Faraday rotation from API echos

The enhanced echos produced by the API effect can also be used to measure Faraday rotation, i.e., the propagation time difference between the two characteristic modes of radio wave propagation, which is dependent on several plasma parameters, but most importantly electron density. The artificial periodic irregularities are useful here, as they allow us to get strong enhanced backscatter from this region, which would otherwise give such a strong return.

Here is our first Faraday-rotation measurement from API echos. This is still a fairly raw data product, which must be further inverted into an electron density profile.

Faraday rotation measured using enhanced ionospheric backscatter from heating induced artificial periodic irregularities. This allows us to measure small electron densities, which are otherwise difficult to measure. The delay between O and X modes of propagation is given in radians, as a fraction of a radar probing wave cycle.

More post to come, as we have one more week of measurements to go.

Mesospheric velocities with API

One advantage of the API method is that it allows high resolution vertical velocities to be measured between 50-120 km. Here is an example plot from our 9.12.2011 measurement. Around noon (7000 seconds in to the measurement) there are also some tropospheric echos.

Vertical velocities in the 60-120 km region measured using the API method with the EISCAT heater. 

API

This week me and Antti Kero are up in Tromsø doing artificial periodic irregularity (API) experiments with the EISCAT heater, which has recently been upgraded to include a radar receiver.

The idea with the experiments is to first produce a standing wave in the ionosphere, which heats up the plasma, and creates small enhancements of electron density at lambda/2 intervals. These irregularities are then probed with short radar pulses, which are used to probe the decay time of the irregularities. From this, it is possible to determine several ionospheric parameters: ionospheric chemistry, electron density, winds, and neutral density.

5.423 MHz X-mode API echo. There is also a meteor head echo accompanied with a decaying trail echo in this measurement.

We managed to actually get an API echo on our first try! Here is a picture from our second day of measurements. This is an API echo with 5.423 MHz at X-mode heating and probing polarization. We were also lucky to observe a meteor head echo and associated trail echo.

GNU Chirp Sounder

This monday is yet another release from me. I'm publishing my code for ionospheric chirp sounding that I've been working on for a while. In lack of a better name, I'm calling it the GNU Chirp Sounder. It is a software defined radio based receiver for monitoring ionospheric sounders. The software is based on gnuradio and relies on Ettus research USRP2 and USRP N210 based digital receivers. The receiver can be used to receive the whole HF band (typically at 25 MHz bandwidth) simultaneously, and to receive multiple chirp sounders around the world simultaneously. The current receiver can be also used to perform single or dual polarization (channel) soundings. The dual channel mode can be used to determine the polarization form vertical soundings, or for angle of arrival measurements for horizontal soundings. It would also be trivial to add more channels for e.g., imaging or full polarization measurements.

The software consumes about 70% of one Intel Core i7 processor core, meaning that with a quad core processor, it is possible to simultaneously receive four different sounders, assuming that they are within the 25 MHz band that the receiver is tuned to.

What can I do with this? First of all, it is possible to use it to standard oblique ionospheric soundings, such as this one, between Sodankylä and Cyprus:

Oblique ionogram between Cyprus and Sodankylä. The Cyprus over the horizon radar installation transmits between 8 and 35 MHz with a 5 minute repetition period. This can be used to deduce ionospheric electron densities along the propagation path. 

Also, it is possible to perform standard vertical ionospheric sounding, such as this one:

Vertical ionospheric sounding with the Sodankylä Ionosonde. The receiver is approximately 1 km from the transmitter. 

In the case of vertical soundings, it is possible use two antennas to determine the polarization of the vertical electromagnetic wave:

The phase difference between H and V polarizations. The greenish corresponds to O-mode of propagation and the blue corresponds to X-mode of propagation. 

As the receiver is relatively inexpensive and the hardware contains a GPS reference, it would be possible to perform 3D imaging of the ionosphere with this receiver, provided that there are enough of them.

GNU Ionospheric Tomography Receiver (Jitter)

I am releasing the ionospheric tomography receiver that I have been
working on for a couple of years. The receiver is not related to the
GNU Digital Beacon Receiver provided by Prof. Mamoru Yamamoto, which
can be used for a similar purpose. The receiver looks at Russian
Tsykada, US Radcal, and DMPSF15 satellites that transmit 150 and 400
MHz beacon signals. The FORMOSAT satellites should also be usable,
although they do not seem to transmit anything above Northern
Scandinavia, so I haven't been able to test them. From these signals,
it is possible to derive the relative propagation delay between 150
and 400 MHz (other frequencies can also be used with the receiver).
The propagation delay is approximately related with the line integral
of the ionospheric electron density between the satellite and the
receiver. With a large enough network of receivers, it is possible to
perform limited angle tomography of the ionospheric electron density,
which is what my group is planning to do with this receiver.

The receiver package that I am now releasing is intended for use with
such a large scale tomography receiver chain. The receiver is capable
of autonomous operation and it can observe multiple beacon satellites
simultaneously. The receiver is written with C++ and the relative
propagation delay calculation is performed using GNU R. A normal PC
will be able to perform all of these calculations, although you will
need about 4 GB of memory for the phase curve calculation part.

I am not releasing any tomographic reconstruction software yet, but
this might happen in the future.

Here is the web site for my receiver, which I am choosing to call
Jitter (GNU Ionospheric Tomography Receiver):

http://www.sgo.fi/~j/jitter/web/

The software is still in alpha stage, but it has been used with
several test receivers for several months of continuous operation. I
have used it successfully with usrp1 and two WBX daughterboards, but
also recently I have managed to get USRPN210 and TVRX2 working. The
receiver can be locked to a GPS reference, but this is not necessary.
If you are interested using my receiver, feel free to contact me.

LOFAR Jülich DE605

Another international LOFAR station has been commissioned. Yesterday, 5th October 2011, the LOFAR station "DE605" was officially opened at Jülich in Germany. Their press statement read:

Bochum, Bremen, Jülich, 5 October 2011 – At last! The LOFAR station DE605 was officially opened on the campus of Forschungszentrum Jülich. The antenna station is an integral part of the digital radio telescope and European large-scale project LOFAR (Low Frequency Array). It will contribute to measuring long-wave radio signals from the dawn of the universe. LOFAR DE605 was built in cooperation between Ruhr University Bochum, Jacobs University Bremen and Forschungszentrum Jülich with funds provided by the Federal Ministry of Education and Research (BMBF).

This new station is one of five located in Germany. There are now eight operational international stations and, although there is still a lot of commissioning work to be done, it is great to see that the network has expanded so far so fast. The photograph below shows the newly completed station.

LOFAR station DE605. Image: Forschungszentrum Jülich

The opening of the facility is great news not just for the Bochum/Bremen/Jülich team, but also for the entire LOFAR facility. The addition of new baselines will improve the image quality and the additional collecting area will provide a valuable sensitivity improvement.

Congratulations to the German team on the completion of their new station!

Press-releases:

• http://www.fz-juelich.de/SharedDocs/Pressemitteilungen/UK/EN/2011/11-10-05lofar.html (IN ENGLISH)
• http://www.astro.ruhr-uni-bochum.de/lofar-station/ (IN GERMAN)

LOFAR-UK observes the active sun

Over the last few days, our Sun has been very active. Apart from the dedicated solar instruments monitoring this activity, we've also been able to observe it with the LOFAR radio telescope at Chilbolton. This facility comprises two fields of small antennas, that link together to perform as a powerful VHF radio telescope. Although it is also possible to link the LOFAR system in the UK with others across Europe, on the 28th September 2011, it was operating independently.



At 16:10 UTC (= 5:10 pm local time), LOFAR Chilbolton was used to record an 'all-sky' image. Even before calibration and analysis, this image shows the bright objects in the radio sky. On the left-hand side of the image there are, from top to bottom, Cassiopeia-A (a supernova remnant), Cygnus-A (a radio-galaxy) and the broader region which is the centre of our own Galaxy. However, to the right of the image, the Sun can be seen with extremely strong intensity, just before it set below the western horizon.

The image shown was made using the UK608 LBA. It demonstrates what a LOFAR station is capable of, even when working alone and independent of the main International LOFAR network. The image is not calibrated nor cleaned in anyway. The observations were made by Menno Norden (ASTRON), and the report by Derek McKay-Bukowski (STFC).

The web release is also available from STFC's "RAL Space" website: http://www.stfc.ac.uk/RALSpace/Areas+of+expertise/Astronomy/LOFAR-UK/In+the+news/37611.aspx

Updated Science Image

Today I updated the figure (on the right panel) that shows the different science cases that can be addressed with KAIRA. I have added two more cases: Relative TEC tomography, and tropospheric echos.

Relative TEC tomography involves several different LOFAR stations simultaneously measuring the total electron content, which is basically the difference in ionospheric propagation delay. Because LOFAR can do this with radio stars, in theory it should be possible to perform these kinds of measurements on a huge number of directions simultaneously. This is a huge advantage compared to satellite tomography, which can only measure the total electron content between ground based stations and satellites. This large number of rays might make it possible to do extremely high resolution 3D ionospheric tomography with LOFAR. Currently the best location for doing this would be near the core of the array, where there are many stations located nearby.

The tropospheric echo science case was added after discussions with Ingemar Häggström of EISCAT, who told me that he often sees tropospheric echos in Kiruna, both from the VHF and UHF. With multistatic receivers, this would potentially allow tropospheric wind and scattering spectrum measurements.

HBA Rack installation

Last week Peter Gruppen ja Jan-Pieter de Reijer from ASTRON came by, and did the cabling work and receiver rack installation at the KAIRA site.

The HBA cables going into the digital receivers. 

Beamformer cables going into the switch. 

 Now if we could just get summators (which are in short supply right now) before snowfall, we could start measuring. 

LBA protype v 2.0

LBA platform version 2.0. Photo: Markku Postila

Here is another prototype of the raised LBA antenna platform designed by Markku Postila and Toivi Iinatti. This one allows arbitrary rotation of the antenna and has easily adjustable legs. The picture also shows the LBA antenna installed on top of the platform.

However, it seems that we will need to extend the ground plane with this type of a design. Recent antenna modeling results by Johan Borg of LTU suggests that this configuration has the effect of broadening of beam significantly at higher frequencies.

NEC model of the antenna with a raised ground sheet.

40 MHz beam pattern of an individual element. Slight broadening at 40 MHz, but nothing too serious.

80 MHz beam pattern of an individual element. The raised platform causes a broadening of the beam and a deeper minima at the zenith.

Optical fiber

Laying the last meter of fiber to the RF container. Photo: Markku Postila.

Yesterday there was good news, and some bad news. First of all, the fiber cabling work has now been completed. Whenever we want to, we can start using the internet over fiber cable! However, in terms of bad news, we didn't get any summators, and we are thus not able to start using the antenna yet. There seems to be some kind of a problem with the production of the summators, and nobody knows of a delivery date. If this is delayed a month more, it will be too cold and snowy to install them this winter, and we will have to wait for next summer before we can measure with the system.

TEC Measurements with LOFAR

TEC Measurements with LOFAR. Copyright: Mark Aartsen. 

Today the ASTRON daily image featured a very interesting image in terms of geophysics. It featured a total electron content measurement produced by listening to a strong radio source using four different stations. This is interesting because if one can produce such measurements using many different radio sources, it would be possible to use this for three dimensional ionospheric tomography. 

LBA Prototype platform

The LBA platform prototype, built by Markku Postila and Toivi Iinatti. 

We are now already thinking about the LBA build next year. This will be challenging because we cannot level the field, as it will probably cause erosion and water quality problems with the nearby Siilasjärvi lake, were all the water drains to. We are now focusing on a design which involves similar raised platforms, which hopefully will allow easy position adjustments, when permafrost moves our antennas around. Another reason for the platform is that we will need to use them in order to level out the antennas, if we cannot level out the ground. In anticipation of this, Johan Borg of LTU is currently investigating the effect of such a platform on the beam pattern of an individual antenna.

Summators and signal processing installation

Wireless internet mast installed on the RF-container. The Barracks have also been moved next to the RF container. 

This week ASTRON will visit the KAIRA site and install the summators inside the HBA tiles. After this, they will install the A/D converters and beamformers inside the container. After this, we they will hopefully test all of the antennas, and we can hopefully start calibrating the antenna. With Ilkka now in the Netherlands, this is a prime opportunity for us to learn how to use the signal processing hardware.

Lego radio antennas — Part 1

Putting the KAIRA site together has been sometimes described as being akin to assembling flat-pack furniture or building Lego. Ah... if only it was that simple! Despite the failings of this analogy, it is a fun one. And it can certainly be a great way to demonstrate or explain concepts. So, in this article (the first of a little special series), we'll take a look a some examples of miniature radio antennas built from that famous Danish toy: Lego!

And, in order to be topical, our first example comes from the SKA (Square Kilometre Array) community. Although the SKA will have aperture arrays like KAIRA, it will also have sets of conventional radio dishes as well.

Currently, there are a variety of prototypes for antennas (both dishes and phased arrays), which are being considered for the two main site candidates of the final SKA project. At this stage, it has not been decided where the final telescope system will be located, but the list has been shortened to two strong candidates: Southern Africa and Australasia.

Our Lego model today is from the second of the two: the anzSKA project.

Here are the pieces...


And the build sequence...


And the result!


More models of other systems are to follow on this web log (including some stunning ones of KAIRA and LOFAR). We'll also be using them to show how they can be used to explain radio astronomy and engineering concepts to children. So stay in touch for our future instalments!

Path to SKA-low Workshop

This week, there is the "Path to SKA-low Workshop". SKA is the Square Kilometre Array (see our two previous articles: LINK1, LINK2), and this important meeting focuses on the aperture arrays that will be used in this up-coming radio telescope.

As part of the SKA's Aperture Array Verification Programme (AAVP), there will be reports on the latest science and system results from low-frequency pathfinders (LOFAR, MWA, KAIRA, etc.), discussions on the major system design challenges, and progress reports on the crucial technologies and techniques. Not only will KAIRA be contributing its experience to the discussions, but we hope to learn some important lessons to assist with the EISCAT_3D project.

The workshop is being hosted by the International Centre for Radio Astronomy Research (ICRAR), Perth, Western Australia from the 6th to 9th September, 2011.

Link: http://www.icrar.org/news/ska-low

EISCAT International Workshop

The 15th EISCAT International Workshop will be held in Qingdao, China, 5-9 September 2011, hosted by the China Research Institute of Radiowave Propagation (CRIRP). A ceremony for the 30th anniversary of the EISCAT measurements will highlight the Workshop.

There will be a presentation about KAIRA on Friday.

Looking forward to an interesting week.

EISCAT_3D Executive Meeting

The EISCAT_3D Executive Board held its third physical meeting at the Sodankylä Geophysical Observatory (SGO) on Wednesday and Thursday of this week (31.8.-1.9.2011).

A full report is available on the EISCAT_3D web log.

LOFAR Data School

This week, there has been the LOFAR data school, held in the United Kingdom. A report is available on the LOFAR-UK web log (along with some photographs).

Official website: http://www.jb.man.ac.uk/meetings/LOFAR_UK/

Azure hawker

Azure hawker

There are only three or four species of dragonfly in the area. This is one of the biggest at 60-65 mm.


Photo and text: Stuart Keenan

Bluethroat

Bluethroat

One of the most beautiful birds in the area, common in the birchwoods. Recent research has shown the Kilpisjärvi area to be the best place so far found for these birds with up to 50 breeding pairs per square kilometer.

Photo and text: Stuart Keenan

Thistles

Thistles

Photo: Stuart Keenan

Lichen forest

The lichens pictured here belong to the genus "cladonia" which contains a number of species important to reindeer as a primary food source .They are found in more open tundra like habitats and are extremely cold tolerant.

Cadonia lichen

Photo and text: Stuart Keenan

Hooded crow

This ex hoodie has been killed and eaten by a Perigrine Falcon usually they only leave the wings attached to the breastbone. This is the first time I've found prey remains with the legs as well.

Hooded crow remains.

Photo and text: Stuart Keenan

Lemmings

Lemmings are arctic rodents similar to voles. In Kilpisjärvi they were very plentiful this year. Our nature enthusiast Stuart managed to get quite a few very nice photographs of these creatures during his stay. Today we post the best of them.

There are many species of Lemming found around the arctic, this one is the Norweigian Lemming and this year it was very common. The population of Lemmings vary in a cycle during which they increase in number until the habitat can no longer support so many and they are forced to migrate en masse to find food. In some years the population reaches plague proportions when the ground is literally crawling with them. During these plague years large numbers of them die crossing rivers, lakes and other obstacles, which led to the myth of 'mass suicide' this myth was further enhanced by a so called wildlife documentary made in 1958 by Walt Disney in which hundreds of lemmings were captured and taken to the top of a cliff in Canada, they were then forced to run on to a revolving turntable which hurled them off the cliff thus creating the myth.

Cute lemming. Copyright Stuart Keenan.

I discovered they are not always so cute though and some would squeek loudly as I past and actually challenge me if I moved closer.

Angry lemming. Copyright Stuart Keenan.

It goes without saying that this year, there also seem to be quite a lot of young well fed owls. On my way back to Sodankylä I counted at least 15 young owls beside the road, preying for voles and lemmings that were brave enough to attempt crossing a road -- a common sight this year as well.

Calm lemming. Copyright Stuart Keenan.

At first I thought the reason for the aggressive behaviour of some lemmings might be because they were females with young nearby, but one of the biologists told me that breeding lemmings tend to be pretty quiet and its more likely to be individuals on the move, perhaps when they are on the move they are more vulnerable to predators and therefore more defensive.

Agitated lemming. Copyright Stuart Keenan.

Very agitated lemming. Copyright Stuart Keenan.

Photos and text: Stuart Keenan.

Reindeer, Part Two

Silhouette of a reindeer hiding under the HBA tiles.  

A while back, we had a post about Reindeer at the KAIRA site. This time we will post some more pictures of these animals which have found shelter under our HBA field.

At any given moment during our cabling work, there were at least two reindeer under our antenna. They arrived on the day that we accidentally brought in some salted gritting sand to fill in one of the cable trenches. At first, they were a bit shy, but on the second week they were already feeling very much at home. 

So far there haven't been any problems with the reindeer. They have been very relaxed, and have grown to be very used to the kind people who obviously are building them a huge reindeer shelter. Little do they know that we have already put up most of the fence posts for our reindeer fence, which will hopefully be in place by the time mating season starts. This is when the reindeer get more aggressive, and might pose more risk to our radar receiver. At SGO, we have previously had problems with reindeer that think various antennas are competing male reindeer with fancy antlers that just call out for a duel. Already now there is a small risk of one of these guys getting their antlers tangles up in some of the cables, even though they are fairly well protected by the cable trays and ducts. 

Reindeer breastfeeding. 

Another one hiding under the tiles.

Yet another one hiding under the tiles.

Photo credits: Mikko Orispää.

Saana

A view of KAIRA with Saana behind it.
Photo credits: Mikko Orispää.  

Saana, the landmark of Kilpisjärvi is mentioned in many of the posts on this blog already, but today we will offer several more pictures of it. It is approximately 1030 m from the sea level and approximately 500 m above the KAIRA site. It is also one of the most well known fells in Finland. Nearly every Finn would recognize it from an image from its unique steep south east slope.

Our radar has an excellent view to this fell, which is evident from the picture on the left, taken from the northern side of the field. The picture below shows Saana from the south.

Saana viewed from the south.
Photo credits: Stuart Keenan

Siilasjärvi

The Pikku-Malla fell, as seen from Siilasjärvi.
Photo credits: Mikko Orispää. 

There is a lake called Siilasjärvi several hundred meters away from the array. On this lake, the water is crystal clear, with a shallow sandy beach and a spectacular view to the neighbouring fells Saana and Pikku-Malla. On the day that we went down to the lake, there were also several young merlin on a lookout for lemmings.

A small hut storing fishing nets on the shore of Siilasjärvi.
Photo credits: Mikko Orispää.  

First light

Electrifying the RF container.

We have first light! Well, not in the conventional astronomical instrument sense no, but we have now managed to get electricity to the RF container, where all of the signal processing will be located.

The cable mausoleum has also got a roof on top of it, which will prevent any snow and ice forming in our cables. Let's hope that the mausoleum will also be rodent-proof once we get the walls on it.

Hooking up to the mains.

Doing the mausoleum roofing work. The owl nesting box can also be seen on top of the RF container.

Nightly visitors

I had been told there was a pair of hawk owls breeding somewhere near the KAIRA site, then Derek and Juha saw them one night when they were on site late, so I went along the next night to try to see them and managed to get a few photos.

On a couple of nights we had spotted several young northern hawk owls flying around the site. There was even a photograph of one of them earlier. The young birds were just learning to fly and did all kinds of silly crash landings and such while they where checking us out.

Stuart managed to get some very nice pictures on one of the nights. In Finnish these birds are called "hiiripöllö", which means mouse owl.

On his last day, Stuart even built a nesting box for the owls. Their presence will hopefully keep the small rodents from chewing on our cables.

Hawk owl sitting on top of the 3D cameras.

After warching two adults and one young flying around from perch to
perch for a while I decided to try to attract the bird on the cameras
a bit closer by making mouse noises, it worked and it came closer
to investigate.

Making the mouse noises worked well for the owls but backfired on
me the next day. At lunchbreak I went into the forest to look for the
nest site and was away a bit longer than I should have been when I
heard a strange owl in the distance, so I headed straight for it. When I
emerged from the trees I was back at the KAIRA site where I found
Derek making owl calls, the theory being mouse calls attracted
the owls for Stuart so maybe owl calls will attract Stuart back to
work. It worked like a charm and I was back at work before I knew
I had been fooled. Well done Derek

Photos: Stuart Keenan

Stuart Keenan

Stuart Keenan (pictured below) was helping us to build the KAIRA array this summer for four weeks. He has helped out with many of the international LOFAR stations too. He is also an avid nature enthusiast -- he even helped out some biologists at the Kilpisjärvi Biological station to find merlin nesting sites. During this month, we will be posting some of the nature photographs that he took during his stay in Kilpisjärvi. We will also his descriptions of the photos.

Stuart Keenan on top of Pikku-Malla with the KAIRA site seen on the background. 

For the last two years I have worked on the Installation of LOFAR stations across Europe, and this year I had the chance to join the team at Kilpisjärvi for the installation of the KAIRA / LOFAR station in north west Finland. I was exited about this for two reasons, first I love working on LOFAR and having a small part to play in this fantastic project, and second I had never been to the arctic before, so being a bit of a wildlife freak I had a chance see some species new to me. During my stay I was out at weekends, evenings and lunch breaks with my camera trying to photograph nature around the KAIRA site. Over the coming weeks you can see some of what I found. Hope you like it.

This is one four young merlins, reared in an old hooded crow's nest about 200 m from the KAIRA site.

Young merlin.

Young merlin.

Photos and text: Stuart Keenan.