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Tresvig, Johan Ludvig & Lindem, Torfinn
(2011).
Demonstration of a Space Weather experiment on a nano-satellite.
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Balk, Helge; Kubecka, Jan & Lindem, Torfinn
(2010).
Detection, tracking and sizing of fish in data from DIDSON multibeam sonars.
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Lindem, Torfinn
(2010).
CubeSTAR – UiO-studenter bygger fremtidens satellitt for overvåking av romvær.
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Lindem, Torfinn
(2010).
Studentsatellitten CubeSTAR.
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Tresvig, Johan Ludvig & Lindem, Torfinn
(2009).
CubeSTAR - Vision: Demonstrate a new Space Weather satellite concept.
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Bekkeng, Tore Andre; Bekkeng, Jan Kenneth; Lindem, Torfinn; Moen, Jøran Idar; Pedersen, Arne & Trondsen, Espen
(2009).
Development Of A Multi-Needle Langmuir Probe System.
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Balk, Helge; Lindem, Torfinn & Kubecka, Jan
(2009).
New Cubic Cross filter detector for multi beam data recorded with DIDSON acoustic camera.
Show summary
Target detection in data from the DIDSON (dual-frequency identification sonar) is a challenge. Due to high frequency and many narrow beams, the system can take series of acoustic pictures of passing fish. The entire body can be seen, but are often blurred, and unclear in individual pictures. It is only when series of pictures are played like a film, one can see that the target actually is a swimming fish. Hence, a good target detector should not be based on individual frames, but on using series of pictures.
The Crossfilter detector was originally designed for single, dual and split beam systems, to detect targets under very low signal to noise ratio conditions. In order to operate on multi beam data, we have added an extra dimension to the detecting filters. This resulted in a detector that takes into account both the time, range, and width dimensions in the data during the detection process. The talk will focus on the principles and construction of the detector, and discuss experience and test results.
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Balk, Helge & Lindem, Torfinn
(2009).
Target detection in data from splitbeam echosounders and multi beam sonar systems.
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Lindem, Torfinn
(2008).
CubeStar - en teknologisk utfordring.
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Lindem, Torfinn
(2008).
CubeStar - Layout og kravspesifikasjoner.
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Lindem, Torfinn
(2008).
"Space Technology" ved UiO.
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Lindem, Torfinn
(2008).
Instrumentering for CubeSat.
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Balk, Helge & Lindem, Torfinn
(2008).
Analysis and visualization of data from DIDSON Acoustic camera.
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Lindem, Torfinn
(2008).
Fysikk og Teknologi - Elektronikk.
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Balk, Helge; Lindem, Torfinn; Kubecka, Jan & Frank, Knudsen
(2007).
Recent advances in solving signal-to-noise problems in fisheries acoustics.
Show summary
Signal is what we want while noise is disturbance that prevents us from getting it. Especially for horizontal application, surface, bottom, temp profile and unknown fish aspect can cause breakdown in the sonar equations and serious fish stock estimation errors. Noise may blur, hide or misshape the signal so that the echoes cannot be recognised by traditional detectors. Noise may also appear as disguised target echoes and bias the estimates.
We will never be able to conquer the noise. However, new detectors and target noise separators, aspect de-convolution, multi beam and multi frequency will be discussed as ways to push the limits.
Principle, later improvements and experience with the cross filter detector will be presented.
We have measured and modelled sound propagation in lakes and rivers and will show how temperature gradients influence on the sound propagation and how this break down the assumptions behind the sonar equations and cause errors in the TS and Sv estimates.
Experiences with multi beam and multi frequency are relatively new in fresh water and we will be shared as well.
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Balk, Helge & Lindem, Torfinn
(2007).
Analysis of data from high frequency identification sonars combined with data from conventional echo-sounders.
Show summary
High frequent identification sonars have short range and produce a waste amount of data similar to videos. Analysis of these data is commonly done by watching the recorded videos. For surveys lasting for days or weeks this method is too time consuming. By generating traditional echograms from the sonar data and by combing the sonar data with data from long range split beam echo-sounders, we can improve the systems range and decrease the analysis time.
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Strande, Mona & Lindem, Torfinn
(2006).
Fysikere lover trådløs strøm.
[Newspaper].
Teknisk Ukeblad.
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Jemterud, Torkild & Lindem, Torfinn
(2006).
Verd å vite - Radio kringkasting 100 år.
[Radio].
NRK.
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Lindem, Torfinn & Larsen, Kristoffer Vikebak
(2006).
Vektløse studenter.
[Internet].
forskning.no.
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Balk, Helge; Lindem, Torfinn & Kubecka, Jan
(2006).
Enhenced Crossfilter detector for target detection in data from echosounders.
Show summary
Acoustic target detection is commonly carried out with single echo detectors based on echo length. These detectors test one ping at a time and look for echoes with an assumed correct echo length. Obtained echoes are filtered with a set of criteria such as phase stability and position in the beam before they are accepted. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the echo length detector to produce fractionated tracks from fish surrounded by numerous noise detections.
Echo length detectors utilize only small portions of the information available in a split beam echogram. By including information from more than one ping and from the background reverberation, a more robust fish detector has been designed. This detector, called the Cross Filter Detector (CFD), has been further improved by extracting detection power from the variance in the echogram
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Lindem, Torfinn; Øye, Henning Karleif & Farnes, Jarle
(2006).
MEMS Accelerometer and Gyro Experiment in Sounding Rockets.
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Balk, Helge; Lindem, Torfinn & Kubecka, Jan
(2005).
New single echo detection methods for shallow water fisheries acoustics.
Show summary
Acoustic target detection is commonly carried out with parametric single echo detectors. These detectors test one ping at a time and look for echoes fulfilling a set of criteria such as echo duration and shape. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the parametric detector to produce fractionated tracks from fish surrounded by numerous noise detections.
Parametric detectors utilize only small portions of the information available in a split beam echogram. By including information from more than one ping and from the background reverberation, a more robust fish detector has been designed. This detector, called the Cross Filter Detector (CFD), has now been further improved by applying the variance in the split beam echo sounders phase measurements
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Balk, Helge; Lindem, Torfinn & Kubecka, Jan
(2005).
New single echo detection methods for shallow water fishery acoustics.
Show summary
Acoustic target detection is commonly carried out with parametric single echo detectors. These detectors test one ping at a time and look for echoes fulfilling a set of criteria such as echo duration and shape. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the parametric detector to produce fractionated tracks from fish surrounded by numerous noise detections.
Parametric detectors utilize only small portions of the information available in a split beam echogram. By including information from more than one ping and from the background reverberation, a more robust fish detector has been designed. This detector, called the Cross Filter Detector (CFD), has now been further improved by applying the variance in the split beam echo sounders phase measurements.
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Balk, Helge & Lindem, Torfinn
(2003).
Fish detection based on spectral differences in the echogram's range and temporal domain.
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Balk, Helge & Lindem, Torfinn
(2003).
A new method for single target detection.
Show summary
We have experienced that fish detection in sonar-data from horizontally
aligned transducers in shallow water can be difficult. Studying
recorded material revealed that echoes from fish often took on high
ping to ping variations in variables like pulse length, intensity,
shape and phase. Common single-echo detection methods based on a set of
echo criteria tended to overlook echoes from fish. At the same time
echoes from unwanted targets and fluctuations in the background
reverberation was accepted. Hence, tracking and fish counting became
difficult.
To overcome these problems, we have developed a detection method based
on 2-dimensional low-pass filters. By adjusting the cut-off frequency
in the time and range domain, noise can be removed, ping to ping
variation in signals from fish reduced, and local background
reverberation level can be found. By subtracting the echo intensity in
the filtered echograms, traces from fish can be detected.
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Balk, Helge & Lindem, Torfinn
(2002).
Fish detection in rivers with split-beam sonars.
Show summary
Split beam echo sounders can monitor fish in shallow rivers. Positioned horizontally and normal to the river current at a suited place, the conical sound beam can cover major parts of the river cross-section and register passing fish. However, the nearby bottom and surface, air bubbles and debris makes the analysis of the echo sounder data difficult. A summary of our experiences from the last six years with data collection and analysis is presented. Methods such as single echo detections, tracking, cross filter detection and classification are discussed.
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Lindem, Torfinn; Balk, Helge; Vibeke, J. & Kjølerbakken, K. M.
(2002).
Measurements of acoustic sound fields in a shallow river.
Show summary
From earlier experiments, we have seen indications that the sound field
from a horizontally aligned transducer in shallow water can differ from
the expected field. If this is the case, this phenomenon can have an
impact on the measured target strength from fish and on sample volume.
Hence, it has been important to verify this by controlled experiments.
We have measured the sound field produced by a Simrad ES120-4, 120kHz,
4x10 deg. split beam transducer in a shallow river. These measurements
were made with a Reason hydrophone. Reflections from the bottom and
surface made modifications to the transmitted pulse. In some positions
they could even form separate pulses. These pulses were sufficiently
separated in time to produce phantom echoes. Results from this
experiment will be presented.
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Lindem, Torfinn
(2001).
EK60 Split Beam Sonar used in shallow water.
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Lindem, Torfinn
(2001).
Sound measurements under ice.
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Lindem, Torfinn; Balk, Helge & Knudsen, Frank R.
(1999).
Sound propagation through shallow water can make unexpected modification to fish echoes recorded with a horizontally looking split beam system.
Show summary
In several experiments with a 120kHz split beam system we have noticed
that shallow water in some cases can make dramatic changes to the echo
signal. Especially angular measurements in the vertical plane can be
modified by surface and bottom boundaries. Erroneous measurements of
position and target strength can be the result if beam mapping is not
done in a proper way before recording of actual fish echoes. The paper
will demonstrate some of these problems.
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Lindem, Torfinn
(1999).
Data from single beam sonar analysed with HADAS. (Hydro Acoustic Data Acquisition System).
Show summary
Experiences from using indirect statistical methods to analyse single beam data. A review.
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Balk, Helge & Lindem, Torfinn
(1999).
Fish tracking in shallow water by image processing.
Show summary
Automatic fish-counting in data from Split-beam sonar is traditionally
performed by a single echo detection (SED) process followed by some
sort of neighborhood tracking. The SED process uses parameters such as
echo pulse duration and phase deviation to remove echoes with non-valid
angular measurements. The neighborhood tracking algorithm then combines
echoes close in time and position. This method works fine on split-beam
data from vertically mounted sonar in open water. However, with
horizontally mounted sonars in shallow water rivers this method tends
to fail. In order to use hydroacoustics in shallow river fish stock
assessment, improvements on the counting method has to be done with
methods from image processing.
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Lindem, Torfinn & Balk, Helge
(1999).
Problems related to Time Varied Gain (TVG) and phase measurements with elliptic split beam transducers in shallow water, - some observations.
Show summary
For several years split beam echo sounders have been used by biologists
for counting fish in shallow water. Special elliptic transducers have
been developed by manufactures like SIMRAD and HTI for use in
horizontal applications in shallow lakes and rivers. For some time
users have reported ¿strange behaviour¿ of their systems in some
locations. During our ongoing project for new counting methods for fish
in rivers we have experienced these effects and can demonstrate how
boundary effects from surface and bottom can give unexpected
modifications to echo signals received with split beam systems.
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Balk, Helge & Lindem, Torfinn
(1998).
Hydroacoustic fish counting in rivers and shallow waters, with focus on problems related to tracking in horizontal scanning sonars.
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Juell, Jon-Erik; Lindem, Torfinn; Fosseidengen, Jan Erik; Al Houari, Driss & Bjordal, Åsmund
(1995).
Hydroakustisk overvåkning og styring av lakseproduksjon.
Fisken og Havet.
ISSN 0071-5638.
Show summary
Effektiv og miljøvennlig merdproduksjon av laks krever gode overvåknings- og kontrollrutiner. Utprøving av det kombinerte overvåknings- og fóringssystemet "Merdøye" indikerte at aktiv bruk av systemet i kommersielt oppdrett reduserer sjansen for fórspill (inkl. antibiotika). Systemets automatiske appetittfóring, basert på hydroakustisk måling av laksens beiterespons, fungerte uavhengig av merdstørrelse. Systemet er videre spesielt nyttig for generell overvåkning av laks i store dype enheter.
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Lindem, Torfinn
(2003).
Comparison and development of assessment methods for pelagic fish stocks in northern great lakes.
Nordisk ministerråd.
Show summary
At the moment, hydro-acoustics is the most important fish stock assessment method not depending on catches or reconstruction from other historical data. The precision of the method is found to be fairly good (Bagenal & al., 1982). Assessment of pelagic fish stocks for research and management purposes is one of the most important practices of hydro-acoustics in Finnish lakes. For more than ten years hydro-acoustics has been the only method used to monitor the pelagic fish-stock in the three largest Swedish lakes. In Norway, the method is used in many lakes both for monitoring and research purposes. In Russia, hydro-acoustics has proved to be almost the only available stock assessment method because the availability of statistics collapsed with the centralised fishery in the beginning of 1990's. Our intention was to undertake an inter-calibration to evaluate the ways hydro-acoustics is used in the four countries.