Tuesday, February 24, 2009

Acoustics Session at SUT 2009

The Acoustics Session at the Subsea Technical Conference 2009 was chaired by Scott Elsom of L3 Nautronix with presentations by Grant Pusey, PhD student at CMST, Curtin University followed by Sandro Ghiotto, also of L3 Nautronix, and myself, coincidentally an alumnus of L3 Nautronix, representing ACOSP.

Grant spoke about transmission of acoustic signals over horizontal ranges, his work being part of the CSIRO Wealth from Oceans Flagship Cluster on Pipelines. The move towards platform-free fields, deeper water, rugged terrain and subsea facilities drives the need for reliable subsea communications and fault reporting.

The aims of Grant's work include characterisation of the horizontal performance of commercial underwater acoustic modems, and to demonstrate the feasibility of real-time measurement retrieval from subsea facilities. Vertical propagation is relatively easy, exhibiting clean impulse response of first and second reflections with hemispherical propagation.

Deep horizontal propagation, on the other hand, has sound concentrated in surface duct, or within a deep sound channel, and a complex impulse response that requires protocols which have been adapted to avoid intersymbol interference. Grant carried out simulation using the Bellhop program for Gaussian beam ray tracing on a sound field, based on bathymetry from 40km off Rottnest. The result was a negligible decrease in amplitude, i.e. low transmission loss (TL). The plan is to simulate underwater acoustic channels to be able to predict responses for various protocols and symbol codings.

He reported the preliminary results from a sea trial, an upwardly refracting sound speed profile (SSP) from measured conductivity, temperature and density (CTD) measured in situ; the raw data showing snapping shrimp and minke whales as well as recorder clipping. There was a bandwidth discrepancy and need to fix the wrong location of the the recorder using recorder localisation, exploiting TL=20log(R) spherical spreading. Modem reception was unsuccessful over about 500m; ambient noise recorder extremely helpful in assessing performance; 32-sample FFT average over 5ms, 15-24kHz comm band.

Sandro spoke about underwater acoustic surveillance networking - Autonomous Surveillance Sonar Network (AUSSNet) as a Defence Capability Technology Demonstrator (CTD) project. Credits for the paper included my friends and colleagues Mal Cifuentes, Eve Clark and Richard Jarvis among others from L3, in addition to Sandro, and two authors from DSTO (Defence Science Technology Organisation).

The objective of AUSSNet is to report on acoustic events during windows of opportunity while remaining covert. The system comprises a ship-based control and monitoring system (CMS), shore-based CMS, cluster of sensor node hydrophone elements, popup gateway connected to comms satellite via an Iridium RF channel; low frequency passive array (and active source); mid-frequency (14kHz) acoustic modem; array data processing - recording, beam forming, broadband detection, event characterisation and data reduction.

The 150m flexible line array is lightweight, low noise, 48 low frequency 10-500Hz anti-aliasing frequency, three heading sensors used to determine the array orientation. A "ping" shaped signal gives times of arrival for each sensor node used to estimate hydrophone element positions - calibrated for beam forming. Surveillance mode requires acoustic data - data recording and beam forming giving "speckle diagrams", i.e. freq-time, freq-bearing and bearing-time; subset of FRAZ (freq-azimuth) - only send bearing and frequency components of interest, needs about 1kbps bandwidth after data reduction. Track data fusion (lat, long) of events (targets) over about 1 year.

Background and issues:
  • Sea Wasp CTD - sensor array and RF buoy.
  • HAIL (Hydro-Acoustic Information Link) CTD - reliable, long range, low data rate.
  • AUSSNet shift from HAIL rate x10 at 1/10 range.
  • Propagation analysis.
  • Time independent/reverberation analysis.
  • Short symbol swamped by multipath interference.
  • Real-time channel simulator for water depth and surface roughness.
  • Array position calibration signals - loud, low frequency sound, good amplitude and phase linearity (to avoid harmonics), about 150 dB re 1µPa, bandwidth about 80Hz (really?) in 1 cubic foot (!).
  • Popup gateway buoy mechanical design; about 50kg buoyancy.
  • DC modem (used in trucks), single core shield and sea water return.
  • Stabilisation floats for popup gateway UHF comms.
  • Jetty shallow water (20m) and deeper water (150m) demo.
  • Batteries about 300h (2 weeks).
I spoke about subsea networks for system integration to identify an-oft neglected subject that presents first-class problems of the same level as subsea multiphase processing, pipeline reliability and flow assurance in the oil and gas industry - clearly distinguishing ACOSP from my company Systec Engineering. ACOSP is a not-for-profit that seeks to fill the gap between academia and industry. Like ITF, we are interested in facilitating applied research to solve actual problems and to deliver solutions in the security, marine, oil and gas industries.

Western Australia has a uniquely rich marine environment which must be protected - even as it is exploited commercially for fisheries, tourism, oil and gas industries, and scientific exploration. Wireless technologies are flexible to deploy for various applications for which we can envisage a common architecture.

Underwater communications - copper and fibre cable, electromagnetic and acoustic each has pros and cons for any given application. Pitched at a fairly high level, the only technical slide was to introduce m-sequences,(*) also called pseudo-random noise because its energy is spread across a broad band, hence "spread spectrum" - enables covert communications.

A common network architecture can be conceived that satisfies the requirements for subsea systems integration for a range of applications, increasing flexibility and lowering costs. A common problem of subsea processing in oil and gas is subsea integration, i.e. software and systems integration in the subsea environment. One application scenario is a subsea communication network involving multiple submerged surface and other participants - true network needs standards for discovery, channel and protocol negotiation.

Over the course of the conference, we have been hearing there are numerous technical problems needing innovative solutions in subsea processing - particular interest in software and systems integration to support automation. Safety and integrity in remote or hazardous locations is drawing towards "intelligent" autonomous systems. Subsea systems are particularly difficult to access after they have been commissioned so very good telemetry is important for pipelines and multiphase systems.

Message in a bottle (MIB) is a concept invented by Chris Skinner, also an initiator of ACOSP, to collect spatially and temporally diverse data, cheaply, by exploiting miniaturisation and adhoc networking to deploy a low-cost network. The bottles will be dropped in the ocean and will disperse over a wide area with winds and tides. WiFi networking, GPS position and time stamps will slowly deliver data to fill gaps in very large, marine datasets.

Data fusion is important to enable informed decision making, including in the oil and gas industry. We envisage GIS overlays of relevant datasets to fulfil the need for comprehensive sea floor, environmental and benthic(**) studies. In order to deliver these and other innovations, ACOSP has been conceived as a science-based, not-for-profit to facilitate technology transfer and to undertake projects for industry.



(*) m-sequence - maximum length sequence generated by linear feedback shift register; m registers gives sequence of length 2^m - 1.
(**) benthic - living on the sea floor; cf. pelagic fauna meaning organisms that live in the open ocean.

[I note a creative suggestion by Scott Elson in private conversation for the development of a system of commercial acoustic sensors for monitoring fields during extraction operations to optimise flow and extraction, where it is usual to survey 1-2 times per year.]

Sunday, February 22, 2009

Innovative Approaches to Submarine Detection and Ranging

This article is a little late coming and was to have been preceded by a number of articles which were to have gone into some detail ab out acoustic signal processing from fundamentals to advanced concepts. I was forced to withdraw for business reasons but had intended to present a paper at Undersea Defence Technology (UDT Pacific) Conference in Sydney from 4-6 Nov 2008.

The premise of this paper has been raised by a number of parties, including myself at the Defence White Paper 2008 public consultation in Perth, that an innovative approach is needed in submarine detection and anti-submarine warfare (ASW) to deal with the asymmetry in numbers of unfriendly submarines likely to patrol the northern seas of Australia over the next few decades.

The paper was to describe a research programme being proposed to investigate innovative approaches to the detection of submerged submarines. The problem of submarine detection has been approached for many years using a number of conventional techniques based on Doppler processing and array processing in the frequency and time domains.

I claimed that it is highly plausible that increased detection sensitivity can be achieved by applying a combination of techniques from the disciplines of underwater acoustics and target characterisation, advanced signal processing and systems engineering. The paper was to review past and current approaches and then goes on to describe signal processing techniques used in radio-frequency and other array systems that could be adapted for use with acoustic signal processing in the underwater domain.

When time allows, it will be worthwhile describing the current state of the art and to review the current literature. The plan was for the paper to review the theoretical basis of signal processing in both time and frequency domains and to summarise the critical equations and parameters that apply. The interesting part was to follow with a review of the applications in RF, optical and acoustic processing that use similar approaches and then to discuss their suitability for acoustic signal processing for submarine detection.

The techniques include beam formers with pilot signals, a technique from radio-astronomy known as Very Long Baseline Interferometry (VLBI), adaptive signal processing for sound and vibration monitoring (also used for active control of sound and vibration) from architectural acoustics and mechanical engineering.

The systems engineering aspects of the solution that needs to be deployed in order to be able to carry out the signal processing in real-time are non-trivial and themselves pose a first-problem. The paper was to conclude with a proposal for a programme of research and development to be undertaken with the objective being the delivery of a new generation of innovative ASW systems.

Sunday, November 12, 2006

Problem Solving

How to Solve Mathematical Problems

It is useful for engineers of all stripes to build a personal toolkit of problem solving techniques. This small volume serves admirably as an introduction to problem solving and is chockful of motivating examples demonstrating each technique.

The Towers of Hanoi, for example, is a staple problem for budding computer scientists to cut their teeth on algorithm design. By exploring several different approaches to this and other problams, the author Wayne A. Wickelgren takes the challenge away from low-level algorithm implementation and into the realm of selecting an approach.

Wickelgren explores depth first, induction, sub goal and working backwards among others in an ongoing discussion with the reader. First he sets the problem and gives a clue as to how to solve it, then adds more information until, almost magically, the path towards the answer becomes obvious to the reader.

At least it is while reading the lively discourse. Readers will need to put in a lot of practice in order to be able apply these methods in their everyday work. That said, the intent is to empower the reader by adding more tools to his kit of problem-solving techniques.

Software engineers need to know a lot more than just computer science and I heartily recommend this read to anyone who wants to excel in problem solving in a professional setting.

Friday, November 03, 2006

Systems and Signals

The beginning is founded in mathematics and the end in grounded in applications that are rather sophisticated. So perhaps it is best to start in the middle. The foundation stone for systems is grounded in analysis. System technology is what Systec is all about and system analysis is the fashion of work.

The lapis philosophorum or Philosophers Stone might be the dream material for alchemists even if it does not exist. Luckily for us, there is a raft of tools at our disposal for analysing systems from differential equations, matrix and state-space analysis in the fields of control systems and signal processing. Signals are the germ from which our systems grow.

It is perhaps wise when starting in the middle to allude to the beginning and to take a focussed look into the end. To find our feet and to remain grounded. Differential equations of the ordinary and partial kind are incredibly useful tools if sometimes unwieldy. Various transformations into algebraic representations, homeomorphisms in equivalent phase spaces, give us one or more matrix equations instead of a set of differential terms.

For example, Fourier and Laplace transformations provide algebraic terms in linear combinations of s=jw (the complex value j=-1) being s, s^2 and so on to represent the frequency domain response of a system.

There are neat relationships between this formulation and similar expressions in quantum mechanics for representing spin states of fermions and bosons (we will examine elsewhere) that I mention to emphasise the universality of expressions like e^{-st) or e^{-jwt} and related symmetry relations.

The sorts of applications we are envisaging include adaptive control systems for process control, active noise reduction, smart structures like flexible space structures, cardiac regulation, flight control systems, synthetic aperture and other signal processing techniques. The generic approaches are usually categorised into conventional feeedback control systems based on single channel techniques including bode plots for stability and step responses.

Modern control theory embraces multiple input and multiple output channels, utilising state-space control theory to describe such systems using systems of matrix equations called state equations that have solutions for, naturally enough, the evolution of the state of the system in phase space.

Alternative approaches that have started to gain traction in recent years include H-infinity analysis for robustness analysis and whole families of optimisation approaches for solving the parameters of state equations.

One of the most successul approaches takes an altogether different approach to any of these and uses adaptive signal processing to build adaptive feedforward controllers that mimic and counter the behaviour of disturbances on the system that has been modelled online.

The model and controller take the form of finite or indefinite in time impulse response or lattice fillters with various combinations of poles and zeros, perhaps not provably, globally stable. In contrast, conventional regulators in feedback control systems are provably, globally stable in many instances.

Some of the most exciting applications of adaptive signal processing and adaptive feedforward control include some of those mentioned earlier like adaptive noise control and adaptive array processing for synthetic aperture radar and sonar. These are the areas that I have the most experience in and show the most promise as real developments today in addition to having much scope for further improvement in the future.

Sunday, October 29, 2006

Engineer and Physicist

My initial technical background, aside from years of programming, science and electronics at high school, was an undergraduate degree in Electronic Engineering with coverage from dynamics, statics and electrodynamics to photonics, control and communications. Great stuff. Some luck helped me into Physics, Astrophysics and Acoustics with interludes in Hearing Physiology and Radio Astronomy. So how did I end up in Software Engineering?

Basically I did an apprenticeship under one of the smartest and charismatic people you could hope to meet. I give credit for my initial grounding in Engineering to my family, for my postgrad work to Pan and David, and for my career to Harry (*). The focus was on object-oriented design for underwater acoustics systems in a defence environment. Many of the concepts from my Honours and PhD work in holography, coherent optics, digital signal processing, classical multichannel feedback and adaptive feedforward control found new application in acoustic cameras, zoom FFT, inverse synthetic aperture and other areas within the field where I served my apprenticeship.

This blog is about the technical background and exploration of many of the concepts in the public domain and fundamental science and engineering that I apply, when necessary, in my day-to-day work. A big part of my role is opening up engineers' minds to a broader problem-solving toolkit than they may have from their own limited experience. Technical leadership in and R&D environment, with the emphasis on D, is a wonderfully stimulating and rewarding domain in which to work.

(*) Harry passed away in tragic circumstances. I owe a debt of gratitude to him, Ross and other at Nix for the opportunities and learning experiences they provided me with at that time.