Sunday, 30 September 2012

Durin's Day

One evening, as the leaden winter grey skies were gathering to hang heavy over the chill Lappish air, there was a small gap in the clouds. And the light of the setting sun shone through to light up Saana... the lonely mountain.

The lonely mountain, as seen from the KAIRA site.

It reminded me of this passage from an old children's book...

"Stand by the grey stone when the thrush knocks," read Elrond, "and the setting sun with the last light of Durin's Day will shine upon the key-hole."
"Durin, Durin!" said Thorin. "He was the father of the fathers of the eldest race of Dwarves, the Longbeards, and my first ancestor: I am his heir."
"Then what is Durin's Day?" asked Elrond.
"The first day of the dwarves' New Year," said Thorin, "is as all should know the first, day of the last moon of Autumn on the threshold of Winter. We still call it Durin's Day when the last moon of Autumn and the sun are in the sky together. But this will not help us much, I fear, for it passes our skill in these days to guess when such a time will come again."

-- J.R.R.Tolkien, "The Hobbit", 1937.
We didn't see any dwarves that day nor, thankfully, any dragons. Yet still this is a magical place; one of the few remaining quiet places in the world. And winter is at hand.

Saturday, 29 September 2012

Last days of autumn

Autumn is a lovely time of year in Lapland (actually... every time of year is lovely in Lapland). But in autumn, the landscape turns to bright yellow, then flame red, then rust... before it finally succumbs to the winter snows. We had a particularly lovely day the other day, so here are some photographs...

Looking across the KAIRA LBA. Saana is visible in the background. (Photo. D. McKay-Bukowski)

From the base of the mound, looking north. The HBA array catches the glorious afternoon sunshine. (Photo: D. McKay-Bukowski)

Friday, 28 September 2012

Using LOFAR LBA and HBA arrays simultaneously

Most LOFAR stations have two antenna fields. These are the Low-Band Antenna (LBA) array and the High-Band Antenna (HBA) array. The LBA and HBA antennas are capable of receiving a range of 10-90 and 110-270 MHz respectively. However, the signal processing is arranged slightly differently. Each "channel" in the signal processing system has three inputs into which the antennas are connected. These receiver units (RCUs), then have several signal paths that can be used, each of which switch in different filters. Thus, the different antennas and frequency bands can be selected and sampled.

Because each individual receiver unit has both an LBA and an HBA polarisation connected to it, it is not possible to observe with both of these simultaneously. Furthermore, because of the filters used in the RCUs, it is not possible to observe across the entire frequency range of the HBA simultaneously either. You must select one of these "RCU modes" for each given channel.

  •  RCU mode 3 = LBA input, 10-90 MHz filters
  •  RCU mode 5 = HBA input, 110-190 MHz filters
  •  RCU mode 7 = HBA input, 210-270 MHz filters

There are other modes as well, such as RCU mode 6, which uses not just different filters, but also a different clock rate (160, rather than 200 MHz) in order to sample the frequencies around 200 MHz (which would normally be aliased in the other modes). The point is that each RCU is limited to a single mode. This has led some to believe that LOFAR stations cannot observe with the LBA and HBA simultaneously.

Actually... they can.

Here at KAIRA, we have been using combinations of modes. In what we refer to as "RCU mode 357", we have been observing with RCU modes 3, 5 and 7 simultaneously. The modes are interleaved, so that there is still a distribution of antennas for each; this allows beam-forming to take place.

Of course, this is not for the faint-hearted, and one must be careful in mapping the channels and powering-up the HBA tiles in such a way so as not to cause power supply failures. However, when done correctly, multiple beams across all bands can be formed giving frequency coverage over nearly the entire VHF band. Here is one of our first results:

A small sample of data from KAIRA, ranging from ~15 to ~275 MHz. Each frequency channel has been divided by the median to bring out the radio features. The bright arcs are ionospheric scintillation during observations of Cas A. (D. McKay-Bukowski, J. Vierinen, and R. Fallows.)
Already we have seen some interesting things and we will be reporting these over the next few days.

Wednesday, 26 September 2012

First full set of subbands on the LBA

This evening, we managed to get good spectra from all LBAs at KAIRA. This has been a challenging task with blood, sweat and tears... right to the end, due to the problems with cable-ground shielding. But we've managed to do it and therefore proudly present our first full set of LBA bandpasses.

LBA spectra from all 96 RCUs. (Click to enlarge)

We still have more work to do on this and, in fact, we will need to do further subband testing to ensure that we have flushed out all the cabling problems. However this is another step for the project and we can now look forward to facing the next task!

Monday, 24 September 2012

First international experiment

This week, KAIRA is participating for the first time in a large international experiment with baselines of several thousands of kilometres. Apart from hoping to discover some new science, this is a perfect opportunity to link EISCAT, LOFAR and, of course, KAIRA itself. As the week goes on, we will be incorporating more and more stations into this experiment network. However, on this first night of observing the following stations are involved (from North to South): 
  • ESR (EISCAT Svalbard Radar, Norway)
  • KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array, Finland)
  • Kiruna (EISCAT receiver station, Sweden)
  • SE607 (LOFAR station at Onsala, Sweden)
  • DE604 (LOFAR station at Potsdam, Germany)
  • UK608 (LOFAR station at Chilbolton, UK)
  • FR606 (LOFAR station at Nançay, France)
 Putting them on the map, the layout is as follows:

The observations are being carried out at approximately 224 MHz (thus allowing us to use the Kiruna receiver station). The ESR is at a different frequency, but is providing concurrent observing. And both KAIRA and UK608 are using special observing modes. Kiruna and the ESR are conventional, mechanically-steered, parabolic dishes. The others are all LOFAR-design phased arrays.

People often ask if KAIRA is part of the LOFAR network. The answer to this is: "not directly". Although there are plans to one day link KAIRA directly to LOFAR via a high speed link, we are currently running as an independent station. While this gives us the flexibility to pursue our own science programme, it certainly does not preclude us from participating in large, important, observing experiment — like this one.

Oh... and we should also point out that although this particular experiment is being run by ASTRON, this work is being done by ASTRON staff at the KAIRA site!

Stay tuned as the week goes on as, no doubt, we will have some very interesting things to report!

Sunday, 23 September 2012

Fixing cable shielding problems

Last week we reported on the work that we were doing on the LBA array bandpasses. We had noted a few problems and have been looking into the cause of them. It turns out that some of the cables with joins did not have a correct ground shielding. The break in the shield occurs at this join location and is caused by resin from the water-proofing forming an insulator gap in the connector threads.

To fix the problem, we need to very careful remove the housing and clean out the resin. The cable connectors can then be re-fitted and tested.

A cleaned and reconnected LBA cable join.

Carrying out this work is extremely time-consuming. Each cable join is taking between 15-30 minutes to clean out and test. Still, it is important work, and it is essential in order to get the entire LBA operational.

Friday, 21 September 2012

Tiled panorama of the HBA

Just a nice photograph to end the week. This is actually a sequence of photographs taken by Thomas Ulich from the top of the High-Band Antenna array. In the background are the mountains of the surrounding landscape, radiant in their autumnal colours. Click on the image to see an enlargement.

Tiled panorama of the KAIRA HBA array. Photo: Th. Ulich.
Have a nice weekend!

Thursday, 20 September 2012

LBA bandpass spectra

We're now in the midst of some intensive commissioning of KAIRA. Some of us have been working on the High-Band Antenna array and others on the Low-Band Antenna array. Today's report is about this LBA work. One of the things we need to do as part of the commissioning is confirm that all the low-noise amplifiers are generating the correct signal outputs. For each aerial there are two outputs: one for each polarisation. The bandpass spectrum for each of them should look the same (a bandpass spectrum is a plot showing the intensity received as a function of frequency, for the range of frequencies that the system can measure). In the case of a good working antenna, we should see something like this:

The lowest part (0-10 MHz) is filtered out. Then there are various short-wave radio frequency interference. The main part of the band rises up and then tapers off at the top end (going into the FM radio band).

However, we've noted that some channels are a bit weak:

And some channels are quite low across the main part of the band.

Before we can operate the entire LBA array, we will need to resolve these problems and ensure that all components are working correctly. This sort of trouble-shooting and tuning is a standard part of commissioning these sorts of systems.

Tuesday, 18 September 2012

LBA amplifier unit

Today we have a photograph of the top of one of the Low-Band Antenna (LBA) aerials. This is the so-called "LNA cap". The black disc at the top of the post in the photograph contains the two Low-Noise Amplifier (LNA) units which increase the strength of the very weak electrical signal that is created from the reception of the radio waves. The LNA cap is potted with resin to keep the unit water proof. The output from the LNAs goes into coaxial cables that run down the grey post. The red band on the post is just the label... in this case "01" (aerial L01 in the sequence L00-L47). There is also a bar code on the label for easy scanning and identification of the aerial.

The LNA cap of LBA aerial L01. Photo: D. McKay-Bukowski.

First pulsar observation at KAIRA

PSR J0332+5434 observed in single station mode with 31 subbands. The dedispersed signal is shown on the left hand side, while the original incoming signal is shown on the top right hand side. The lower left plot shows the power averaged across all 31 subbands after dedispersion. 

Today we tried something different. Instead of looking at our local atmospheric plasma, we looked at interstellar plasma, this time with the help of a pulsar. In this case, PSR J0332+5434, which is the brightest pulsar in the northern hemisphere.

Pulsars are highly magnetized neutron stars that spin very fast. This spinning movement causes broadband radio emissions due to acceleration of charged particles in the magnetic field of the pulsar. Due to the fact that the interstellar space contains charged particles (although typically less than one electron per cubic centimeter), the broadband pulse arrives to us at different delays for different frequencies. This is a familiar phenomena for radio waves travelling in our own local ionosphere, although the scales are much larger. In order to improve signal statistics, this dispersion has to be measured and also corrected for before averaging over wider bandwidths.

Monday, 17 September 2012

Using the LBA as a solar radio emission receiver

A five minute dynamic spectrum recorded using a single LOFAR LBA antenna element. Apart from the HF radio band, the most prominent features are several type 3 solar radio bursts and the FMCW over-the-horizon radar signal transmitted from Cyprus.
We have now started comissioning the LBA antennas. Although there still are several problems, we have managed to get over half of the antennas up and running. Here is a plot from one of the first spectral measurements done using this new antenna field. Here is a spectrum recorded using a single LBA antenna element over a 100 MHz band with one second time resolution. We happened to catch several solar radio bursts. We also regularly see the FMCW over-the-horizon radar located in Cyprus, so we are confident that the system is working as expected.

Friday, 7 September 2012

Cloud formation

Just a nice photograph to finish the week. This cloud formation was spotted over Pikku Malla. (Photograph by A. Jutila.)

Sunday, 2 September 2012

Special lid

In order to service the HBA tiles, it is necessary to be able to reach into them. Normally, LOFAR stations are built directly on the ground. This means you can open up a tile, step into it and then work on the parts.

For KAIRA, this is not an option. The reason is that there is insufficient support under each tile to allow a person to stand inside it.

As a result, we need to use a special tile lid that we can place on an open cell. From there, we can get in sufficiently close to the parts to work on them, without actually needing to step into the tile itself. Just for the record, here are some photographs of the special tile lid.

Top view. (Photo: A. Jutila)

And the underside. (Photo: A. Jutila)

Saturday, 1 September 2012

The reindeer problem

At KAIRA, we have a reindeer fence to keep the larger animals from getting in amongst the aerials. Apart from the RF-implications and possible damage to our experiments, we also want to prevent the animals from being hurt, should they get caught in any wires or frames. And we know that they do like visiting, given half a chance!

As we have reported before, a fence has been placed around the entire area. It is a wire mesh, some 2 metres high.

A small section of the reindeer fence. (Photo: A. Jutila)

At the site entrance, there is an even larger gate. Although we haven't featured this since the foundations went in, we can assure you that it is in place and working.

And stay out! A mother and calf are thwarted by the main gate. (Photo: A. Jutila)

So, given all this anti-reindeer security, we remain perplexed as to how the creatures continue to get into the KAIRA area.

Cute is no excuse... you are still on the LBA field! (Photo: A. Jutila)
And it is not just the little ones. Even large reindeer are getting in as well. Given the number of times we have checked the gate, perimeter and all, the only logical conclusion that we can draw is that they are flying in.

But flying reindeer? Really?