital Converter (ADC). Forimproved fault tolerance, we included a heartbeat mechanismto monitor the system health as shown in fig. 5. The AcousticAcquisitionSystem is a multi threaded software component.

Fig. 3. System Component Overview

The Card Interface thread reads the data from the ADC inblocks of 655.36ms per channel.This corresponds to a 1MBstorage in memory. The data from the ADC is convertedfrom EBDIC format and is stored into a buffers that isretrieved from a pre-allocated circular buffer pool by theSignal Conditioning thread. The buffer is then sent byasynchronous TCP/IP connection to the TCP/IP server inthe Signal Processing System in Surface Unit 1 see fig. 3.If an connection to the Signal Processing server cannot beestablished the buffer is immediately returned to the circularbuffer pool.This is possible due the asynchronous clientwhich reports that the connection state is not establishedinstead of waiting for the connection to complete. In the casewhere the connection is established the Signal Conditioningthread returns the buffer to the pool after the socket write bythe TCP/IP client.This system runs in a real time constraints.It is fatal failure if the Card Interface thread cannot get abuffer from the circular buffer pool. The size of the poolitself is adjusted to account for a transient nominal load onthe CPU.

C. Navigation Sensor System

The Navigation Sensor System runs a thread per sensorwhich reads the navigation sensors in the AutoLARS on aperiodic basis. The data from the sensors is filtered and storedin a cache. This cache is then queried by the clients. The dataof the navigation sensors is the current heading, depth , pitchand roll of the AutoLARS system. The Navigation SensorSystems is a TCP/IP server which waits for the data requestsfrom the AUV Positioning System, the Path Planning Systemand the Control System which are the TCP/IP clients to theserver.

Fig. 4. Hydrophone placement on the AutoLARS system

Fig. 5. Acoustic Acquisition System Sequence Diagram

D. Control System

The Control System for the AutoLARS uses a ProportionalIntegral Derivative (PID) controller. The Control System ofthe AutoLARS takes the required heading, depth and thesway and surge distances as set points from the Path PlanningSystem. The Control System then uses the six thrusters tosatisfy the commands of the Path Planning System. TheControl System uses the data from the Navigation SensorSystem to close the feedback loop. The Control System runsa TCP/IP client thread to connect to the Navigation SensorSystem. The Control System runs a TCP/IP server thread tointerface with the Path Planning System.

E. Signal Processing System

The AutoLARS fig. 4 uses four commercial off the shelf(COTS) Reson TC4013 hydrophones. One COTS pinger

operating at 20kHz frequency is fixed butting hydrophoneone. The transponder on the AUV is a COTS chirp basedtransponder with a frequency range of (23-27)kHz. Thepinger has a signal duration of 4ms and the transponder hasa signal duration of 10ms. The Signal Processing Systemruns as a server to the Acoustic Acquisition System fig. 6.The Signal Processing System starts the Range Aggregatorthread. This thread starts the Pinger Detector and the fourTransponder Detector threads for each of the hydrophonesin the system. The data from all the hydrophones is readby the server for processing. The Signal Processing Systemuses the same circular buffer interface used in the AcousticAcquisition System. This ensures that the cost of memoryallocation is paid upfront and not during the processing ofthe signal. The Pinger Detector thread and the TransponderDetector threads process the acoustic data in blocks. Thismethod of processing should be able to handle scenarioswhere the signal is spilt between two consecutive blocksof data. The goal of the signal processing by the detectorthreads is to find the start of the signal from the transponderand pinger by correlation. The detector threads prependsto the current block, the data from the previous blockfor finding the correlation. The data prepended from theprevious block should be at least the size of the signal.The AutoLARS system prepends the previous block data of20ms. The detectors uses a running index over the blocks.The correlation index chosen is the one with the largermagnitude of the correlation when the detection is within theambiguous zone striding two blocks. This zone is detectedby the two correlation index delta being less than the size ofthe signal under consideration. The detector threads informthe range aggregator on the detection of the transponder andthe pinger signals. On a detection of a pinger signal i therange aggregator sends the range information between theprevious ping i−1 and the transponder signals correspondingto pinger signal i−1 to the AUV Positioning System

The Logging thread is a consumer of data from theAcoustic Acquisition System. The acoustic data is storedwith the meta data which include timing parameters andchannel numbers. The data is stored in the same blockformat as was sent by the Acoustic Acquisition System. Thisfacilitates the subsequent testing of the Signal ProcessingSystem with the stored data as the input. The processing fromthe data acquired from a specific sea trial can be repetitivelyprocessed by the Signal Processing System to refine theprocess of detection.

F. AUV Positioning System

The AUV Positioning System receives the range data formthe Signal Processing System on the TCP/IP Interface fig.7. The range data is the filtered to remove outliers and theTriangulation thread is notified. The Triangulation thread getsthe telemetry data from the Navigation Sensor System fig.3. The Telemetry data is the depth, the heading, the rolland pitch of the AutoLARS. The hydrophone positions aretransformed from the Body coordinate System to the NEDcoordinate system see fig. 8. The details of the solution are

Fig. 6. Signal Processing System Sequence Diagram

Fig. 7. AUV Positioning System Sequence Diagram

explained in section III. The AUV Positioning System isa client to the Path Planning System. The Path PlanningSystem receives the AUV position in polar co-ordinates fromthe AUV Positioning System.

G. Path Planning System

The Path Planning System runs as a server to the AUVpositioning System and is a client to the Control System. ThePath Planning System receives the AUV coordinates from theAUV Positioning System and this data is input to the FiniteState Machine (FSM). The FSM states direct the AutoLARSon a state specific strategy for intercepting the AUV. Thestrategies would include commands to the Control Systemto proceed in sway or surge for a specific distance. It couldalso tell the Control System to change the heading of theAutoLARS. The Navigation Sensor System will provide thesensor parameters of depth,pitch,roll and heading to the PathPlanning System. The Path Planning System will utilize thisdata along with the data from the AUV Positioning Systemto effect the interception of the AUV.

Fig. 8. Body Fixed Coordinate System

III. SOLUTION FOR AUV POSITION

The input to the AUV positioning system is the number ofsamples corresponding to the time difference of transmissionof the pinger and the receipt of the transponder signal by thehydrophone. The pinger mounted adjacent to the hydrophoneH1 (fig. 4) is a free running pinger operating at 20kHz. Thedistance between the hydrophone H1 and the transponder(τ) on the AUV is half of the roundtrip time from the pingerto the transponder and back to the hydrophone H1. Afterreceiving the signal from the pinger the transponder (τ)responds after a predefined time given by the manufacturerdatasheet. The fig. 4 shows the placement of the hydrophonesand the pinger on the AutoLARS system.

LIST OF SYMBOLS

(xn,yn,zn) Location of hydrophone Hn;n = {1, . . . ,4}(xτ ,yτ ,zτ) Location of the transponder τ

sn Number of samples from the start of the pingerto the detection by hydrophone Hn at position(xn,yn,zn)

tn Time corresponding to the samples of hydrophoneHn; sn

sru Speed of sound in waterρ̂cn Distance between the hydrophone Hn and the as-

sumed position of the transponder τ

ρetat Distance corresponding to the error for the turn-around time in transponder

ρtat uttat is the distance corresponding to the turn-around time

ρn Distance from the hydrophone Hn to the transpon-der τ

ob Origin in the BODY coordinate system. The originis chosen to coincide with the point at the centreof the hoop on the AutoLARS

on Origin in the NED coordinate systemsr Sampling rate of the ADCtetat Error in turn-around time(in seconds). This is the

delta between the actual time and the expectedtime of transmission

ttat Turn-around time (in seconds)vt Rt

f v f where vt ,v f are the positions in the twocoordinate systems and Rt

f is the Rotation Matrixto convert from one coordinate system to the other

Za AUV depthZl AutoLARS depth

The heading information is used to orient the AutoLARSsystem towards the AUV. The roll (φ ) and pitch (θ ) in-duced by the motion of AutoLARS are considered as meredisturbance in the system. The Hydrophone Hn locationsneed to be transformed by the rotational matrix Rn

b forthe roll and pitch from body frame to North East Down(NED) coordinate system. While the heading is calculatedby numerical methods described later.The depth of the AUV and AutoLARS system are knownapriori. The AUV is programmed to maintain a specific depthwith an expectation that it is capable of having a good depthcontrol.

The difference in depth between the AUV (corresponds tothe transponder τ depth) and the AutoLARS is given by

zτ = Za−Zl (1)

• For H1:

us1 = 2ρ1 +ρtat +ρetat

ρc1 =us1− srρtat

sr2

ρe =ρetat

2ρ1 = ρc1−ρe

(2)

• For Hn, n = 2, . . . ,4:

usn = ρn +ρ1 +ρtat +ρetat

ρcn =2usn−us1− srρtat

2sr

ρe =ρetat

2ρn = ρcn−ρe

(3)

The following non linear equations ∀n = {1..4} give thedistance between the AUV and the hydrophone n.

ρn =√(xτ − xn)2 +(yτ − yn)2 +(zτ − zn)2 (4)

Assuming initial values of xτ ,yτ ,zτ ,ρe which is the esti-mated position of the transponder τ .• For H1:

ρ̂c1 =√(x1− xτ)2 +(y1− yτ)2 +(z1− zτ)2 +ρe (5)

The distance using the measured values by the SignalProcessing System is given by

ρc1 =us1− srρtat

2sr(6)

• For Hn, n = 2, . . . ,4:

ˆρcn =√(xn− xτ)2 +(yn− yτ)2 +(zn− zτ)2 +ρe (7)

The distance using the measured values by the SignalProcessing System is given by

ρcn =2usn−us1− srρtat

2sr(8)

The error between the measured and computed distance is

δρcn = ρcn− ρ̂cn (9)

Using Taylors series to address the non-linearities. For Hy-drophones Hn.

δρcn =(xn− xτ)δxτ +(yn− yτ)δyτ +(zn− zτ)δ zτ

dn+δρe

(10)

dn =√(xn− xτ)2 +(yn− yτ)2 +(zn− zτ)2 (11)

The equations are formed with the form Ax = B where thesolution is of the form x = (AT A)−1AT B.

• Shallow Water In the experiments with shallow watersof a lake there was a noticeable error in the turn aroundtime while the AUV depth was stable. For this scenariowith zτ is known from (1).

δρcn =(xn− xτ)δxτ +(yn− yτ)δyτ

dn+δρe (12)

(x1−xτ )

d1

(y1−yτ )d1

1(x2−xτ )

d2

(y2−yτ )d2

1(x3−xτ )

d3

(y3−yτ )d3

1(x4−xτ )

d4

(y4−yτ )d4

1

δxτ

δyτ

δρe

=

δρc1δρc2δρc3δρc4

(13)

Iteration is repeated till δe is less than some predeter-mined precision.

δe =√

δxτ2 +δyτ

2 +δρe2 (14)

• Sea WaterIn experiments in the sea waters the error in turnaround time was not noticeable, then with δρe = 0 andassuming zτ is unknown.

(x1−xτ )

d1

(y1−yτ )d1

(z1−zτ )d1

(x2−xτ )d2

(y2−yτ )d2

(z2−zτ )d1

(x3−xτ )d3

(y3−yτ )d3

(z3−zτ )d1

(x4−xτ )d4

(y4−yτ )d4

(z4−zτ )d1

δxτ

δyτ

δ zτ

=

δρc1δρc2δρc3δρc4

(15)

Iteration is repeated till δe is less than some predeter-mined precision.

δe =√

δxτ2 +δyτ

2 +δ zτ2 (16)

Fig. 9. AUV with transponder strapped to its bottom

IV. EXPERIMENT SETUP AND RESULTS

The experiments were carried out in shallow waters ofPandan reservoir in Singapore to test the path planning andthe positioning systems. A transponder was attached to thesmall AUV for the tests see fig. 9. The AutoLARS systemused in the tests did not have a capture mechanism. Theobjective of the experiment was to validate the AUV trackingand the trajectory employed for interception.

The AUV towed the transponder at a constant depth of0.75 meter. The AUV maintained a constant speed of oneknot and constant heading while towing the transponder. TheAutoLARS was set up at approximately 30 meter from thestarting point of AUV. The AutoLARS has umbilical of 12meter. The Path Planning System received the heading deltafrom the AUV Positioning System. This is the heading ofthe AUV with respect to the LARS as shown in fig. 10.The Path Planning System uses this delta along with theactual heading of the AutoLARS from the Navigation SensorSystem to compute the heading that the AutoLARS shouldturn towards for an interception. The plot as shown in fig.11 shows the heading difference vs distance between theAUV and AutoLARS for three distinct interceptions. Theinterception of the AUV is at the point when the difference indistance between the AUV and AutoLARS is zero. The AUVtransponder is a point source and at the time of interceptionthere may be a heading difference due to the point of entryof the AUV into the hoop see fig. 10 and the placement ofthe hydrophone constellation see fig. 4. This is true whenthe AUV does not enter at the center (ob) of the hoop.

V. CONCLUSION

The experiments were successfully carried out by using thesystem architecture described above. The distributed modelleveraged on the high bandwidth of the TCP/IP interfacesto move the large volume of the acoustic data between thedifferent subsystems while meeting real time constraints.This allowed us to handle the data on the surface platformswithout constraints imposed by the embedded systems. Byusing overlapped block data we were able to handle thesignal processing with block based functions provided by thestandard signal processing libraries in real time.In the futurework in order to further exploit the distributed architecture,

Fig. 10. Heading of the AUV with respect to AutoLARS

05101520253035−60

−40

−20

0

20

40

Distance (meters)

Headin

g D

iffe

rence

(Degre

es)

Experiment 1

Experiment 2

Experiment 3

Fig. 11. AUV heading with respect to AutoLARS and the distance betweenthe AUV and AutoLARS

we plan to provide fault tolerance in the system. We will beusing the TCP/IP interfaces for providing the fault tolerance.We will also be looking at using the redundancy in the hy-drophone constellation to achieve robustness in the solutionof the AUV position in presence of error of detection in oneor more hydrophones.

ACKNOWLEDGMENT

We would like to thank several colleagues and collab-orators who have been instrumental in this projects suc-cess. Alain Maguer, Pierro Guerrini, Jim Osse, John Potter,S.Biagini from NURC for initial collaboration, We wouldlike to express our gratitude to Harold Tay. Finally, theauthors would also like to thank Public Utilities Board (PUB)management at Pandan Reservoir, Singapore for allowing usto conduct our field experiments.

REFERENCES

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[2] Hyrdoid Inc, . (2012). Underwater mobile docking of autonomousunderwater vehicles. In Oceans, 2012, pages 1–15. In UnderwaterTechnology Symposium (UT), 2013 IEEE International, pages 1–7.

[3] Hayashi, E., Kimura, H., Tam, C., Ferguson, J., Laframboise, J.-M.,Miller, G., Kaminski, C., and Johnson, A. (2013). Customizing anautonomous underwater vehicle and developing a launch and recoverysystem.

[4] Pai, S.; Guerrini,P.; Potter,J.; Maguer,A.; Chitre,M.; Biagini,S. Au-tonomous Initial Capture System for AUV Recovery Proceedings ofthe Underwater Acoustic Measurements: Technologies and Results, 3rdinternational conference and exhibition, Nafplion, Greece, 21-26 June2009.

[5] Osse T.j, Pai S, Maguer A, Chitre M, DeBoni M, Guerrini P, Potter J.Autonomous Launch and Recovery System (AutoLars) for AUVs, Eu-ropean conference on Underwater Acoustics (ECUA), Istanbul, Turkey,5-9 July 2010

[6] Maguer A, Pai S, Chitre M, DeBoni M, Guerrini P, Osse J.AutonomousLaunch And Recovery System (AutoLARS) for AUVs , MAST 2010:Rome,10-11 Nov 2010