Torfinn Lindem

• Bekkeng, Tore Andre; Helgeby, Espen Sørlie; Pedersen, Arne; Trondsen, Espen; Lindem, Torfinn & Moen, Jøran Idar (2019). Multi-Needle Langmuir Probe System for Electron Density Measurements and Active Spacecraft Potential Control on CubeSats. IEEE Transactions on Aerospace and Electronic Systems. ISSN 0018-9251. 55(6), p. 2951–2964. doi: 10.1109/TAES.2019.2900132. Full text in Research Archive
• Tresvig, Johan Ludvig & Lindem, Torfinn (2016). Investigating the Coupling Effects between CubeSTAR's Communication Antenna and Deployable Payload Sensors [Measurements Corner]. IEEE Antennas & Propagation Magazine. ISSN 1045-9243. 58(1), p. 90–94. doi: 10.1109/MAP.2015.2501234. Full text in Research Archive
• Tresvig, Johan Ludvig & Lindem, Torfinn (2015). Design of a communication system for a nanosatellite space weather mission. IEEE Aerospace and Electronic Systems Magazine. ISSN 0885-8985. 29(11), p. 22–29. doi: 10.1109/MAES.2014.130177. Full text in Research Archive
• Tresvig, Johan Ludvig & Lindem, Torfinn (2015). A miniaturized subpayload system used for measurements in the ionosphere - A design concept. IEEE Aerospace and Electronic Systems Magazine. ISSN 0885-8985. 30(4), p. 24–28. doi: 10.1109/MAES.2015.140154. Full text in Research Archive
• Bekkeng, Tore Andre; Jacobsen, Knut Stanley; Bekkeng, Jan Kenneth; Lindem, Torfinn; Lebreton, JP & Moen, Jøran Idar [Show all 7 contributors for this article] (2010). Design of a multi-needle Langmuir probe system. Measurement Science and Technology. ISSN 0957-0233. 21(8). doi: 10.1088/0957-0233/21/8/085903. Full text in Research Archive
• Balk, Helge & Lindem, Torfinn (2000). Improved fish detection in data from split-beam sonar. Aquatic Living Resources. ISSN 0990-7440. 13, p. 297–303. Full text in Research Archive Show summary

  Hydroacostic split-beam techniques have been applied to enumerate salmon migrating in the river Tana (northern Norway) during summer 1998 and 1999. Analysing data by single echo detection and tracking was difficult. Missing echoes in tracks from fish, combined with noise from the single echo detector was seen as a reason for this. An improved counting method is presented. Contours from moving targets are detected by image analysis. Then, detected single echoes within these contours are combined into tracks. This procedure reduces problems related to noise and to tracking fishing with few accepted single echoes.

• Rudstam, Lars G.; Lindem, Torfinn & LaBar, G. (1999). Methodology for Target Strength Measurements (With special reference to in situ techniques for fish and mikro-necton) The single beam analysis. In Ona, E (Eds.), ICES Cooperative Research Report No.235. International Council for the Exploration of the Sea, Copenhagen K Denmark. p. 6–13. Full text in Research Archive Show summary

  This report has been produced as a result of discussions in the Fisheries Acoustics Science and Technology (FAST) Working Group of the International Council of the Exploration of the Sea (ICES). Following discussions in the FAST Working Group, it was proposed that a study group on Target Strength Methodology be formed, which was recommended by the Fish Capture Committee. This resulted in ICES Resolution C Res 1992 2:11:¿A Study group on Target strength Methodology is established under the Chaiship of E.Ona (Norway) and will meet in Gothenburg, Sweden on April 19 1993 to prepare a report, with a view to publication in the ICES Cooperative Report Series on the methodology for Target Strength measurements with special reference to in situ techniques for fish and micro- nekton¿

• Rudstam, Lars G.; Hansson, Sture; Lindem, Torfinn & Einhouse, Donald W. (1999). Comparison of target strength distributions and fish densities obtained with split and single beam echo sounders. Fisheries Research. ISSN 0165-7836. p. 207–214. Full text in Research Archive Show summary

  We compared results from analysis of split-beam and single-beam acoustic surveys using two data sets, one from the Baltic Sea (both systems at 70kHz) and one from Lake Erie(70kHz single-beam and 120 kHz split-beam. First,we show that there is a bias towards smaller targets associated with implementation of a commonly used algorithm for deconvolving target strength distributions from single beam (modified Craig-Forbes). This bias is 0.8dB for a circular transducer. Target strength distributions obtained by split and single beam methods were very similar for the Baltic Sea data resulting in almost identical overall average TS. This was also true for epilimnetic targets strength distributions in Lake Erie. However, hypolimnetic target strength distributions still yielded smaller target strengths with single than with the two split beam units.

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• Grande, Jøran; Birkeland, Roger; Lindem, Torfinn; Schlanbusch, Rune; Houge, Torbjørn & Mathisen, Stian Vik [Show all 7 contributors for this article] (2013). EDUCATIONAL BENEFITS AND CHALLENGES FOR THE NORWEGIAN STUDENT SATELLITE PROGRAM. Full text in Research Archive
• Tresvig, Johan Ludvig & Lindem, Torfinn (2011). Demonstration of a Space Weather experiment on a nano-satellite. Full text in Research Archive
• Lindem, Torfinn (2010). Studentsatellitten CubeSTAR. Full text in Research Archive
• Balk, Helge; Kubecka, Jan & Lindem, Torfinn (2010). Detection, tracking and sizing of fish in data from DIDSON multibeam sonars. Full text in Research Archive
• Lindem, Torfinn (2010). CubeSTAR – UiO-studenter bygger fremtidens satellitt for overvåking av romvær. Full text in Research Archive
• Bekkeng, Tore Andre; Bekkeng, Jan Kenneth; Lindem, Torfinn; Moen, Jøran Idar; Pedersen, Arne & Trondsen, Espen [Show all 7 contributors for this article] (2009). Development Of A Multi-Needle Langmuir Probe System. Full text in Research Archive
• Balk, Helge & Lindem, Torfinn (2009). Target detection in data from splitbeam echosounders and multi beam sonar systems. Full text in Research Archive
• Tresvig, Johan Ludvig & Lindem, Torfinn (2009). CubeSTAR - Vision: Demonstrate a new Space Weather satellite concept. Full text in Research Archive
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2009). New Cubic Cross filter detector for multi beam data recorded with DIDSON acoustic camera. Full text in Research Archive 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.

• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2008). Split beam versus multi beam for monitoring migrating fish in rivers and shallow lakes. Full text in Research Archive
• Balk, Helge & Lindem, Torfinn (2008). Analysis and visualization of data from DIDSON Acoustic camera. Full text in Research Archive
• Lindem, Torfinn (2008). CubeStar - en teknologisk utfordring. Full text in Research Archive
• Lindem, Torfinn (2008). "Space Technology" ved UiO. Full text in Research Archive
• Lindem, Torfinn (2008). CubeStar - Layout og kravspesifikasjoner. Full text in Research Archive
• Lindem, Torfinn (2008). Instrumentering for CubeSat. Full text in Research Archive
• Lindem, Torfinn (2008). Fysikk og Teknologi - Elektronikk. Full text in Research Archive
• Balk, Helge & Lindem, Torfinn (2007). Analysis of data from high frequency identification sonars combined with data from conventional echo-sounders. Full text in Research Archive 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.

• Balk, Helge; Lindem, Torfinn; Kubecka, Jan & Frank, Knudsen (2007). Recent advances in solving signal-to-noise problems in fisheries acoustics. Full text in Research Archive 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.

• Lindem, Torfinn; Øye, Henning Karleif & Farnes, Jarle (2006). MEMS Accelerometer and Gyro Experiment in Sounding Rockets. Full text in Research Archive
• Strande, Mona & Lindem, Torfinn (2006). Fysikere lover trådløs strøm. [Journal]. Teknisk Ukeblad. Full text in Research Archive
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2006). Enhenced Crossfilter detector for target detection in data from echosounders. Full text in Research Archive 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

• Lindem, Torfinn & Larsen, Kristoffer Vikebak (2006). Vektløse studenter. [Internet]. forskning.no. Full text in Research Archive
• Jemterud, Torkild & Lindem, Torfinn (2006). Verd å vite - Radio kringkasting 100 år. [Radio]. NRK. Full text in Research Archive
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2005). New single echo detection methods for shallow water fishery acoustics. Full text in Research Archive 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.

• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2005). New single echo detection methods for shallow water fisheries acoustics. Full text in Research Archive 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

• Balk, Helge & Lindem, Torfinn (2003). A new method for single target detection. Full text in Research Archive 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.

• Balk, Helge & Lindem, Torfinn (2003). Fish detection based on spectral differences in the echogram's range and temporal domain. Full text in Research Archive
• Lindem, Torfinn; Balk, Helge; Vibeke, J. & Kjølerbakken, K. M. (2002). Measurements of acoustic sound fields in a shallow river. Full text in Research Archive 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.

• Balk, Helge & Lindem, Torfinn (2002). Fish detection in rivers with split-beam sonars. Full text in Research Archive 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.

• Lindem, Torfinn (2001). EK60 Split Beam Sonar used in shallow water. Full text in Research Archive
• Lindem, Torfinn (2001). Sound measurements under ice. Full text in Research Archive
• 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. Full text in Research Archive 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.

• Balk, Helge & Lindem, Torfinn (1999). Fish tracking in shallow water by image processing. Full text in Research Archive 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.

• Lindem, Torfinn (1999). Data from single beam sonar analysed with HADAS. (Hydro Acoustic Data Acquisition System). Full text in Research Archive Show summary

  Experiences from using indirect statistical methods to analyse single beam data. A review.

• 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. Full text in Research Archive 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.

• Balk, Helge & Lindem, Torfinn (1998). Hydroacoustic fish counting in rivers and shallow waters, with focus on problems related to tracking in horizontal scanning sonars. Full text in Research Archive
• Juell, Jon-Erik; Lindem, Torfinn; Fosseidengen, Jan Erik; Houari, Driss Al & Bjordal, Åsmund (1995). Hydroakustisk overvåkning og styring av lakseproduksjon. Fisken og Havet. ISSN 0071-5638. Full text in Research Archive 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.

• Lindem, Torfinn (2003). Comparison and development of assessment methods for pelagic fish stocks in northern great lakes. Nordisk ministerråd. Full text in Research Archive 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.

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