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A daily view of all the goings-on at ASTRON and JIVE.

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    © Onsala Space Observatory

    An interesting pattern appeared in the data rate output from the Onsala LOFAR station (SE607) when survey observations in an ILT(*) project were started. It is simply a coincidence of varying observation times and a beating with the rate sample time and the plot resolution. But it did cause some unwarranted worry about link stability at first.

    (*) ILT: International LOFAR Telescope


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    © Mao et al. 2015

    Wide-band radio polarimetry is a unique probe of magnetized gas in galaxies. Faraday rotation and depolarization effects can be used to derive both large-scale and small-scale magnetic field properties. In this talk, I will present new Jansky Very Large Array polarization observations of the Whirlpool galaxy (see image above) and what they tell us about the large-scale magnetic field and turbulence in M51's halo. In addition, I will describe on-going efforts to measure magnetic fields in HVCs and interacting galaxies.

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    © Pieter Benthem

    After a few months of intensive testing at the ICRAR laboratoires, we are now deploying the RF-over-Fibre (RFoF) upgrade on the LFAA Aperture Array Verification System 0.5 (AAVS0.5). This array is situated on the Murchison Radio-astronomy Observatory (Western Australia), next to the MWA and ASKAP telescopes.

    The picture shows 2 modified MWA beam former boxes (with green caps on), housing the 16 RFoF links. These boxes handle the X-polarization. One box is the transmitter and the other the receiver, with the fibre (black surface cabling, laid out in a loop behind the boxes) in between, replacing the coaxial cabling. The third box is unmodified, and handles the Y-polarization.

    The ASTRON hardware, as can be seen on the daily image of 05-01-2015, is driving this development.

    In the coming days, we will wrap up the installation and start testing the array. Before we disassembled the coaxial cabling, we performed a reference measurement on the bright radio source Hydra A, which we will be repeated when the array is up and running.

    The Low Frequency Aperture Array (LFAA) covers the lowest frequency band of the SKA, from 50MHz up to 350MHz, as described in the Baseline Design. To transport the dual polarised antenna signals over the required distance of up to 10 kilometres towards the processing facilities, RFoF technology is used.

    Many thanks for all the work done by the ICRAR and ASTRON team!


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    © ASTRON

    At the recent opening of the new and refurbished buildings at ASTRON, the State Secretary of OCW, Drs. Sander Dekker unveiled a historical timeline of ASTRON. The timeline depicts some of the main events that have occurred with an emphasis on new developments rather than organisation changes. Of course, as well as commenting on the events presented in the timeline, we can also argue about what is missing - space limitations meant that several key events did not make it into the final cut.

    The timeline begins with 3 important contributions that were essential for the development of radio science and techniques. The first was the publication by James Clerk Maxwell of his theory of electro-magnetism almost exactly 150 years ago. 20 years later, Heinrich Hertz became the first person to generate man-made radio waves, confirming Maxwell's theory. Some ten years after that, Guglielmo Marconi began to use radio waves as a way of enabling long-distance communication. And 20 years after that, radar was invented by Robert Watson Watt changing the course of European history. The rest, as they say, is history. Today these fundamental developments underpin everything we do at ASTRON, and radio science itself pervades all of society - our mobile phones, our GPS systems, civilian and military radar, global satellite communications, wifi and the internet.

    The timeline is of course incomplete - there is a bright future ahead for ASTRON and there is still quite some space on the wall to detail our continuing story of discovery in radio astronomy!

    Many thanks to those that shared their images used in the timeline, incl. Tom Oosterloo, Joeri van Leeuwen, Michiel van Haarlem, Huib van Langevelde, Richard Porcas. Also many thanks to the many people who helped in retrieving images from the archive etc.


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  • 03/29/15--17:00: Dwingeloo wildlife
  • © Tom Oosterloo/Cees Bassa

    Being located in a national park, Astron is surrounded by interesting wildlife. Right next to the Dwingeloo dish is a small wooded bank that is well known to be the location where many vipers (adders) choose to hide for the winter. One tradition is to try to spot the first adders waking up after a long winter of hibernation. This usually happens around the first week of March and if you pass the telescope in this period, you can often see people staring into the bush, trying to spot one. They are hard to see (there is one in the picture), but sometimes you can see several lying in the sun, warming up before going out to hunt for food.

    Recently, a wolf was spotted very close to the WSRT. People did not go out to see that animal�


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    © Vlad Kondratiev

    PSR J0815+4611, or "Candi2" is the 13th pulsar (a "baker's dozen") discovered with LOFAR, and it's the first found in a targeted search.

    It was first identified by Ger de Bruyn in the deep EOR observations of the 3C196 field as a point source with very high polarisation fraction (∼50%) and steep spectrum (index Navarro, de Bruyn, Frail et al. 1995, ApJ, 455, 55).

    We performed the follow-up 1-h HBA observation with the full core tied-array beam. We searched the dedispersed data for periodic and single-pulse signals and to our excitement we found the pulsar! It's a long-period pulsar with the period P = 434 ms and dispersion measure of 11.28 pc/cm^3; the latter corresponds to a distance of only about 400 pc. From the beamformed data we derived a rotation measure of +3.35 rad/m^2, a high fraction of linear polarisation (>50%), a mean flux density of about 8 mJy, and a very steep spectral index of -2.6. These parameters all agree precisely with what was previously inferred from the EOR images. Thus, the pulsar must be Candi2!

    The left figure shows the polarimetric image at a Faraday depth of +3.5 rad/m^2 (uncorrected for ionosphere). Candi2 is in the middle with other diffuse features being from polarised Galactic foreground emission with a similar Faraday depth. On the right is the diagnostic plot from our pulsar search, showing the pulse profile (repeated twice) as a function of time and frequency.

    All the LOFAR pulsar discoveries so far can be found on the LOFAR Tied-Array All-Sky Survey (LOTAAS) web-page here.


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    © Astron

    "In theory there is no difference between theory and practice, but in practice there is"(*). This is why Gijs, Sjouke and Sieds (left to right on the photo) make mock-up boards first, before ordering the actual printed circuit boards. This allows us to verify whether the boards will really fit together in the rack as specified.

    The mock-up boards are made in-house by Albert van Duin. The photo shows one for the OEB (Optic to Electrical Board), the COBI (Correlator Backplane Interfaces) and the miniPAC (mini version of PAC Power and Clock distribution). The Apertif correlator will use these new boards in a subrack, together with 8 UniBoards and 1 PAC board. We already have the UniBoard and PAC boards because they are also used in the Apertif beamformer subrack.


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    © ESA, NASA & Felix Mirabel

    Accreting neutron stars have been studied intensively, although the investigations of the accretion physics involved have focused on systems with relatively high accretion rates: typically >1% of the Eddington mass accretion rate. However, a wide variety of physics is involved in sub-Eddington accretion flows such as the physics of radiative inefficient accretion flows, physics of jets, influence of stellar magnetic fields and surfaces on the accretion process, and instabilities in the accretion flow.

    Over the last decade a lot of new and exciting observational information has been obtained for systems accreting at rates that are orders of magnitude lower than commonly observed. During this colloquium I will present an overview of our current understanding of sub-Eddington accretion flows in neutron-star low-mass X-ray binaries and I will also highlight the differences and similarities between those systems and the transitional millisecond pulsars (i.e., during their accretion phase), which are a recently recognized special class of neutron-star binaries that can accrete at very low rates.


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    © Science journal and Wolfgang Steffen (Instituto de Astronomía, UNAM)

    The current paradigm of star formation through accretion disks, and magnetohydrodynamically driven gas ejections, predicts the development of collimated outflows, rather than expansion without any preferential direction.

    We have observed and reported in Science the evolution of the collimation of a radio jet emitted from the high-mass protostar W75N(B)-VLA2. A pair of radio images of the young star, made 18 years apart, has revealed a dramatic difference in morphology that is providing us with a unique, "real-time" look at how massive stars develop in the earliest stages of their formation. The first image obtained with the Very Large Array (VLA) in 1996 shows a compact source of a hot, ionised wind ejected from the young star (top left panel). The 2014 image, observed with the Jansky VLA, shows that ejected wind deformed into a distinctly elongated outflow (left bottom panel).

    Furthermore, the magnetic field around W75N(B)-VLA2 developed a preferred direction aligned with the large scale magnetic field in the region. Now we have observed that also the outflow is in the same direction, indicating that magnetic forces are important in the formation of this star.

    We think that the young star is forming in a dense, gaseous environment, and is surrounded by a doughnut-shaped, dusty torus. The star has episodes in which it ejects a hot, ionised wind for several years. According to a simple model, that wind can initially expand in all directions, forming a spherical shell around the star. Later, the wind hits the dusty torus, which slows it. Wind expanding outward along the poles of the torus, where there is less resistance, moves more quickly, resulting in an elongated shape for the outflow.

    Image caption:

    Left panels: Comparison of the K band continuum emission of VLA 2 in epoch 1996 (top) and 2014 (bottom). Image Credit: Science journal.

    Right panels: 3D hydrodynamic simulation and visualization (generated in the Shape software) of an episodic, short-lived, originally isotropic outflow ejection (top) from the massive protostar W75N(B)-VLA 2, whose morphology evolves into a collimated outflow (bottom) as it expands within a toroid of dust and gas. Image Credit: Wolfgang Steffen, Instituto de Astronomía, UNAM.


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  • 04/05/15--17:00: Dome face-to-face meeting
  • © ASTRON/H.J.Stiepel

    Twice a year, representatives of the Dome team come together to share knowledge and discuss progress on a face-to-face basis. This year's first meeting was held March 25-27th at ASTRON. Present were 32 colleagues from ASTRON, IBM Netherlands at ASTRON, IBM ZRL and SKA South Africa.

    At this meeting, an overview was presented of the current status of the SKA project and of the SKA Preliminary Design Reviews. After these presentations, each workstream reported on their progress, relevance to SKA, and plans for going forward. Overall, we could conclude significant and important progress in all areas. This is also witnessed by the impressive number of 42 publications in 2014 and 14 publications in the pipeline. In addition, each workstream reported on planned publications.

    On the SKA front, just after the re-baselining, we can report on significant impact on several areas of the SKA program. This includes design work for the SDP consortium in which ASTRON-DOME leads two central work-packages, and the RFoF work which is currently being tested in Australia. In addition,

  • the "algorithms and machines" workstream (P1) has created a working tool for design space exploration: it was reported that over 350 design alternatives of a specific processor could be evaluated within 50 seconds.

  • The "access parttern" workstream (P2) has working tools for storage system sizing and is asked to move forward with incorporating SKA's regional data centers.

  • The "photonics" workstream (P3) showed first measurement results of an all-optical beamformer.

  • In the "microserver" workstream (P4), South Africa reported on their approach for MeerKat, using small, existing Nvidea Tegra cards and apply oil-cooling because of their short time to deployment (end 2016). Although the delivery of T4240 microservers for use in the Users Platform is delayed, the microserver makes steady progress and the 8+1 system is ready for test.

  • The "accellerator" workstream (P5) reported on an impressive set of implementations of the correlator in GPU/DSP systems. Near-memory accelerators are moving to prototype their implementation on Power8.

  • The "novel algorithms" workstream (P6) reported on novel algorithms and good initial results and performance for calibration and imaging. The latter one uses the Gram-Schmidt approach and abolishes gridding and FFT.

  • Finally, the "real-time communications" workstream (P7) reported on direct remote memory access being operational for SSD devices and a reduction in power of at least a factor of 2.

    At the end of the second meeting day, the team also met with companies and a knowledge institute interested in the Dome R&D, and potentially interested in joining the Dome Users Platform. All in all, this was an excellent meeting which showed great progress and a truly collaborative atmosphere.


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    © JIVE

    For upcoming ASTRON/JIVE colloqiua, click here

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    © Francesco de Gasperin

    LOFAR-LBA phase solutions of 3C196 of a 6h observation obtained using LoSoTo (the LOFAR Solution Tool) referenced to CS001. Every panel is one station, on the X axis the observing time in hours, on the Y axis the observing frequency in MHz (22-70 MHz).

    Ionosphere and clock drifts drive most of the variation, both these effects are time dependent. As a consequence remote stations, which have independent clocks and observe through a significantly different ionosphere, change phase faster then

    core stations.

    The combined effect of clock and ionosphere is clearly marked also by the fast phase rotation of remote stations. For some of the remotest stations a ionospheric-induced frequency dependency of the phase solutions is also visible (e.g last panel). White patches are flags applied by LoSoTo using amplitude-only information (see subsequent daily image).


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    © Dieter Engels

    Distances to AGB stars with optically thick circumstellar shells cannot be determined using optical parallaxes. However, for stars with OH 1612 MHz maser emission emanating from their circumstellar shells, distances can be determined by the phase-lag method. This method combines a linear diameter obtained from a phase-lag measurement with an angular diameter obtained from interferometry.

    The phase-lag of the variable emission from the back and front sides of the shells is determined for 20 OH/IR stars in the galactic disk. It is measured on the base of a monitoring program with the Nancay radio telescope ongoing for more than 6 years. Recent eMERLIN and eVLA observations of several stars provided angular diameters.

    The new distances for the sample will provide an improved estimate of the mass range from which these highly obscured stars descend from. I will discuss the new distances obtained and the uncertainties inherent in the method.


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    © ASTRON/H.J.Stiepel

    Twice a year, representatives of the Dome team come together to share knowledge and discuss progress on a face-to-face basis. This year's first meeting was held March 25-27th at ASTRON. Present were 32 colleagues from ASTRON, IBM Netherlands at ASTRON, IBM ZRL and SKA South Africa.

    At this meeting, an overview was presented of the current status of the SKA project and of the SKA Preliminary Design Reviews. After these presentations, each workstream reported on their progress, relevance to SKA, and plans for going forward. Overall, we could conclude significant and important progress in all areas. This is also witnessed by the impressive number of 42 publications in 2014 and 14 publications in the pipeline. In addition, each workstream reported on planned publications.

    On the SKA front, just after the re-baselining, we can report on significant impact on several areas of the SKA program. This includes design work for the SDP consortium in which ASTRON-DOME leads two central work-packages, and the RFoF work which is currently being tested in Australia. In addition,

  • the "algorithms and machines" workstream (P1) has created a working tool for design space exploration: it was reported that over 350 design alternatives of a specific processor could be evaluated within 50 seconds.

  • The "access parttern" workstream (P2) has working tools for storage system sizing and is asked to move forward with incorporating SKA's regional data centers.

  • The "photonics" workstream (P3) showed first measurement results of an all-optical beamformer.

  • In the "microserver" workstream (P4), South Africa reported on their approach for MeerKat, using small, existing Nvidea Tegra cards and apply oil-cooling because of their short time to deployment (end 2016). Although the delivery of T4240 microservers for use in the Users Platform is delayed, the microserver makes steady progress and the 8+1 system is ready for test.

  • The "accellerator" workstream (P5) reported on an impressive set of implementations of the correlator in GPU/DSP systems. Near-memory accelerators are moving to prototype their implementation on Power8.

  • The "novel algorithms" workstream (P6) reported on novel algorithms and good initial results and performance for calibration and imaging. The latter one uses the Gram-Schmidt approach and abolishes gridding and FFT.

  • Finally, the "real-time communications" workstream (P7) reported on direct remote memory access being operational for SSD devices and a reduction in power of at least a factor of 2.

    At the end of the second meeting day, the team also met with companies and a knowledge institute interested in the Dome R&D, and potentially interested in joining the Dome Users Platform. All in all, this was an excellent meeting which showed great progress and a truly collaborative atmosphere.


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    © Daniele Michilli / PWG

    Michilli, Hessels, Cooper, Stappers, Kondratiev & van Leeuwen

    Rotating RAdio Transients (RRATs; McLaughlin et al. 2006) are

    sporadically emitting pulsars. Finding more RRATs is important in

    order to have a complete picture of the radio-emitting neutron star

    population. Also, understanding why their behavior is in some cases

    quite different compared to "steady" pulsars is important for

    understanding the pulsar radio-emission mechanism.

    LOFAR's first RRAT discovery is shown in this plot, where the pulses

    from the neutron star are highlighted in blue. The bottom panel of

    the plot shows all the significant pulses detected in beam 56 of

    sub-array pointing 2 of observation L202425. These are plotted as a

    function of dispersion measure and time. The discovered RRAT has a

    dispersion measure close to 78 pc/cm3 and a period of 2.23 s (or some

    integer fraction of this). About ten bright single pulses have been

    detected from the source in the one-hour discovery observation.

    The discovery has been made as part of the LOFAR Tied-Array All-Sky

    Survey (LOTAAS), an ongoing survey for pulsars and fast radio

    transients, which has previously discovered another 13 new pulsars

    (www.astron.nl/lotaas). The irregular emission of RRATs makes them

    difficult to detect in periodicity searches. For my PhD I am

    developing new techniques to sift through the LOTAAS data in order to

    find more individual dispersed pulses. This is a very challenging

    task because each LOTAAS pointing contains 222 beams, each with

    thousands of frequency channels, and millions of time samples!


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    © ASTRON

    A recent AJDI (23-03-2015) shortly mentions the influence of RFI on the design of the WLNT and on noise measurements at specific RFI frequencies. Obviously RFI in the 500 MHz to 1500 MHz frequency range (even at 20 dB below the system noise level) will deteriorate the measured noise performance at the RFI-frequencies. Strong RFI may even render measurements impossible. As long as the RFI-levels do not drive the receiver into saturation, the solution is to ‘flag’ the affected frequency bins and ignore them in further processing. However, due to the inherent non-linearity of receiver systems, RFI may produce intermodulation products at other in-band frequencies. Although these intermods will be at a much lower level than the original RFI-frequencies, they may still influence the noise measurements. In particular second order products in wideband systems like the WLNT may have a large impact. To keep their effect on noise measurements to below 1%, the intermods should remain at least 20 dB under the system noise level.

    Based on the properties of our WLNT receiver system (gain, second order intercept point) and measured RFI-levels at the measurement site (see the RFI-spectrum on display), we calculated the levels of in-band intermodulation products for RFI-signals below 1000 MHz. As an example the total power plot shows the expected intermodulation products between 1220 MHz and 1270 MHz (lower, magentic line). Also shown is the output spectrum (upper, blue line) of the 2x2 array looking at broadside. Notice that the calculated intermods are between 10 and 15 dB below the system noise level and will influence the noise measurement. Some in-band signals are visible in the blue line, as well as a noisy structure and a spike above 1250 MHz. The noise temperature plot clearly shows the effect of the in-band signals on the measured array noise temperature (blue line). It also shows a considerable increase in noise temperature due to intermods between 1230 and 1240 MHz.

    In our WLNT-receivers the last amplifier stage sees the largest RFI-levels and thus will be dominant in the production of intermods. One may illustrate the effect of the intermods on the noise by reducing their level, either placing a 6 dB attenuator or a bandpass filter before the last amplifier stage. This will reveal which signals are due to intermodulation and which are original in-band signals. Indeed the total power spectrum plots (red line with filter and green line with attenuator) show a much cleaner and flatter response. At the same time the noise temperature plots at frequencies where the intermods occur show a considerable improvement with the attenuator and filter, while at in-band RFI-frequencies noise peaks remain. These results explain the anomalies in noise temperature at certain frequencies that we found in previous measurements and underline the importance of an optimally dimensioned RF-chain.


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    © NOVA Optical IR Instrumentation Group, Rijksuniversiteit Groningen, Isaac Newton Group of Telescopes (ING)

    WEAVE is a new multi-object survey spectrograph for the 4.2-m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos, on La Palma in the Canary Islands. It will allow astronomers to take optical spectra of up to ~1000 targets over a two-degree field of view in a single exposure (MOS), or to carry out integral-field spectroscopy using 20 deployable mini integral-field units (mIFUs) or one large fixed integral-field unit (LIFU). WEAVE's fibre-fed spectrograph comprises two arms, one optimised for the blue and one for the red, and offers two possible spectroscopic resolutions, 5000 and 20,000.

    The principal science goals are twofold:

  • Milky Way archaeology: exploiting Gaia's scientific legacy.

  • Galaxy evolution and cosmology: exploiting SKA Pathfinders (e.g. LOFAR and APERTIF on the WSRT).

    The Final Design Review of the Spectrograph of WEAVE took place last 17 and 18 March at ASTRON. The team successfully passed this important milestone. The picture shows the Review Board, the WEAVE management team and the Spectrograph team in the backyard of ASTRON.

    The Spectrograph team is a collaboration between NOVA Optical IR Instrumentation Group at ASTRON (Optomechanical Design and Production, Management), RAL in the UK (Optical Design), JMU Liverpool in the UK (Detectors), IAC at Tenerife Spain (Control Hardware), ING at La Palma Spain (Control Software), INAF Italy (VPH Gratings and Control Software). Dutch PI is Scott Trager, Kapteyn Astronomical Institute, RUG.

    http://www.ing.iac.es/weave/index.html

    http://www.astron.nl/dailyimage/main.php?date=20130319


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    © Francesco de Gasperin

    LOFAR-LBA amplitude solutions on 3C196 of a 6h observation plotted using LoSoTo (the LOFAR Solution Tool). Every panel is one station, on the X axis the observing time in hours, on the Y axis the observing frequency in MHz (22-70 MHz). The bandpass effect is dominant and underline the strong peak of LBA bandpass at 58 MHz. The small panel shows unflagged data with many outliers that were identified and flagged with LoSoTo. Good amplitude solutions obtained using a good model (such as in this case) should be relatively constant in time. This is not the case for phases (see daily image of yesterday).

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    © DG

    Standard theoretical models for the formation and launching of the relativistic jets of Active Galactic Nuclei (AGNs) predict the development of a helical magnetic-field component, due to the combination of the rotation of the central supermassive black hole and its accretion disk and the jet outflow. One elegant way to detect such helical magnetic fields observationally is through Faraday rotation images of the jets: since the Faraday rotation depends on the line-of-sight magnetic-field component, the presence of a helical field component should give rise to a systematic gradient in the observed Faraday rotation across the jet. Furthermore, Monte Carlo simulations have demonstrated that this effect is detectable even across very narrow jets. Monotonic transverse Faraday-rotation gradients have now been observed across the parsec-scale (VLBI) jets of 27 AGNs, providing firm evidence that they carry helical magnetic fields. Reversals in the direction of these transverse Faraday-rotation gradients that have been observed in several jets provide the first observational evidence for the 'return field', which forms a nested-helical-field structure together with the 'outgoing' field. Intriguing asymmetry in the orientations of the 27 observed transverse Faraday rotation gradients suggests that the direction of the longitudinal field component and the direction of the rotation of the central accretion disk are coupled.

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  • 04/16/15--17:00: When water and EM waves meet
  • © ASTRON / NIOZ, 2015

    The Royal Netherlands Institute for Sea Research (NIOZ) is the national oceanographic institution. Just like in Radio Astronomy, specialized instrumentation is needed for oceanographic research. At the NIOZ, the Marine Electronics Department specializes in electronics development for marine research equipment. Recently, they developed a sensor to measure sediment movement. This sensor periodically measures the height of the sediment and stores the data on a memory card. When an operator visits the sensor, the measured data is transferred to a PC over a Wi-Fi radio link.

    However, the engineers at NIOZ found out that radio waves behave quite different from the water waves they are used to. They could not get the radio link to work and turned to ASTRON. At the ASTRON RadioLab, we performed some measurements and isolated the problem. With some quick fixes, we managed to demonstrate a proof of concept to improve the stability of the link, to great satisfaction of our NIOZ colleagues. Fascinated by RF technology, two people from NIOZ participated in the March edition of the ASTRON RF course.


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